概要
本章对Java.util.concurrent包中的ConcurrentSkipListMap类进行详细的介绍。内容包括:
ConcurrentSkipListMap介绍
ConcurrentSkipListMap原理和数据结构
ConcurrentSkipListMap函数列表
ConcurrentSkipListMap源码分析(JDK1.7.0_40版本)
ConcurrentSkipListMap示例
转载请注明出处:http://www.cnblogs.com/skywang12345/p/3498556.html
ConcurrentSkipListMap介绍
ConcurrentSkipListMap是线程安全的有序的哈希表,适用于高并发的场景。
ConcurrentSkipListMap和TreeMap,它们虽然都是有序的哈希表。但是,第一,它们的线程安全机制不同,TreeMap是非线程安全的,而ConcurrentSkipListMap是线程安全的。第二,ConcurrentSkipListMap是通过跳表实现的,而TreeMap是通过红黑树实现的。
关于跳表(Skip List),它是平衡树的一种替代的数据结构,但是和红黑树不相同的是,跳表对于树的平衡的实现是基于一种随机化的算法的,这样也就是说跳表的插入和删除的工作是比较简单的。
ConcurrentSkipListMap原理和数据结构
ConcurrentSkipListMap的数据结构,如下图所示:
说明:
先以数据“7,14,21,32,37,71,85”序列为例,来对跳表进行简单说明。
跳表分为许多层(level),每一层都可以看作是数据的索引,这些索引的意义就是加快跳表查找数据速度。每一层的数据都是有序的,上一层数据是下一层数据的子集,并且第一层(level 1)包含了全部的数据;层次越高,跳跃性越大,包含的数据越少。
跳表包含一个表头,它查找数据时,是从上往下,从左往右进行查找。现在“需要找出值为32的节点”为例,来对比说明跳表和普遍的链表。
情况1:链表中查找“32”节点
路径如下图1-02所示:
需要4步(红色部分表示路径)。
情况2:跳表中查找“32”节点
路径如下图1-03所示:
忽略索引垂直线路上路径的情况下,只需要2步(红色部分表示路径)。
下面说说Java中ConcurrentSkipListMap的数据结构。
(01) ConcurrentSkipListMap继承于AbstractMap类,也就意味着它是一个哈希表。
(02) Index是ConcurrentSkipListMap的内部类,它与“跳表中的索引相对应”。HeadIndex继承于Index,ConcurrentSkipListMap中含有一个HeadIndex的对象head,head是“跳表的表头”。
(03) Index是跳表中的索引,它包含“右索引的指针(right)”,“下索引的指针(down)”和“哈希表节点node”。node是Node的对象,Node也是ConcurrentSkipListMap中的内部类。
ConcurrentSkipListMap函数列表
// 构造一个新的空映射,该映射按照键的自然顺序进行排序。 ConcurrentSkipListMap() // 构造一个新的空映射,该映射按照指定的比较器进行排序。 ConcurrentSkipListMap(Comparator<? super K> comparator) // 构造一个新映射,该映射所包含的映射关系与给定映射包含的映射关系相同,并按照键的自然顺序进行排序。 ConcurrentSkipListMap(Map<? extends K,? extends V> m) // 构造一个新映射,该映射所包含的映射关系与指定的有序映射包含的映射关系相同,使用的顺序也相同。 ConcurrentSkipListMap(SortedMap<K,? extends V> m) // 返回与大于等于给定键的最小键关联的键-值映射关系;如果不存在这样的条目,则返回 null。 Map.Entry<K,V> ceilingEntry(K key) // 返回大于等于给定键的最小键;如果不存在这样的键,则返回 null。 K ceilingKey(K key) // 从此映射中移除所有映射关系。 void clear() // 返回此 ConcurrentSkipListMap 实例的浅表副本。 ConcurrentSkipListMap<K,V> clone() // 返回对此映射中的键进行排序的比较器;如果此映射使用键的自然顺序,则返回 null。 Comparator<? super K> comparator() // 如果此映射包含指定键的映射关系,则返回 true。 boolean containsKey(Object key) // 如果此映射为指定值映射一个或多个键,则返回 true。 boolean containsValue(Object value) // 返回此映射中所包含键的逆序 NavigableSet 视图。 NavigableSet<K> descendingKeySet() // 返回此映射中所包含映射关系的逆序视图。 ConcurrentNavigableMap<K,V> descendingMap() // 返回此映射中所包含的映射关系的 Set 视图。 Set<Map.Entry<K,V>> entrySet() // 比较指定对象与此映射的相等性。 boolean equals(Object o) // 返回与此映射中的最小键关联的键-值映射关系;如果该映射为空,则返回 null。 Map.Entry<K,V> firstEntry() // 返回此映射中当前第一个(最低)键。 K firstKey() // 返回与小于等于给定键的最大键关联的键-值映射关系;如果不存在这样的键,则返回 null。 Map.Entry<K,V> floorEntry(K key) // 返回小于等于给定键的最大键;如果不存在这样的键,则返回 null。 K floorKey(K key) // 返回指定键所映射到的值;如果此映射不包含该键的映射关系,则返回 null。 V get(Object key) // 返回此映射的部分视图,其键值严格小于 toKey。 ConcurrentNavigableMap<K,V> headMap(K toKey) // 返回此映射的部分视图,其键小于(或等于,如果 inclusive 为 true)toKey。 ConcurrentNavigableMap<K,V> headMap(K toKey, boolean inclusive) // 返回与严格大于给定键的最小键关联的键-值映射关系;如果不存在这样的键,则返回 null。 Map.Entry<K,V> higherEntry(K key) // 返回严格大于给定键的最小键;如果不存在这样的键,则返回 null。 K higherKey(K key) // 如果此映射未包含键-值映射关系,则返回 true。 boolean isEmpty() // 返回此映射中所包含键的 NavigableSet 视图。 NavigableSet<K> keySet() // 返回与此映射中的最大键关联的键-值映射关系;如果该映射为空,则返回 null。 Map.Entry<K,V> lastEntry() // 返回映射中当前最后一个(最高)键。 K lastKey() // 返回与严格小于给定键的最大键关联的键-值映射关系;如果不存在这样的键,则返回 null。 Map.Entry<K,V> lowerEntry(K key) // 返回严格小于给定键的最大键;如果不存在这样的键,则返回 null。 K lowerKey(K key) // 返回此映射中所包含键的 NavigableSet 视图。 NavigableSet<K> navigableKeySet() // 移除并返回与此映射中的最小键关联的键-值映射关系;如果该映射为空,则返回 null。 Map.Entry<K,V> pollFirstEntry() // 移除并返回与此映射中的最大键关联的键-值映射关系;如果该映射为空,则返回 null。 Map.Entry<K,V> pollLastEntry() // 将指定值与此映射中的指定键关联。 V put(K key, V value) // 如果指定键已经不再与某个值相关联,则将它与给定值关联。 V putIfAbsent(K key, V value) // 从此映射中移除指定键的映射关系(如果存在)。 V remove(Object key) // 只有目前将键的条目映射到给定值时,才移除该键的条目。 boolean remove(Object key, Object value) // 只有目前将键的条目映射到某一值时,才替换该键的条目。 V replace(K key, V value) // 只有目前将键的条目映射到给定值时,才替换该键的条目。 boolean replace(K key, V oldValue, V newValue) // 返回此映射中的键-值映射关系数。 int size() // 返回此映射的部分视图,其键的范围从 fromKey 到 toKey。 ConcurrentNavigableMap<K,V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) // 返回此映射的部分视图,其键值的范围从 fromKey(包括)到 toKey(不包括)。 ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey) // 返回此映射的部分视图,其键大于等于 fromKey。 ConcurrentNavigableMap<K,V> tailMap(K fromKey) // 返回此映射的部分视图,其键大于(或等于,如果 inclusive 为 true)fromKey。 ConcurrentNavigableMap<K,V> tailMap(K fromKey, boolean inclusive) // 返回此映射中所包含值的 Collection 视图。 Collection<V> values()
ConcurrentSkipListMap源码分析(JDK1.7.0_40版本)
ConcurrentSkipListMap.java的完整源码如下:
1 /* 2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 * 13 * 14 * 15 * 16 * 17 * 18 * 19 * 20 * 21 * 22 * 23 */ 24 25 /* 26 * 27 * 28 * 29 * 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent; 37 import java.util.*; 38 import java.util.concurrent.atomic.*; 39 40 /** 41 * A scalable concurrent {@link ConcurrentNavigableMap} implementation. 42 * The map is sorted according to the {@linkplain Comparable natural 43 * ordering} of its keys, or by a {@link Comparator} provided at map 44 * creation time, depending on which constructor is used. 45 * 46 * <p>This class implements a concurrent variant of <a 47 * href="http://en.wikipedia.org/wiki/Skip_list" target="_top">SkipLists</a> 48 * providing expected average <i>log(n)</i> time cost for the 49 * <tt>containsKey</tt>, <tt>get</tt>, <tt>put</tt> and 50 * <tt>remove</tt> operations and their variants. Insertion, removal, 51 * update, and access operations safely execute concurrently by 52 * multiple threads. Iterators are <i>weakly consistent</i>, returning 53 * elements reflecting the state of the map at some point at or since 54 * the creation of the iterator. They do <em>not</em> throw {@link 55 * ConcurrentModificationException}, and may proceed concurrently with 56 * other operations. Ascending key ordered views and their iterators 57 * are faster than descending ones. 58 * 59 * <p>All <tt>Map.Entry</tt> pairs returned by methods in this class 60 * and its views represent snapshots of mappings at the time they were 61 * produced. They do <em>not</em> support the <tt>Entry.setValue</tt> 62 * method. (Note however that it is possible to change mappings in the 63 * associated map using <tt>put</tt>, <tt>putIfAbsent</tt>, or 64 * <tt>replace</tt>, depending on exactly which effect you need.) 65 * 66 * <p>Beware that, unlike in most collections, the <tt>size</tt> 67 * method is <em>not</em> a constant-time operation. Because of the 68 * asynchronous nature of these maps, determining the current number 69 * of elements requires a traversal of the elements, and so may report 70 * inaccurate results if this collection is modified during traversal. 71 * Additionally, the bulk operations <tt>putAll</tt>, <tt>equals</tt>, 72 * <tt>toArray</tt>, <tt>containsValue</tt>, and <tt>clear</tt> are 73 * <em>not</em> guaranteed to be performed atomically. For example, an 74 * iterator operating concurrently with a <tt>putAll</tt> operation 75 * might view only some of the added elements. 76 * 77 * <p>This class and its views and iterators implement all of the 78 * <em>optional</em> methods of the {@link Map} and {@link Iterator} 79 * interfaces. Like most other concurrent collections, this class does 80 * <em>not</em> permit the use of <tt>null</tt> keys or values because some 81 * null return values cannot be reliably distinguished from the absence of 82 * elements. 83 * 84 * <p>This class is a member of the 85 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 86 * Java Collections Framework</a>. 87 * 88 * @author Doug Lea 89 * @param <K> the type of keys maintained by this map 90 * @param <V> the type of mapped values 91 * @since 1.6 92 */ 93 public class ConcurrentSkipListMap<K,V> extends AbstractMap<K,V> 94 implements ConcurrentNavigableMap<K,V>, 95 Cloneable, 96 java.io.Serializable { 97 /* 98 * This class implements a tree-like two-dimensionally linked skip 99 * list in which the index levels are represented in separate 100 * nodes from the base nodes holding data. There are two reasons 101 * for taking this approach instead of the usual array-based 102 * structure: 1) Array based implementations seem to encounter 103 * more complexity and overhead 2) We can use cheaper algorithms 104 * for the heavily-traversed index lists than can be used for the 105 * base lists. Here's a picture of some of the basics for a 106 * possible list with 2 levels of index: 107 * 108 * Head nodes Index nodes 109 * +-+ right +-+ +-+ 110 * |2|---------------->| |--------------------->| |->null 111 * +-+ +-+ +-+ 112 * | down | | 113 * v v v 114 * +-+ +-+ +-+ +-+ +-+ +-+ 115 * |1|----------->| |->| |------>| |----------->| |------>| |->null 116 * +-+ +-+ +-+ +-+ +-+ +-+ 117 * v | | | | | 118 * Nodes next v v v v v 119 * +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ 120 * | |->|A|->|B|->|C|->|D|->|E|->|F|->|G|->|H|->|I|->|J|->|K|->null 121 * +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ +-+ 122 * 123 * The base lists use a variant of the HM linked ordered set 124 * algorithm. See Tim Harris, "A pragmatic implementation of 125 * non-blocking linked lists" 126 * http://www.cl.cam.ac.uk/~tlh20/publications.html and Maged 127 * Michael "High Performance Dynamic Lock-Free Hash Tables and 128 * List-Based Sets" 129 * http://www.research.ibm.com/people/m/michael/pubs.htm. The 130 * basic idea in these lists is to mark the "next" pointers of 131 * deleted nodes when deleting to avoid conflicts with concurrent 132 * insertions, and when traversing to keep track of triples 133 * (predecessor, node, successor) in order to detect when and how 134 * to unlink these deleted nodes. 135 * 136 * Rather than using mark-bits to mark list deletions (which can 137 * be slow and space-intensive using AtomicMarkedReference), nodes 138 * use direct CAS'able next pointers. On deletion, instead of 139 * marking a pointer, they splice in another node that can be 140 * thought of as standing for a marked pointer (indicating this by 141 * using otherwise impossible field values). Using plain nodes 142 * acts roughly like "boxed" implementations of marked pointers, 143 * but uses new nodes only when nodes are deleted, not for every 144 * link. This requires less space and supports faster 145 * traversal. Even if marked references were better supported by 146 * JVMs, traversal using this technique might still be faster 147 * because any search need only read ahead one more node than 148 * otherwise required (to check for trailing marker) rather than 149 * unmasking mark bits or whatever on each read. 150 * 151 * This approach maintains the essential property needed in the HM 152 * algorithm of changing the next-pointer of a deleted node so 153 * that any other CAS of it will fail, but implements the idea by 154 * changing the pointer to point to a different node, not by 155 * marking it. While it would be possible to further squeeze 156 * space by defining marker nodes not to have key/value fields, it 157 * isn't worth the extra type-testing overhead. The deletion 158 * markers are rarely encountered during traversal and are 159 * normally quickly garbage collected. (Note that this technique 160 * would not work well in systems without garbage collection.) 161 * 162 * In addition to using deletion markers, the lists also use 163 * nullness of value fields to indicate deletion, in a style 164 * similar to typical lazy-deletion schemes. If a node's value is 165 * null, then it is considered logically deleted and ignored even 166 * though it is still reachable. This maintains proper control of 167 * concurrent replace vs delete operations -- an attempted replace 168 * must fail if a delete beat it by nulling field, and a delete 169 * must return the last non-null value held in the field. (Note: 170 * Null, rather than some special marker, is used for value fields 171 * here because it just so happens to mesh with the Map API 172 * requirement that method get returns null if there is no 173 * mapping, which allows nodes to remain concurrently readable 174 * even when deleted. Using any other marker value here would be 175 * messy at best.) 176 * 177 * Here's the sequence of events for a deletion of node n with 178 * predecessor b and successor f, initially: 179 * 180 * +------+ +------+ +------+ 181 * ... | b |------>| n |----->| f | ... 182 * +------+ +------+ +------+ 183 * 184 * 1. CAS n's value field from non-null to null. 185 * From this point on, no public operations encountering 186 * the node consider this mapping to exist. However, other 187 * ongoing insertions and deletions might still modify 188 * n's next pointer. 189 * 190 * 2. CAS n's next pointer to point to a new marker node. 191 * From this point on, no other nodes can be appended to n. 192 * which avoids deletion errors in CAS-based linked lists. 193 * 194 * +------+ +------+ +------+ +------+ 195 * ... | b |------>| n |----->|marker|------>| f | ... 196 * +------+ +------+ +------+ +------+ 197 * 198 * 3. CAS b's next pointer over both n and its marker. 199 * From this point on, no new traversals will encounter n, 200 * and it can eventually be GCed. 201 * +------+ +------+ 202 * ... | b |----------------------------------->| f | ... 203 * +------+ +------+ 204 * 205 * A failure at step 1 leads to simple retry due to a lost race 206 * with another operation. Steps 2-3 can fail because some other 207 * thread noticed during a traversal a node with null value and 208 * helped out by marking and/or unlinking. This helping-out 209 * ensures that no thread can become stuck waiting for progress of 210 * the deleting thread. The use of marker nodes slightly 211 * complicates helping-out code because traversals must track 212 * consistent reads of up to four nodes (b, n, marker, f), not 213 * just (b, n, f), although the next field of a marker is 214 * immutable, and once a next field is CAS'ed to point to a 215 * marker, it never again changes, so this requires less care. 216 * 217 * Skip lists add indexing to this scheme, so that the base-level 218 * traversals start close to the locations being found, inserted 219 * or deleted -- usually base level traversals only traverse a few 220 * nodes. This doesn't change the basic algorithm except for the 221 * need to make sure base traversals start at predecessors (here, 222 * b) that are not (structurally) deleted, otherwise retrying 223 * after processing the deletion. 224 * 225 * Index levels are maintained as lists with volatile next fields, 226 * using CAS to link and unlink. Races are allowed in index-list 227 * operations that can (rarely) fail to link in a new index node 228 * or delete one. (We can't do this of course for data nodes.) 229 * However, even when this happens, the index lists remain sorted, 230 * so correctly serve as indices. This can impact performance, 231 * but since skip lists are probabilistic anyway, the net result 232 * is that under contention, the effective "p" value may be lower 233 * than its nominal value. And race windows are kept small enough 234 * that in practice these failures are rare, even under a lot of 235 * contention. 236 * 237 * The fact that retries (for both base and index lists) are 238 * relatively cheap due to indexing allows some minor 239 * simplifications of retry logic. Traversal restarts are 240 * performed after most "helping-out" CASes. This isn't always 241 * strictly necessary, but the implicit backoffs tend to help 242 * reduce other downstream failed CAS's enough to outweigh restart 243 * cost. This worsens the worst case, but seems to improve even 244 * highly contended cases. 245 * 246 * Unlike most skip-list implementations, index insertion and 247 * deletion here require a separate traversal pass occuring after 248 * the base-level action, to add or remove index nodes. This adds 249 * to single-threaded overhead, but improves contended 250 * multithreaded performance by narrowing interference windows, 251 * and allows deletion to ensure that all index nodes will be made 252 * unreachable upon return from a public remove operation, thus 253 * avoiding unwanted garbage retention. This is more important 254 * here than in some other data structures because we cannot null 255 * out node fields referencing user keys since they might still be 256 * read by other ongoing traversals. 257 * 258 * Indexing uses skip list parameters that maintain good search 259 * performance while using sparser-than-usual indices: The 260 * hardwired parameters k=1, p=0.5 (see method randomLevel) mean 261 * that about one-quarter of the nodes have indices. Of those that 262 * do, half have one level, a quarter have two, and so on (see 263 * Pugh's Skip List Cookbook, sec 3.4). The expected total space 264 * requirement for a map is slightly less than for the current 265 * implementation of java.util.TreeMap. 266 * 267 * Changing the level of the index (i.e, the height of the 268 * tree-like structure) also uses CAS. The head index has initial 269 * level/height of one. Creation of an index with height greater 270 * than the current level adds a level to the head index by 271 * CAS'ing on a new top-most head. To maintain good performance 272 * after a lot of removals, deletion methods heuristically try to 273 * reduce the height if the topmost levels appear to be empty. 274 * This may encounter races in which it possible (but rare) to 275 * reduce and "lose" a level just as it is about to contain an 276 * index (that will then never be encountered). This does no 277 * structural harm, and in practice appears to be a better option 278 * than allowing unrestrained growth of levels. 279 * 280 * The code for all this is more verbose than you'd like. Most 281 * operations entail locating an element (or position to insert an 282 * element). The code to do this can't be nicely factored out 283 * because subsequent uses require a snapshot of predecessor 284 * and/or successor and/or value fields which can't be returned 285 * all at once, at least not without creating yet another object 286 * to hold them -- creating such little objects is an especially 287 * bad idea for basic internal search operations because it adds 288 * to GC overhead. (This is one of the few times I've wished Java 289 * had macros.) Instead, some traversal code is interleaved within 290 * insertion and removal operations. The control logic to handle 291 * all the retry conditions is sometimes twisty. Most search is 292 * broken into 2 parts. findPredecessor() searches index nodes 293 * only, returning a base-level predecessor of the key. findNode() 294 * finishes out the base-level search. Even with this factoring, 295 * there is a fair amount of near-duplication of code to handle 296 * variants. 297 * 298 * For explanation of algorithms sharing at least a couple of 299 * features with this one, see Mikhail Fomitchev's thesis 300 * (http://www.cs.yorku.ca/~mikhail/), Keir Fraser's thesis 301 * (http://www.cl.cam.ac.uk/users/kaf24/), and Hakan Sundell's 302 * thesis (http://www.cs.chalmers.se/~phs/). 303 * 304 * Given the use of tree-like index nodes, you might wonder why 305 * this doesn't use some kind of search tree instead, which would 306 * support somewhat faster search operations. The reason is that 307 * there are no known efficient lock-free insertion and deletion 308 * algorithms for search trees. The immutability of the "down" 309 * links of index nodes (as opposed to mutable "left" fields in 310 * true trees) makes this tractable using only CAS operations. 311 * 312 * Notation guide for local variables 313 * Node: b, n, f for predecessor, node, successor 314 * Index: q, r, d for index node, right, down. 315 * t for another index node 316 * Head: h 317 * Levels: j 318 * Keys: k, key 319 * Values: v, value 320 * Comparisons: c 321 */ 322 323 private static final long serialVersionUID = -8627078645895051609L; 324 325 /** 326 * Generates the initial random seed for the cheaper per-instance 327 * random number generators used in randomLevel. 328 */ 329 private static final Random seedGenerator = new Random(); 330 331 /** 332 * Special value used to identify base-level header 333 */ 334 private static final Object BASE_HEADER = new Object(); 335 336 /** 337 * The topmost head index of the skiplist. 338 */ 339 private transient volatile HeadIndex<K,V> head; 340 341 /** 342 * The comparator used to maintain order in this map, or null 343 * if using natural ordering. 344 * @serial 345 */ 346 private final Comparator<? super K> comparator; 347 348 /** 349 * Seed for simple random number generator. Not volatile since it 350 * doesn't matter too much if different threads don't see updates. 351 */ 352 private transient int randomSeed; 353 354 /** Lazily initialized key set */ 355 private transient KeySet keySet; 356 /** Lazily initialized entry set */ 357 private transient EntrySet entrySet; 358 /** Lazily initialized values collection */ 359 private transient Values values; 360 /** Lazily initialized descending key set */ 361 private transient ConcurrentNavigableMap<K,V> descendingMap; 362 363 /** 364 * Initializes or resets state. Needed by constructors, clone, 365 * clear, readObject. and ConcurrentSkipListSet.clone. 366 * (Note that comparator must be separately initialized.) 367 */ 368 final void initialize() { 369 keySet = null; 370 entrySet = null; 371 values = null; 372 descendingMap = null; 373 randomSeed = seedGenerator.nextInt() | 0x0100; // ensure nonzero 374 head = new HeadIndex<K,V>(new Node<K,V>(null, BASE_HEADER, null), 375 null, null, 1); 376 } 377 378 /** 379 * compareAndSet head node 380 */ 381 private boolean casHead(HeadIndex<K,V> cmp, HeadIndex<K,V> val) { 382 return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); 383 } 384 385 /* ---------------- Nodes -------------- */ 386 387 /** 388 * Nodes hold keys and values, and are singly linked in sorted 389 * order, possibly with some intervening marker nodes. The list is 390 * headed by a dummy node accessible as head.node. The value field 391 * is declared only as Object because it takes special non-V 392 * values for marker and header nodes. 393 */ 394 static final class Node<K,V> { 395 final K key; 396 volatile Object value; 397 volatile Node<K,V> next; 398 399 /** 400 * Creates a new regular node. 401 */ 402 Node(K key, Object value, Node<K,V> next) { 403 this.key = key; 404 this.value = value; 405 this.next = next; 406 } 407 408 /** 409 * Creates a new marker node. A marker is distinguished by 410 * having its value field point to itself. Marker nodes also 411 * have null keys, a fact that is exploited in a few places, 412 * but this doesn't distinguish markers from the base-level 413 * header node (head.node), which also has a null key. 414 */ 415 Node(Node<K,V> next) { 416 this.key = null; 417 this.value = this; 418 this.next = next; 419 } 420 421 /** 422 * compareAndSet value field 423 */ 424 boolean casValue(Object cmp, Object val) { 425 return UNSAFE.compareAndSwapObject(this, valueOffset, cmp, val); 426 } 427 428 /** 429 * compareAndSet next field 430 */ 431 boolean casNext(Node<K,V> cmp, Node<K,V> val) { 432 return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); 433 } 434 435 /** 436 * Returns true if this node is a marker. This method isn't 437 * actually called in any current code checking for markers 438 * because callers will have already read value field and need 439 * to use that read (not another done here) and so directly 440 * test if value points to node. 441 * @param n a possibly null reference to a node 442 * @return true if this node is a marker node 443 */ 444 boolean isMarker() { 445 return value == this; 446 } 447 448 /** 449 * Returns true if this node is the header of base-level list. 450 * @return true if this node is header node 451 */ 452 boolean isBaseHeader() { 453 return value == BASE_HEADER; 454 } 455 456 /** 457 * Tries to append a deletion marker to this node. 458 * @param f the assumed current successor of this node 459 * @return true if successful 460 */ 461 boolean appendMarker(Node<K,V> f) { 462 return casNext(f, new Node<K,V>(f)); 463 } 464 465 /** 466 * Helps out a deletion by appending marker or unlinking from 467 * predecessor. This is called during traversals when value 468 * field seen to be null. 469 * @param b predecessor 470 * @param f successor 471 */ 472 void helpDelete(Node<K,V> b, Node<K,V> f) { 473 /* 474 * Rechecking links and then doing only one of the 475 * help-out stages per call tends to minimize CAS 476 * interference among helping threads. 477 */ 478 if (f == next && this == b.next) { 479 if (f == null || f.value != f) // not already marked 480 appendMarker(f); 481 else 482 b.casNext(this, f.next); 483 } 484 } 485 486 /** 487 * Returns value if this node contains a valid key-value pair, 488 * else null. 489 * @return this node's value if it isn't a marker or header or 490 * is deleted, else null. 491 */ 492 V getValidValue() { 493 Object v = value; 494 if (v == this || v == BASE_HEADER) 495 return null; 496 return (V)v; 497 } 498 499 /** 500 * Creates and returns a new SimpleImmutableEntry holding current 501 * mapping if this node holds a valid value, else null. 502 * @return new entry or null 503 */ 504 AbstractMap.SimpleImmutableEntry<K,V> createSnapshot() { 505 V v = getValidValue(); 506 if (v == null) 507 return null; 508 return new AbstractMap.SimpleImmutableEntry<K,V>(key, v); 509 } 510 511 // UNSAFE mechanics 512 513 private static final sun.misc.Unsafe UNSAFE; 514 private static final long valueOffset; 515 private static final long nextOffset; 516 517 static { 518 try { 519 UNSAFE = sun.misc.Unsafe.getUnsafe(); 520 Class k = Node.class; 521 valueOffset = UNSAFE.objectFieldOffset 522 (k.getDeclaredField("value")); 523 nextOffset = UNSAFE.objectFieldOffset 524 (k.getDeclaredField("next")); 525 } catch (Exception e) { 526 throw new Error(e); 527 } 528 } 529 } 530 531 /* ---------------- Indexing -------------- */ 532 533 /** 534 * Index nodes represent the levels of the skip list. Note that 535 * even though both Nodes and Indexes have forward-pointing 536 * fields, they have different types and are handled in different 537 * ways, that can't nicely be captured by placing field in a 538 * shared abstract class. 539 */ 540 static class Index<K,V> { 541 final Node<K,V> node; 542 final Index<K,V> down; 543 volatile Index<K,V> right; 544 545 /** 546 * Creates index node with given values. 547 */ 548 Index(Node<K,V> node, Index<K,V> down, Index<K,V> right) { 549 this.node = node; 550 this.down = down; 551 this.right = right; 552 } 553 554 /** 555 * compareAndSet right field 556 */ 557 final boolean casRight(Index<K,V> cmp, Index<K,V> val) { 558 return UNSAFE.compareAndSwapObject(this, rightOffset, cmp, val); 559 } 560 561 /** 562 * Returns true if the node this indexes has been deleted. 563 * @return true if indexed node is known to be deleted 564 */ 565 final boolean indexesDeletedNode() { 566 return node.value == null; 567 } 568 569 /** 570 * Tries to CAS newSucc as successor. To minimize races with 571 * unlink that may lose this index node, if the node being 572 * indexed is known to be deleted, it doesn't try to link in. 573 * @param succ the expected current successor 574 * @param newSucc the new successor 575 * @return true if successful 576 */ 577 final boolean link(Index<K,V> succ, Index<K,V> newSucc) { 578 Node<K,V> n = node; 579 newSucc.right = succ; 580 return n.value != null && casRight(succ, newSucc); 581 } 582 583 /** 584 * Tries to CAS right field to skip over apparent successor 585 * succ. Fails (forcing a retraversal by caller) if this node 586 * is known to be deleted. 587 * @param succ the expected current successor 588 * @return true if successful 589 */ 590 final boolean unlink(Index<K,V> succ) { 591 return !indexesDeletedNode() && casRight(succ, succ.right); 592 } 593 594 // Unsafe mechanics 595 private static final sun.misc.Unsafe UNSAFE; 596 private static final long rightOffset; 597 static { 598 try { 599 UNSAFE = sun.misc.Unsafe.getUnsafe(); 600 Class k = Index.class; 601 rightOffset = UNSAFE.objectFieldOffset 602 (k.getDeclaredField("right")); 603 } catch (Exception e) { 604 throw new Error(e); 605 } 606 } 607 } 608 609 /* ---------------- Head nodes -------------- */ 610 611 /** 612 * Nodes heading each level keep track of their level. 613 */ 614 static final class HeadIndex<K,V> extends Index<K,V> { 615 final int level; 616 HeadIndex(Node<K,V> node, Index<K,V> down, Index<K,V> right, int level) { 617 super(node, down, right); 618 this.level = level; 619 } 620 } 621 622 /* ---------------- Comparison utilities -------------- */ 623 624 /** 625 * Represents a key with a comparator as a Comparable. 626 * 627 * Because most sorted collections seem to use natural ordering on 628 * Comparables (Strings, Integers, etc), most internal methods are 629 * geared to use them. This is generally faster than checking 630 * per-comparison whether to use comparator or comparable because 631 * it doesn't require a (Comparable) cast for each comparison. 632 * (Optimizers can only sometimes remove such redundant checks 633 * themselves.) When Comparators are used, 634 * ComparableUsingComparators are created so that they act in the 635 * same way as natural orderings. This penalizes use of 636 * Comparators vs Comparables, which seems like the right 637 * tradeoff. 638 */ 639 static final class ComparableUsingComparator<K> implements Comparable<K> { 640 final K actualKey; 641 final Comparator<? super K> cmp; 642 ComparableUsingComparator(K key, Comparator<? super K> cmp) { 643 this.actualKey = key; 644 this.cmp = cmp; 645 } 646 public int compareTo(K k2) { 647 return cmp.compare(actualKey, k2); 648 } 649 } 650 651 /** 652 * If using comparator, return a ComparableUsingComparator, else 653 * cast key as Comparable, which may cause ClassCastException, 654 * which is propagated back to caller. 655 */ 656 private Comparable<? super K> comparable(Object key) 657 throws ClassCastException { 658 if (key == null) 659 throw new NullPointerException(); 660 if (comparator != null) 661 return new ComparableUsingComparator<K>((K)key, comparator); 662 else 663 return (Comparable<? super K>)key; 664 } 665 666 /** 667 * Compares using comparator or natural ordering. Used when the 668 * ComparableUsingComparator approach doesn't apply. 669 */ 670 int compare(K k1, K k2) throws ClassCastException { 671 Comparator<? super K> cmp = comparator; 672 if (cmp != null) 673 return cmp.compare(k1, k2); 674 else 675 return ((Comparable<? super K>)k1).compareTo(k2); 676 } 677 678 /** 679 * Returns true if given key greater than or equal to least and 680 * strictly less than fence, bypassing either test if least or 681 * fence are null. Needed mainly in submap operations. 682 */ 683 boolean inHalfOpenRange(K key, K least, K fence) { 684 if (key == null) 685 throw new NullPointerException(); 686 return ((least == null || compare(key, least) >= 0) && 687 (fence == null || compare(key, fence) < 0)); 688 } 689 690 /** 691 * Returns true if given key greater than or equal to least and less 692 * or equal to fence. Needed mainly in submap operations. 693 */ 694 boolean inOpenRange(K key, K least, K fence) { 695 if (key == null) 696 throw new NullPointerException(); 697 return ((least == null || compare(key, least) >= 0) && 698 (fence == null || compare(key, fence) <= 0)); 699 } 700 701 /* ---------------- Traversal -------------- */ 702 703 /** 704 * Returns a base-level node with key strictly less than given key, 705 * or the base-level header if there is no such node. Also 706 * unlinks indexes to deleted nodes found along the way. Callers 707 * rely on this side-effect of clearing indices to deleted nodes. 708 * @param key the key 709 * @return a predecessor of key 710 */ 711 private Node<K,V> findPredecessor(Comparable<? super K> key) { 712 if (key == null) 713 throw new NullPointerException(); // don't postpone errors 714 for (;;) { 715 Index<K,V> q = head; 716 Index<K,V> r = q.right; 717 for (;;) { 718 if (r != null) { 719 Node<K,V> n = r.node; 720 K k = n.key; 721 if (n.value == null) { 722 if (!q.unlink(r)) 723 break; // restart 724 r = q.right; // reread r 725 continue; 726 } 727 if (key.compareTo(k) > 0) { 728 q = r; 729 r = r.right; 730 continue; 731 } 732 } 733 Index<K,V> d = q.down; 734 if (d != null) { 735 q = d; 736 r = d.right; 737 } else 738 return q.node; 739 } 740 } 741 } 742 743 /** 744 * Returns node holding key or null if no such, clearing out any 745 * deleted nodes seen along the way. Repeatedly traverses at 746 * base-level looking for key starting at predecessor returned 747 * from findPredecessor, processing base-level deletions as 748 * encountered. Some callers rely on this side-effect of clearing 749 * deleted nodes. 750 * 751 * Restarts occur, at traversal step centered on node n, if: 752 * 753 * (1) After reading n's next field, n is no longer assumed 754 * predecessor b's current successor, which means that 755 * we don't have a consistent 3-node snapshot and so cannot 756 * unlink any subsequent deleted nodes encountered. 757 * 758 * (2) n's value field is null, indicating n is deleted, in 759 * which case we help out an ongoing structural deletion 760 * before retrying. Even though there are cases where such 761 * unlinking doesn't require restart, they aren't sorted out 762 * here because doing so would not usually outweigh cost of 763 * restarting. 764 * 765 * (3) n is a marker or n's predecessor's value field is null, 766 * indicating (among other possibilities) that 767 * findPredecessor returned a deleted node. We can't unlink 768 * the node because we don't know its predecessor, so rely 769 * on another call to findPredecessor to notice and return 770 * some earlier predecessor, which it will do. This check is 771 * only strictly needed at beginning of loop, (and the 772 * b.value check isn't strictly needed at all) but is done 773 * each iteration to help avoid contention with other 774 * threads by callers that will fail to be able to change 775 * links, and so will retry anyway. 776 * 777 * The traversal loops in doPut, doRemove, and findNear all 778 * include the same three kinds of checks. And specialized 779 * versions appear in findFirst, and findLast and their 780 * variants. They can't easily share code because each uses the 781 * reads of fields held in locals occurring in the orders they 782 * were performed. 783 * 784 * @param key the key 785 * @return node holding key, or null if no such 786 */ 787 private Node<K,V> findNode(Comparable<? super K> key) { 788 for (;;) { 789 Node<K,V> b = findPredecessor(key); 790 Node<K,V> n = b.next; 791 for (;;) { 792 if (n == null) 793 return null; 794 Node<K,V> f = n.next; 795 if (n != b.next) // inconsistent read 796 break; 797 Object v = n.value; 798 if (v == null) { // n is deleted 799 n.helpDelete(b, f); 800 break; 801 } 802 if (v == n || b.value == null) // b is deleted 803 break; 804 int c = key.compareTo(n.key); 805 if (c == 0) 806 return n; 807 if (c < 0) 808 return null; 809 b = n; 810 n = f; 811 } 812 } 813 } 814 815 /** 816 * Gets value for key using findNode. 817 * @param okey the key 818 * @return the value, or null if absent 819 */ 820 private V doGet(Object okey) { 821 Comparable<? super K> key = comparable(okey); 822 /* 823 * Loop needed here and elsewhere in case value field goes 824 * null just as it is about to be returned, in which case we 825 * lost a race with a deletion, so must retry. 826 */ 827 for (;;) { 828 Node<K,V> n = findNode(key); 829 if (n == null) 830 return null; 831 Object v = n.value; 832 if (v != null) 833 return (V)v; 834 } 835 } 836 837 /* ---------------- Insertion -------------- */ 838 839 /** 840 * Main insertion method. Adds element if not present, or 841 * replaces value if present and onlyIfAbsent is false. 842 * @param kkey the key 843 * @param value the value that must be associated with key 844 * @param onlyIfAbsent if should not insert if already present 845 * @return the old value, or null if newly inserted 846 */ 847 private V doPut(K kkey, V value, boolean onlyIfAbsent) { 848 Comparable<? super K> key = comparable(kkey); 849 for (;;) { 850 Node<K,V> b = findPredecessor(key); 851 Node<K,V> n = b.next; 852 for (;;) { 853 if (n != null) { 854 Node<K,V> f = n.next; 855 if (n != b.next) // inconsistent read 856 break; 857 Object v = n.value; 858 if (v == null) { // n is deleted 859 n.helpDelete(b, f); 860 break; 861 } 862 if (v == n || b.value == null) // b is deleted 863 break; 864 int c = key.compareTo(n.key); 865 if (c > 0) { 866 b = n; 867 n = f; 868 continue; 869 } 870 if (c == 0) { 871 if (onlyIfAbsent || n.casValue(v, value)) 872 return (V)v; 873 else 874 break; // restart if lost race to replace value 875 } 876 // else c < 0; fall through 877 } 878 879 Node<K,V> z = new Node<K,V>(kkey, value, n); 880 if (!b.casNext(n, z)) 881 break; // restart if lost race to append to b 882 int level = randomLevel(); 883 if (level > 0) 884 insertIndex(z, level); 885 return null; 886 } 887 } 888 } 889 890 /** 891 * Returns a random level for inserting a new node. 892 * Hardwired to k=1, p=0.5, max 31 (see above and 893 * Pugh's "Skip List Cookbook", sec 3.4). 894 * 895 * This uses the simplest of the generators described in George 896 * Marsaglia's "Xorshift RNGs" paper. This is not a high-quality 897 * generator but is acceptable here. 898 */ 899 private int randomLevel() { 900 int x = randomSeed; 901 x ^= x << 13; 902 x ^= x >>> 17; 903 randomSeed = x ^= x << 5; 904 if ((x & 0x80000001) != 0) // test highest and lowest bits 905 return 0; 906 int level = 1; 907 while (((x >>>= 1) & 1) != 0) ++level; 908 return level; 909 } 910 911 /** 912 * Creates and adds index nodes for the given node. 913 * @param z the node 914 * @param level the level of the index 915 */ 916 private void insertIndex(Node<K,V> z, int level) { 917 HeadIndex<K,V> h = head; 918 int max = h.level; 919 920 if (level <= max) { 921 Index<K,V> idx = null; 922 for (int i = 1; i <= level; ++i) 923 idx = new Index<K,V>(z, idx, null); 924 addIndex(idx, h, level); 925 926 } else { // Add a new level 927 /* 928 * To reduce interference by other threads checking for 929 * empty levels in tryReduceLevel, new levels are added 930 * with initialized right pointers. Which in turn requires 931 * keeping levels in an array to access them while 932 * creating new head index nodes from the opposite 933 * direction. 934 */ 935 level = max + 1; 936 Index<K,V>[] idxs = (Index<K,V>[])new Index[level+1]; 937 Index<K,V> idx = null; 938 for (int i = 1; i <= level; ++i) 939 idxs[i] = idx = new Index<K,V>(z, idx, null); 940 941 HeadIndex<K,V> oldh; 942 int k; 943 for (;;) { 944 oldh = head; 945 int oldLevel = oldh.level; 946 if (level <= oldLevel) { // lost race to add level 947 k = level; 948 break; 949 } 950 HeadIndex<K,V> newh = oldh; 951 Node<K,V> oldbase = oldh.node; 952 for (int j = oldLevel+1; j <= level; ++j) 953 newh = new HeadIndex<K,V>(oldbase, newh, idxs[j], j); 954 if (casHead(oldh, newh)) { 955 k = oldLevel; 956 break; 957 } 958 } 959 addIndex(idxs[k], oldh, k); 960 } 961 } 962 963 /** 964 * Adds given index nodes from given level down to 1. 965 * @param idx the topmost index node being inserted 966 * @param h the value of head to use to insert. This must be 967 * snapshotted by callers to provide correct insertion level 968 * @param indexLevel the level of the index 969 */ 970 private void addIndex(Index<K,V> idx, HeadIndex<K,V> h, int indexLevel) { 971 // Track next level to insert in case of retries 972 int insertionLevel = indexLevel; 973 Comparable<? super K> key = comparable(idx.node.key); 974 if (key == null) throw new NullPointerException(); 975 976 // Similar to findPredecessor, but adding index nodes along 977 // path to key. 978 for (;;) { 979 int j = h.level; 980 Index<K,V> q = h; 981 Index<K,V> r = q.right; 982 Index<K,V> t = idx; 983 for (;;) { 984 if (r != null) { 985 Node<K,V> n = r.node; 986 // compare before deletion check avoids needing recheck 987 int c = key.compareTo(n.key); 988 if (n.value == null) { 989 if (!q.unlink(r)) 990 break; 991 r = q.right; 992 continue; 993 } 994 if (c > 0) { 995 q = r; 996 r = r.right; 997 continue; 998 } 999 } 1000 1001 if (j == insertionLevel) { 1002 // Don't insert index if node already deleted 1003 if (t.indexesDeletedNode()) { 1004 findNode(key); // cleans up 1005 return; 1006 } 1007 if (!q.link(r, t)) 1008 break; // restart 1009 if (--insertionLevel == 0) { 1010 // need final deletion check before return 1011 if (t.indexesDeletedNode()) 1012 findNode(key); 1013 return; 1014 } 1015 } 1016 1017 if (--j >= insertionLevel && j < indexLevel) 1018 t = t.down; 1019 q = q.down; 1020 r = q.right; 1021 } 1022 } 1023 } 1024 1025 /* ---------------- Deletion -------------- */ 1026 1027 /** 1028 * Main deletion method. Locates node, nulls value, appends a 1029 * deletion marker, unlinks predecessor, removes associated index 1030 * nodes, and possibly reduces head index level. 1031 * 1032 * Index nodes are cleared out simply by calling findPredecessor. 1033 * which unlinks indexes to deleted nodes found along path to key, 1034 * which will include the indexes to this node. This is done 1035 * unconditionally. We can't check beforehand whether there are 1036 * index nodes because it might be the case that some or all 1037 * indexes hadn't been inserted yet for this node during initial 1038 * search for it, and we'd like to ensure lack of garbage 1039 * retention, so must call to be sure. 1040 * 1041 * @param okey the key 1042 * @param value if non-null, the value that must be 1043 * associated with key 1044 * @return the node, or null if not found 1045 */ 1046 final V doRemove(Object okey, Object value) { 1047 Comparable<? super K> key = comparable(okey); 1048 for (;;) { 1049 Node<K,V> b = findPredecessor(key); 1050 Node<K,V> n = b.next; 1051 for (;;) { 1052 if (n == null) 1053 return null; 1054 Node<K,V> f = n.next; 1055 if (n != b.next) // inconsistent read 1056 break; 1057 Object v = n.value; 1058 if (v == null) { // n is deleted 1059 n.helpDelete(b, f); 1060 break; 1061 } 1062 if (v == n || b.value == null) // b is deleted 1063 break; 1064 int c = key.compareTo(n.key); 1065 if (c < 0) 1066 return null; 1067 if (c > 0) { 1068 b = n; 1069 n = f; 1070 continue; 1071 } 1072 if (value != null && !value.equals(v)) 1073 return null; 1074 if (!n.casValue(v, null)) 1075 break; 1076 if (!n.appendMarker(f) || !b.casNext(n, f)) 1077 findNode(key); // Retry via findNode 1078 else { 1079 findPredecessor(key); // Clean index 1080 if (head.right == null) 1081 tryReduceLevel(); 1082 } 1083 return (V)v; 1084 } 1085 } 1086 } 1087 1088 /** 1089 * Possibly reduce head level if it has no nodes. This method can 1090 * (rarely) make mistakes, in which case levels can disappear even 1091 * though they are about to contain index nodes. This impacts 1092 * performance, not correctness. To minimize mistakes as well as 1093 * to reduce hysteresis, the level is reduced by one only if the 1094 * topmost three levels look empty. Also, if the removed level 1095 * looks non-empty after CAS, we try to change it back quick 1096 * before anyone notices our mistake! (This trick works pretty 1097 * well because this method will practically never make mistakes 1098 * unless current thread stalls immediately before first CAS, in 1099 * which case it is very unlikely to stall again immediately 1100 * afterwards, so will recover.) 1101 * 1102 * We put up with all this rather than just let levels grow 1103 * because otherwise, even a small map that has undergone a large 1104 * number of insertions and removals will have a lot of levels, 1105 * slowing down access more than would an occasional unwanted 1106 * reduction. 1107 */ 1108 private void tryReduceLevel() { 1109 HeadIndex<K,V> h = head; 1110 HeadIndex<K,V> d; 1111 HeadIndex<K,V> e; 1112 if (h.level > 3 && 1113 (d = (HeadIndex<K,V>)h.down) != null && 1114 (e = (HeadIndex<K,V>)d.down) != null && 1115 e.right == null && 1116 d.right == null && 1117 h.right == null && 1118 casHead(h, d) && // try to set 1119 h.right != null) // recheck 1120 casHead(d, h); // try to backout 1121 } 1122 1123 /* ---------------- Finding and removing first element -------------- */ 1124 1125 /** 1126 * Specialized variant of findNode to get first valid node. 1127 * @return first node or null if empty 1128 */ 1129 Node<K,V> findFirst() { 1130 for (;;) { 1131 Node<K,V> b = head.node; 1132 Node<K,V> n = b.next; 1133 if (n == null) 1134 return null; 1135 if (n.value != null) 1136 return n; 1137 n.helpDelete(b, n.next); 1138 } 1139 } 1140 1141 /** 1142 * Removes first entry; returns its snapshot. 1143 * @return null if empty, else snapshot of first entry 1144 */ 1145 Map.Entry<K,V> doRemoveFirstEntry() { 1146 for (;;) { 1147 Node<K,V> b = head.node; 1148 Node<K,V> n = b.next; 1149 if (n == null) 1150 return null; 1151 Node<K,V> f = n.next; 1152 if (n != b.next) 1153 continue; 1154 Object v = n.value; 1155 if (v == null) { 1156 n.helpDelete(b, f); 1157 continue; 1158 } 1159 if (!n.casValue(v, null)) 1160 continue; 1161 if (!n.appendMarker(f) || !b.casNext(n, f)) 1162 findFirst(); // retry 1163 clearIndexToFirst(); 1164 return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, (V)v); 1165 } 1166 } 1167 1168 /** 1169 * Clears out index nodes associated with deleted first entry. 1170 */ 1171 private void clearIndexToFirst() { 1172 for (;;) { 1173 Index<K,V> q = head; 1174 for (;;) { 1175 Index<K,V> r = q.right; 1176 if (r != null && r.indexesDeletedNode() && !q.unlink(r)) 1177 break; 1178 if ((q = q.down) == null) { 1179 if (head.right == null) 1180 tryReduceLevel(); 1181 return; 1182 } 1183 } 1184 } 1185 } 1186 1187 1188 /* ---------------- Finding and removing last element -------------- */ 1189 1190 /** 1191 * Specialized version of find to get last valid node. 1192 * @return last node or null if empty 1193 */ 1194 Node<K,V> findLast() { 1195 /* 1196 * findPredecessor can't be used to traverse index level 1197 * because this doesn't use comparisons. So traversals of 1198 * both levels are folded together. 1199 */ 1200 Index<K,V> q = head; 1201 for (;;) { 1202 Index<K,V> d, r; 1203 if ((r = q.right) != null) { 1204 if (r.indexesDeletedNode()) { 1205 q.unlink(r); 1206 q = head; // restart 1207 } 1208 else 1209 q = r; 1210 } else if ((d = q.down) != null) { 1211 q = d; 1212 } else { 1213 Node<K,V> b = q.node; 1214 Node<K,V> n = b.next; 1215 for (;;) { 1216 if (n == null) 1217 return b.isBaseHeader() ? null : b; 1218 Node<K,V> f = n.next; // inconsistent read 1219 if (n != b.next) 1220 break; 1221 Object v = n.value; 1222 if (v == null) { // n is deleted 1223 n.helpDelete(b, f); 1224 break; 1225 } 1226 if (v == n || b.value == null) // b is deleted 1227 break; 1228 b = n; 1229 n = f; 1230 } 1231 q = head; // restart 1232 } 1233 } 1234 } 1235 1236 /** 1237 * Specialized variant of findPredecessor to get predecessor of last 1238 * valid node. Needed when removing the last entry. It is possible 1239 * that all successors of returned node will have been deleted upon 1240 * return, in which case this method can be retried. 1241 * @return likely predecessor of last node 1242 */ 1243 private Node<K,V> findPredecessorOfLast() { 1244 for (;;) { 1245 Index<K,V> q = head; 1246 for (;;) { 1247 Index<K,V> d, r; 1248 if ((r = q.right) != null) { 1249 if (r.indexesDeletedNode()) { 1250 q.unlink(r); 1251 break; // must restart 1252 } 1253 // proceed as far across as possible without overshooting 1254 if (r.node.next != null) { 1255 q = r; 1256 continue; 1257 } 1258 } 1259 if ((d = q.down) != null) 1260 q = d; 1261 else 1262 return q.node; 1263 } 1264 } 1265 } 1266 1267 /** 1268 * Removes last entry; returns its snapshot. 1269 * Specialized variant of doRemove. 1270 * @return null if empty, else snapshot of last entry 1271 */ 1272 Map.Entry<K,V> doRemoveLastEntry() { 1273 for (;;) { 1274 Node<K,V> b = findPredecessorOfLast(); 1275 Node<K,V> n = b.next; 1276 if (n == null) { 1277 if (b.isBaseHeader()) // empty 1278 return null; 1279 else 1280 continue; // all b's successors are deleted; retry 1281 } 1282 for (;;) { 1283 Node<K,V> f = n.next; 1284 if (n != b.next) // inconsistent read 1285 break; 1286 Object v = n.value; 1287 if (v == null) { // n is deleted 1288 n.helpDelete(b, f); 1289 break; 1290 } 1291 if (v == n || b.value == null) // b is deleted 1292 break; 1293 if (f != null) { 1294 b = n; 1295 n = f; 1296 continue; 1297 } 1298 if (!n.casValue(v, null)) 1299 break; 1300 K key = n.key; 1301 Comparable<? super K> ck = comparable(key); 1302 if (!n.appendMarker(f) || !b.casNext(n, f)) 1303 findNode(ck); // Retry via findNode 1304 else { 1305 findPredecessor(ck); // Clean index 1306 if (head.right == null) 1307 tryReduceLevel(); 1308 } 1309 return new AbstractMap.SimpleImmutableEntry<K,V>(key, (V)v); 1310 } 1311 } 1312 } 1313 1314 /* ---------------- Relational operations -------------- */ 1315 1316 // Control values OR'ed as arguments to findNear 1317 1318 private static final int EQ = 1; 1319 private static final int LT = 2; 1320 private static final int GT = 0; // Actually checked as !LT 1321 1322 /** 1323 * Utility for ceiling, floor, lower, higher methods. 1324 * @param kkey the key 1325 * @param rel the relation -- OR'ed combination of EQ, LT, GT 1326 * @return nearest node fitting relation, or null if no such 1327 */ 1328 Node<K,V> findNear(K kkey, int rel) { 1329 Comparable<? super K> key = comparable(kkey); 1330 for (;;) { 1331 Node<K,V> b = findPredecessor(key); 1332 Node<K,V> n = b.next; 1333 for (;;) { 1334 if (n == null) 1335 return ((rel & LT) == 0 || b.isBaseHeader()) ? null : b; 1336 Node<K,V> f = n.next; 1337 if (n != b.next) // inconsistent read 1338 break; 1339 Object v = n.value; 1340 if (v == null) { // n is deleted 1341 n.helpDelete(b, f); 1342 break; 1343 } 1344 if (v == n || b.value == null) // b is deleted 1345 break; 1346 int c = key.compareTo(n.key); 1347 if ((c == 0 && (rel & EQ) != 0) || 1348 (c < 0 && (rel & LT) == 0)) 1349 return n; 1350 if ( c <= 0 && (rel & LT) != 0) 1351 return b.isBaseHeader() ? null : b; 1352 b = n; 1353 n = f; 1354 } 1355 } 1356 } 1357 1358 /** 1359 * Returns SimpleImmutableEntry for results of findNear. 1360 * @param key the key 1361 * @param rel the relation -- OR'ed combination of EQ, LT, GT 1362 * @return Entry fitting relation, or null if no such 1363 */ 1364 AbstractMap.SimpleImmutableEntry<K,V> getNear(K key, int rel) { 1365 for (;;) { 1366 Node<K,V> n = findNear(key, rel); 1367 if (n == null) 1368 return null; 1369 AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot(); 1370 if (e != null) 1371 return e; 1372 } 1373 } 1374 1375 1376 /* ---------------- Constructors -------------- */ 1377 1378 /** 1379 * Constructs a new, empty map, sorted according to the 1380 * {@linkplain Comparable natural ordering} of the keys. 1381 */ 1382 public ConcurrentSkipListMap() { 1383 this.comparator = null; 1384 initialize(); 1385 } 1386 1387 /** 1388 * Constructs a new, empty map, sorted according to the specified 1389 * comparator. 1390 * 1391 * @param comparator the comparator that will be used to order this map. 1392 * If <tt>null</tt>, the {@linkplain Comparable natural 1393 * ordering} of the keys will be used. 1394 */ 1395 public ConcurrentSkipListMap(Comparator<? super K> comparator) { 1396 this.comparator = comparator; 1397 initialize(); 1398 } 1399 1400 /** 1401 * Constructs a new map containing the same mappings as the given map, 1402 * sorted according to the {@linkplain Comparable natural ordering} of 1403 * the keys. 1404 * 1405 * @param m the map whose mappings are to be placed in this map 1406 * @throws ClassCastException if the keys in <tt>m</tt> are not 1407 * {@link Comparable}, or are not mutually comparable 1408 * @throws NullPointerException if the specified map or any of its keys 1409 * or values are null 1410 */ 1411 public ConcurrentSkipListMap(Map<? extends K, ? extends V> m) { 1412 this.comparator = null; 1413 initialize(); 1414 putAll(m); 1415 } 1416 1417 /** 1418 * Constructs a new map containing the same mappings and using the 1419 * same ordering as the specified sorted map. 1420 * 1421 * @param m the sorted map whose mappings are to be placed in this 1422 * map, and whose comparator is to be used to sort this map 1423 * @throws NullPointerException if the specified sorted map or any of 1424 * its keys or values are null 1425 */ 1426 public ConcurrentSkipListMap(SortedMap<K, ? extends V> m) { 1427 this.comparator = m.comparator(); 1428 initialize(); 1429 buildFromSorted(m); 1430 } 1431 1432 /** 1433 * Returns a shallow copy of this <tt>ConcurrentSkipListMap</tt> 1434 * instance. (The keys and values themselves are not cloned.) 1435 * 1436 * @return a shallow copy of this map 1437 */ 1438 public ConcurrentSkipListMap<K,V> clone() { 1439 ConcurrentSkipListMap<K,V> clone = null; 1440 try { 1441 clone = (ConcurrentSkipListMap<K,V>) super.clone(); 1442 } catch (CloneNotSupportedException e) { 1443 throw new InternalError(); 1444 } 1445 1446 clone.initialize(); 1447 clone.buildFromSorted(this); 1448 return clone; 1449 } 1450 1451 /** 1452 * Streamlined bulk insertion to initialize from elements of 1453 * given sorted map. Call only from constructor or clone 1454 * method. 1455 */ 1456 private void buildFromSorted(SortedMap<K, ? extends V> map) { 1457 if (map == null) 1458 throw new NullPointerException(); 1459 1460 HeadIndex<K,V> h = head; 1461 Node<K,V> basepred = h.node; 1462 1463 // Track the current rightmost node at each level. Uses an 1464 // ArrayList to avoid committing to initial or maximum level. 1465 ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>(); 1466 1467 // initialize 1468 for (int i = 0; i <= h.level; ++i) 1469 preds.add(null); 1470 Index<K,V> q = h; 1471 for (int i = h.level; i > 0; --i) { 1472 preds.set(i, q); 1473 q = q.down; 1474 } 1475 1476 Iterator<? extends Map.Entry<? extends K, ? extends V>> it = 1477 map.entrySet().iterator(); 1478 while (it.hasNext()) { 1479 Map.Entry<? extends K, ? extends V> e = it.next(); 1480 int j = randomLevel(); 1481 if (j > h.level) j = h.level + 1; 1482 K k = e.getKey(); 1483 V v = e.getValue(); 1484 if (k == null || v == null) 1485 throw new NullPointerException(); 1486 Node<K,V> z = new Node<K,V>(k, v, null); 1487 basepred.next = z; 1488 basepred = z; 1489 if (j > 0) { 1490 Index<K,V> idx = null; 1491 for (int i = 1; i <= j; ++i) { 1492 idx = new Index<K,V>(z, idx, null); 1493 if (i > h.level) 1494 h = new HeadIndex<K,V>(h.node, h, idx, i); 1495 1496 if (i < preds.size()) { 1497 preds.get(i).right = idx; 1498 preds.set(i, idx); 1499 } else 1500 preds.add(idx); 1501 } 1502 } 1503 } 1504 head = h; 1505 } 1506 1507 /* ---------------- Serialization -------------- */ 1508 1509 /** 1510 * Save the state of this map to a stream. 1511 * 1512 * @serialData The key (Object) and value (Object) for each 1513 * key-value mapping represented by the map, followed by 1514 * <tt>null</tt>. The key-value mappings are emitted in key-order 1515 * (as determined by the Comparator, or by the keys' natural 1516 * ordering if no Comparator). 1517 */ 1518 private void writeObject(java.io.ObjectOutputStream s) 1519 throws java.io.IOException { 1520 // Write out the Comparator and any hidden stuff 1521 s.defaultWriteObject(); 1522 1523 // Write out keys and values (alternating) 1524 for (Node<K,V> n = findFirst(); n != null; n = n.next) { 1525 V v = n.getValidValue(); 1526 if (v != null) { 1527 s.writeObject(n.key); 1528 s.writeObject(v); 1529 } 1530 } 1531 s.writeObject(null); 1532 } 1533 1534 /** 1535 * Reconstitute the map from a stream. 1536 */ 1537 private void readObject(final java.io.ObjectInputStream s) 1538 throws java.io.IOException, ClassNotFoundException { 1539 // Read in the Comparator and any hidden stuff 1540 s.defaultReadObject(); 1541 // Reset transients 1542 initialize(); 1543 1544 /* 1545 * This is nearly identical to buildFromSorted, but is 1546 * distinct because readObject calls can't be nicely adapted 1547 * as the kind of iterator needed by buildFromSorted. (They 1548 * can be, but doing so requires type cheats and/or creation 1549 * of adaptor classes.) It is simpler to just adapt the code. 1550 */ 1551 1552 HeadIndex<K,V> h = head; 1553 Node<K,V> basepred = h.node; 1554 ArrayList<Index<K,V>> preds = new ArrayList<Index<K,V>>(); 1555 for (int i = 0; i <= h.level; ++i) 1556 preds.add(null); 1557 Index<K,V> q = h; 1558 for (int i = h.level; i > 0; --i) { 1559 preds.set(i, q); 1560 q = q.down; 1561 } 1562 1563 for (;;) { 1564 Object k = s.readObject(); 1565 if (k == null) 1566 break; 1567 Object v = s.readObject(); 1568 if (v == null) 1569 throw new NullPointerException(); 1570 K key = (K) k; 1571 V val = (V) v; 1572 int j = randomLevel(); 1573 if (j > h.level) j = h.level + 1; 1574 Node<K,V> z = new Node<K,V>(key, val, null); 1575 basepred.next = z; 1576 basepred = z; 1577 if (j > 0) { 1578 Index<K,V> idx = null; 1579 for (int i = 1; i <= j; ++i) { 1580 idx = new Index<K,V>(z, idx, null); 1581 if (i > h.level) 1582 h = new HeadIndex<K,V>(h.node, h, idx, i); 1583 1584 if (i < preds.size()) { 1585 preds.get(i).right = idx; 1586 preds.set(i, idx); 1587 } else 1588 preds.add(idx); 1589 } 1590 } 1591 } 1592 head = h; 1593 } 1594 1595 /* ------ Map API methods ------ */ 1596 1597 /** 1598 * Returns <tt>true</tt> if this map contains a mapping for the specified 1599 * key. 1600 * 1601 * @param key key whose presence in this map is to be tested 1602 * @return <tt>true</tt> if this map contains a mapping for the specified key 1603 * @throws ClassCastException if the specified key cannot be compared 1604 * with the keys currently in the map 1605 * @throws NullPointerException if the specified key is null 1606 */ 1607 public boolean containsKey(Object key) { 1608 return doGet(key) != null; 1609 } 1610 1611 /** 1612 * Returns the value to which the specified key is mapped, 1613 * or {@code null} if this map contains no mapping for the key. 1614 * 1615 * <p>More formally, if this map contains a mapping from a key 1616 * {@code k} to a value {@code v} such that {@code key} compares 1617 * equal to {@code k} according to the map's ordering, then this 1618 * method returns {@code v}; otherwise it returns {@code null}. 1619 * (There can be at most one such mapping.) 1620 * 1621 * @throws ClassCastException if the specified key cannot be compared 1622 * with the keys currently in the map 1623 * @throws NullPointerException if the specified key is null 1624 */ 1625 public V get(Object key) { 1626 return doGet(key); 1627 } 1628 1629 /** 1630 * Associates the specified value with the specified key in this map. 1631 * If the map previously contained a mapping for the key, the old 1632 * value is replaced. 1633 * 1634 * @param key key with which the specified value is to be associated 1635 * @param value value to be associated with the specified key 1636 * @return the previous value associated with the specified key, or 1637 * <tt>null</tt> if there was no mapping for the key 1638 * @throws ClassCastException if the specified key cannot be compared 1639 * with the keys currently in the map 1640 * @throws NullPointerException if the specified key or value is null 1641 */ 1642 public V put(K key, V value) { 1643 if (value == null) 1644 throw new NullPointerException(); 1645 return doPut(key, value, false); 1646 } 1647 1648 /** 1649 * Removes the mapping for the specified key from this map if present. 1650 * 1651 * @param key key for which mapping should be removed 1652 * @return the previous value associated with the specified key, or 1653 * <tt>null</tt> if there was no mapping for the key 1654 * @throws ClassCastException if the specified key cannot be compared 1655 * with the keys currently in the map 1656 * @throws NullPointerException if the specified key is null 1657 */ 1658 public V remove(Object key) { 1659 return doRemove(key, null); 1660 } 1661 1662 /** 1663 * Returns <tt>true</tt> if this map maps one or more keys to the 1664 * specified value. This operation requires time linear in the 1665 * map size. Additionally, it is possible for the map to change 1666 * during execution of this method, in which case the returned 1667 * result may be inaccurate. 1668 * 1669 * @param value value whose presence in this map is to be tested 1670 * @return <tt>true</tt> if a mapping to <tt>value</tt> exists; 1671 * <tt>false</tt> otherwise 1672 * @throws NullPointerException if the specified value is null 1673 */ 1674 public boolean containsValue(Object value) { 1675 if (value == null) 1676 throw new NullPointerException(); 1677 for (Node<K,V> n = findFirst(); n != null; n = n.next) { 1678 V v = n.getValidValue(); 1679 if (v != null && value.equals(v)) 1680 return true; 1681 } 1682 return false; 1683 } 1684 1685 /** 1686 * Returns the number of key-value mappings in this map. If this map 1687 * contains more than <tt>Integer.MAX_VALUE</tt> elements, it 1688 * returns <tt>Integer.MAX_VALUE</tt>. 1689 * 1690 * <p>Beware that, unlike in most collections, this method is 1691 * <em>NOT</em> a constant-time operation. Because of the 1692 * asynchronous nature of these maps, determining the current 1693 * number of elements requires traversing them all to count them. 1694 * Additionally, it is possible for the size to change during 1695 * execution of this method, in which case the returned result 1696 * will be inaccurate. Thus, this method is typically not very 1697 * useful in concurrent applications. 1698 * 1699 * @return the number of elements in this map 1700 */ 1701 public int size() { 1702 long count = 0; 1703 for (Node<K,V> n = findFirst(); n != null; n = n.next) { 1704 if (n.getValidValue() != null) 1705 ++count; 1706 } 1707 return (count >= Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) count; 1708 } 1709 1710 /** 1711 * Returns <tt>true</tt> if this map contains no key-value mappings. 1712 * @return <tt>true</tt> if this map contains no key-value mappings 1713 */ 1714 public boolean isEmpty() { 1715 return findFirst() == null; 1716 } 1717 1718 /** 1719 * Removes all of the mappings from this map. 1720 */ 1721 public void clear() { 1722 initialize(); 1723 } 1724 1725 /* ---------------- View methods -------------- */ 1726 1727 /* 1728 * Note: Lazy initialization works for views because view classes 1729 * are stateless/immutable so it doesn't matter wrt correctness if 1730 * more than one is created (which will only rarely happen). Even 1731 * so, the following idiom conservatively ensures that the method 1732 * returns the one it created if it does so, not one created by 1733 * another racing thread. 1734 */ 1735 1736 /** 1737 * Returns a {@link NavigableSet} view of the keys contained in this map. 1738 * The set's iterator returns the keys in ascending order. 1739 * The set is backed by the map, so changes to the map are 1740 * reflected in the set, and vice-versa. The set supports element 1741 * removal, which removes the corresponding mapping from the map, 1742 * via the {@code Iterator.remove}, {@code Set.remove}, 1743 * {@code removeAll}, {@code retainAll}, and {@code clear} 1744 * operations. It does not support the {@code add} or {@code addAll} 1745 * operations. 1746 * 1747 * <p>The view's {@code iterator} is a "weakly consistent" iterator 1748 * that will never throw {@link ConcurrentModificationException}, 1749 * and guarantees to traverse elements as they existed upon 1750 * construction of the iterator, and may (but is not guaranteed to) 1751 * reflect any modifications subsequent to construction. 1752 * 1753 * <p>This method is equivalent to method {@code navigableKeySet}. 1754 * 1755 * @return a navigable set view of the keys in this map 1756 */ 1757 public NavigableSet<K> keySet() { 1758 KeySet ks = keySet; 1759 return (ks != null) ? ks : (keySet = new KeySet(this)); 1760 } 1761 1762 public NavigableSet<K> navigableKeySet() { 1763 KeySet ks = keySet; 1764 return (ks != null) ? ks : (keySet = new KeySet(this)); 1765 } 1766 1767 /** 1768 * Returns a {@link Collection} view of the values contained in this map. 1769 * The collection's iterator returns the values in ascending order 1770 * of the corresponding keys. 1771 * The collection is backed by the map, so changes to the map are 1772 * reflected in the collection, and vice-versa. The collection 1773 * supports element removal, which removes the corresponding 1774 * mapping from the map, via the <tt>Iterator.remove</tt>, 1775 * <tt>Collection.remove</tt>, <tt>removeAll</tt>, 1776 * <tt>retainAll</tt> and <tt>clear</tt> operations. It does not 1777 * support the <tt>add</tt> or <tt>addAll</tt> operations. 1778 * 1779 * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator 1780 * that will never throw {@link ConcurrentModificationException}, 1781 * and guarantees to traverse elements as they existed upon 1782 * construction of the iterator, and may (but is not guaranteed to) 1783 * reflect any modifications subsequent to construction. 1784 */ 1785 public Collection<V> values() { 1786 Values vs = values; 1787 return (vs != null) ? vs : (values = new Values(this)); 1788 } 1789 1790 /** 1791 * Returns a {@link Set} view of the mappings contained in this map. 1792 * The set's iterator returns the entries in ascending key order. 1793 * The set is backed by the map, so changes to the map are 1794 * reflected in the set, and vice-versa. The set supports element 1795 * removal, which removes the corresponding mapping from the map, 1796 * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, 1797 * <tt>removeAll</tt>, <tt>retainAll</tt> and <tt>clear</tt> 1798 * operations. It does not support the <tt>add</tt> or 1799 * <tt>addAll</tt> operations. 1800 * 1801 * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator 1802 * that will never throw {@link ConcurrentModificationException}, 1803 * and guarantees to traverse elements as they existed upon 1804 * construction of the iterator, and may (but is not guaranteed to) 1805 * reflect any modifications subsequent to construction. 1806 * 1807 * <p>The <tt>Map.Entry</tt> elements returned by 1808 * <tt>iterator.next()</tt> do <em>not</em> support the 1809 * <tt>setValue</tt> operation. 1810 * 1811 * @return a set view of the mappings contained in this map, 1812 * sorted in ascending key order 1813 */ 1814 public Set<Map.Entry<K,V>> entrySet() { 1815 EntrySet es = entrySet; 1816 return (es != null) ? es : (entrySet = new EntrySet(this)); 1817 } 1818 1819 public ConcurrentNavigableMap<K,V> descendingMap() { 1820 ConcurrentNavigableMap<K,V> dm = descendingMap; 1821 return (dm != null) ? dm : (descendingMap = new SubMap<K,V> 1822 (this, null, false, null, false, true)); 1823 } 1824 1825 public NavigableSet<K> descendingKeySet() { 1826 return descendingMap().navigableKeySet(); 1827 } 1828 1829 /* ---------------- AbstractMap Overrides -------------- */ 1830 1831 /** 1832 * Compares the specified object with this map for equality. 1833 * Returns <tt>true</tt> if the given object is also a map and the 1834 * two maps represent the same mappings. More formally, two maps 1835 * <tt>m1</tt> and <tt>m2</tt> represent the same mappings if 1836 * <tt>m1.entrySet().equals(m2.entrySet())</tt>. This 1837 * operation may return misleading results if either map is 1838 * concurrently modified during execution of this method. 1839 * 1840 * @param o object to be compared for equality with this map 1841 * @return <tt>true</tt> if the specified object is equal to this map 1842 */ 1843 public boolean equals(Object o) { 1844 if (o == this) 1845 return true; 1846 if (!(o instanceof Map)) 1847 return false; 1848 Map<?,?> m = (Map<?,?>) o; 1849 try { 1850 for (Map.Entry<K,V> e : this.entrySet()) 1851 if (! e.getValue().equals(m.get(e.getKey()))) 1852 return false; 1853 for (Map.Entry<?,?> e : m.entrySet()) { 1854 Object k = e.getKey(); 1855 Object v = e.getValue(); 1856 if (k == null || v == null || !v.equals(get(k))) 1857 return false; 1858 } 1859 return true; 1860 } catch (ClassCastException unused) { 1861 return false; 1862 } catch (NullPointerException unused) { 1863 return false; 1864 } 1865 } 1866 1867 /* ------ ConcurrentMap API methods ------ */ 1868 1869 /** 1870 * {@inheritDoc} 1871 * 1872 * @return the previous value associated with the specified key, 1873 * or <tt>null</tt> if there was no mapping for the key 1874 * @throws ClassCastException if the specified key cannot be compared 1875 * with the keys currently in the map 1876 * @throws NullPointerException if the specified key or value is null 1877 */ 1878 public V putIfAbsent(K key, V value) { 1879 if (value == null) 1880 throw new NullPointerException(); 1881 return doPut(key, value, true); 1882 } 1883 1884 /** 1885 * {@inheritDoc} 1886 * 1887 * @throws ClassCastException if the specified key cannot be compared 1888 * with the keys currently in the map 1889 * @throws NullPointerException if the specified key is null 1890 */ 1891 public boolean remove(Object key, Object value) { 1892 if (key == null) 1893 throw new NullPointerException(); 1894 if (value == null) 1895 return false; 1896 return doRemove(key, value) != null; 1897 } 1898 1899 /** 1900 * {@inheritDoc} 1901 * 1902 * @throws ClassCastException if the specified key cannot be compared 1903 * with the keys currently in the map 1904 * @throws NullPointerException if any of the arguments are null 1905 */ 1906 public boolean replace(K key, V oldValue, V newValue) { 1907 if (oldValue == null || newValue == null) 1908 throw new NullPointerException(); 1909 Comparable<? super K> k = comparable(key); 1910 for (;;) { 1911 Node<K,V> n = findNode(k); 1912 if (n == null) 1913 return false; 1914 Object v = n.value; 1915 if (v != null) { 1916 if (!oldValue.equals(v)) 1917 return false; 1918 if (n.casValue(v, newValue)) 1919 return true; 1920 } 1921 } 1922 } 1923 1924 /** 1925 * {@inheritDoc} 1926 * 1927 * @return the previous value associated with the specified key, 1928 * or <tt>null</tt> if there was no mapping for the key 1929 * @throws ClassCastException if the specified key cannot be compared 1930 * with the keys currently in the map 1931 * @throws NullPointerException if the specified key or value is null 1932 */ 1933 public V replace(K key, V value) { 1934 if (value == null) 1935 throw new NullPointerException(); 1936 Comparable<? super K> k = comparable(key); 1937 for (;;) { 1938 Node<K,V> n = findNode(k); 1939 if (n == null) 1940 return null; 1941 Object v = n.value; 1942 if (v != null && n.casValue(v, value)) 1943 return (V)v; 1944 } 1945 } 1946 1947 /* ------ SortedMap API methods ------ */ 1948 1949 public Comparator<? super K> comparator() { 1950 return comparator; 1951 } 1952 1953 /** 1954 * @throws NoSuchElementException {@inheritDoc} 1955 */ 1956 public K firstKey() { 1957 Node<K,V> n = findFirst(); 1958 if (n == null) 1959 throw new NoSuchElementException(); 1960 return n.key; 1961 } 1962 1963 /** 1964 * @throws NoSuchElementException {@inheritDoc} 1965 */ 1966 public K lastKey() { 1967 Node<K,V> n = findLast(); 1968 if (n == null) 1969 throw new NoSuchElementException(); 1970 return n.key; 1971 } 1972 1973 /** 1974 * @throws ClassCastException {@inheritDoc} 1975 * @throws NullPointerException if {@code fromKey} or {@code toKey} is null 1976 * @throws IllegalArgumentException {@inheritDoc} 1977 */ 1978 public ConcurrentNavigableMap<K,V> subMap(K fromKey, 1979 boolean fromInclusive, 1980 K toKey, 1981 boolean toInclusive) { 1982 if (fromKey == null || toKey == null) 1983 throw new NullPointerException(); 1984 return new SubMap<K,V> 1985 (this, fromKey, fromInclusive, toKey, toInclusive, false); 1986 } 1987 1988 /** 1989 * @throws ClassCastException {@inheritDoc} 1990 * @throws NullPointerException if {@code toKey} is null 1991 * @throws IllegalArgumentException {@inheritDoc} 1992 */ 1993 public ConcurrentNavigableMap<K,V> headMap(K toKey, 1994 boolean inclusive) { 1995 if (toKey == null) 1996 throw new NullPointerException(); 1997 return new SubMap<K,V> 1998 (this, null, false, toKey, inclusive, false); 1999 } 2000 2001 /** 2002 * @throws ClassCastException {@inheritDoc} 2003 * @throws NullPointerException if {@code fromKey} is null 2004 * @throws IllegalArgumentException {@inheritDoc} 2005 */ 2006 public ConcurrentNavigableMap<K,V> tailMap(K fromKey, 2007 boolean inclusive) { 2008 if (fromKey == null) 2009 throw new NullPointerException(); 2010 return new SubMap<K,V> 2011 (this, fromKey, inclusive, null, false, false); 2012 } 2013 2014 /** 2015 * @throws ClassCastException {@inheritDoc} 2016 * @throws NullPointerException if {@code fromKey} or {@code toKey} is null 2017 * @throws IllegalArgumentException {@inheritDoc} 2018 */ 2019 public ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey) { 2020 return subMap(fromKey, true, toKey, false); 2021 } 2022 2023 /** 2024 * @throws ClassCastException {@inheritDoc} 2025 * @throws NullPointerException if {@code toKey} is null 2026 * @throws IllegalArgumentException {@inheritDoc} 2027 */ 2028 public ConcurrentNavigableMap<K,V> headMap(K toKey) { 2029 return headMap(toKey, false); 2030 } 2031 2032 /** 2033 * @throws ClassCastException {@inheritDoc} 2034 * @throws NullPointerException if {@code fromKey} is null 2035 * @throws IllegalArgumentException {@inheritDoc} 2036 */ 2037 public ConcurrentNavigableMap<K,V> tailMap(K fromKey) { 2038 return tailMap(fromKey, true); 2039 } 2040 2041 /* ---------------- Relational operations -------------- */ 2042 2043 /** 2044 * Returns a key-value mapping associated with the greatest key 2045 * strictly less than the given key, or <tt>null</tt> if there is 2046 * no such key. The returned entry does <em>not</em> support the 2047 * <tt>Entry.setValue</tt> method. 2048 * 2049 * @throws ClassCastException {@inheritDoc} 2050 * @throws NullPointerException if the specified key is null 2051 */ 2052 public Map.Entry<K,V> lowerEntry(K key) { 2053 return getNear(key, LT); 2054 } 2055 2056 /** 2057 * @throws ClassCastException {@inheritDoc} 2058 * @throws NullPointerException if the specified key is null 2059 */ 2060 public K lowerKey(K key) { 2061 Node<K,V> n = findNear(key, LT); 2062 return (n == null) ? null : n.key; 2063 } 2064 2065 /** 2066 * Returns a key-value mapping associated with the greatest key 2067 * less than or equal to the given key, or <tt>null</tt> if there 2068 * is no such key. The returned entry does <em>not</em> support 2069 * the <tt>Entry.setValue</tt> method. 2070 * 2071 * @param key the key 2072 * @throws ClassCastException {@inheritDoc} 2073 * @throws NullPointerException if the specified key is null 2074 */ 2075 public Map.Entry<K,V> floorEntry(K key) { 2076 return getNear(key, LT|EQ); 2077 } 2078 2079 /** 2080 * @param key the key 2081 * @throws ClassCastException {@inheritDoc} 2082 * @throws NullPointerException if the specified key is null 2083 */ 2084 public K floorKey(K key) { 2085 Node<K,V> n = findNear(key, LT|EQ); 2086 return (n == null) ? null : n.key; 2087 } 2088 2089 /** 2090 * Returns a key-value mapping associated with the least key 2091 * greater than or equal to the given key, or <tt>null</tt> if 2092 * there is no such entry. The returned entry does <em>not</em> 2093 * support the <tt>Entry.setValue</tt> method. 2094 * 2095 * @throws ClassCastException {@inheritDoc} 2096 * @throws NullPointerException if the specified key is null 2097 */ 2098 public Map.Entry<K,V> ceilingEntry(K key) { 2099 return getNear(key, GT|EQ); 2100 } 2101 2102 /** 2103 * @throws ClassCastException {@inheritDoc} 2104 * @throws NullPointerException if the specified key is null 2105 */ 2106 public K ceilingKey(K key) { 2107 Node<K,V> n = findNear(key, GT|EQ); 2108 return (n == null) ? null : n.key; 2109 } 2110 2111 /** 2112 * Returns a key-value mapping associated with the least key 2113 * strictly greater than the given key, or <tt>null</tt> if there 2114 * is no such key. The returned entry does <em>not</em> support 2115 * the <tt>Entry.setValue</tt> method. 2116 * 2117 * @param key the key 2118 * @throws ClassCastException {@inheritDoc} 2119 * @throws NullPointerException if the specified key is null 2120 */ 2121 public Map.Entry<K,V> higherEntry(K key) { 2122 return getNear(key, GT); 2123 } 2124 2125 /** 2126 * @param key the key 2127 * @throws ClassCastException {@inheritDoc} 2128 * @throws NullPointerException if the specified key is null 2129 */ 2130 public K higherKey(K key) { 2131 Node<K,V> n = findNear(key, GT); 2132 return (n == null) ? null : n.key; 2133 } 2134 2135 /** 2136 * Returns a key-value mapping associated with the least 2137 * key in this map, or <tt>null</tt> if the map is empty. 2138 * The returned entry does <em>not</em> support 2139 * the <tt>Entry.setValue</tt> method. 2140 */ 2141 public Map.Entry<K,V> firstEntry() { 2142 for (;;) { 2143 Node<K,V> n = findFirst(); 2144 if (n == null) 2145 return null; 2146 AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot(); 2147 if (e != null) 2148 return e; 2149 } 2150 } 2151 2152 /** 2153 * Returns a key-value mapping associated with the greatest 2154 * key in this map, or <tt>null</tt> if the map is empty. 2155 * The returned entry does <em>not</em> support 2156 * the <tt>Entry.setValue</tt> method. 2157 */ 2158 public Map.Entry<K,V> lastEntry() { 2159 for (;;) { 2160 Node<K,V> n = findLast(); 2161 if (n == null) 2162 return null; 2163 AbstractMap.SimpleImmutableEntry<K,V> e = n.createSnapshot(); 2164 if (e != null) 2165 return e; 2166 } 2167 } 2168 2169 /** 2170 * Removes and returns a key-value mapping associated with 2171 * the least key in this map, or <tt>null</tt> if the map is empty. 2172 * The returned entry does <em>not</em> support 2173 * the <tt>Entry.setValue</tt> method. 2174 */ 2175 public Map.Entry<K,V> pollFirstEntry() { 2176 return doRemoveFirstEntry(); 2177 } 2178 2179 /** 2180 * Removes and returns a key-value mapping associated with 2181 * the greatest key in this map, or <tt>null</tt> if the map is empty. 2182 * The returned entry does <em>not</em> support 2183 * the <tt>Entry.setValue</tt> method. 2184 */ 2185 public Map.Entry<K,V> pollLastEntry() { 2186 return doRemoveLastEntry(); 2187 } 2188 2189 2190 /* ---------------- Iterators -------------- */ 2191 2192 /** 2193 * Base of iterator classes: 2194 */ 2195 abstract class Iter<T> implements Iterator<T> { 2196 /** the last node returned by next() */ 2197 Node<K,V> lastReturned; 2198 /** the next node to return from next(); */ 2199 Node<K,V> next; 2200 /** Cache of next value field to maintain weak consistency */ 2201 V nextValue; 2202 2203 /** Initializes ascending iterator for entire range. */ 2204 Iter() { 2205 for (;;) { 2206 next = findFirst(); 2207 if (next == null) 2208 break; 2209 Object x = next.value; 2210 if (x != null && x != next) { 2211 nextValue = (V) x; 2212 break; 2213 } 2214 } 2215 } 2216 2217 public final boolean hasNext() { 2218 return next != null; 2219 } 2220 2221 /** Advances next to higher entry. */ 2222 final void advance() { 2223 if (next == null) 2224 throw new NoSuchElementException(); 2225 lastReturned = next; 2226 for (;;) { 2227 next = next.next; 2228 if (next == null) 2229 break; 2230 Object x = next.value; 2231 if (x != null && x != next) { 2232 nextValue = (V) x; 2233 break; 2234 } 2235 } 2236 } 2237 2238 public void remove() { 2239 Node<K,V> l = lastReturned; 2240 if (l == null) 2241 throw new IllegalStateException(); 2242 // It would not be worth all of the overhead to directly 2243 // unlink from here. Using remove is fast enough. 2244 ConcurrentSkipListMap.this.remove(l.key); 2245 lastReturned = null; 2246 } 2247 2248 } 2249 2250 final class ValueIterator extends Iter<V> { 2251 public V next() { 2252 V v = nextValue; 2253 advance(); 2254 return v; 2255 } 2256 } 2257 2258 final class KeyIterator extends Iter<K> { 2259 public K next() { 2260 Node<K,V> n = next; 2261 advance(); 2262 return n.key; 2263 } 2264 } 2265 2266 final class EntryIterator extends Iter<Map.Entry<K,V>> { 2267 public Map.Entry<K,V> next() { 2268 Node<K,V> n = next; 2269 V v = nextValue; 2270 advance(); 2271 return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v); 2272 } 2273 } 2274 2275 // Factory methods for iterators needed by ConcurrentSkipListSet etc 2276 2277 Iterator<K> keyIterator() { 2278 return new KeyIterator(); 2279 } 2280 2281 Iterator<V> valueIterator() { 2282 return new ValueIterator(); 2283 } 2284 2285 Iterator<Map.Entry<K,V>> entryIterator() { 2286 return new EntryIterator(); 2287 } 2288 2289 /* ---------------- View Classes -------------- */ 2290 2291 /* 2292 * View classes are static, delegating to a ConcurrentNavigableMap 2293 * to allow use by SubMaps, which outweighs the ugliness of 2294 * needing type-tests for Iterator methods. 2295 */ 2296 2297 static final <E> List<E> toList(Collection<E> c) { 2298 // Using size() here would be a pessimization. 2299 List<E> list = new ArrayList<E>(); 2300 for (E e : c) 2301 list.add(e); 2302 return list; 2303 } 2304 2305 static final class KeySet<E> 2306 extends AbstractSet<E> implements NavigableSet<E> { 2307 private final ConcurrentNavigableMap<E,Object> m; 2308 KeySet(ConcurrentNavigableMap<E,Object> map) { m = map; } 2309 public int size() { return m.size(); } 2310 public boolean isEmpty() { return m.isEmpty(); } 2311 public boolean contains(Object o) { return m.containsKey(o); } 2312 public boolean remove(Object o) { return m.remove(o) != null; } 2313 public void clear() { m.clear(); } 2314 public E lower(E e) { return m.lowerKey(e); } 2315 public E floor(E e) { return m.floorKey(e); } 2316 public E ceiling(E e) { return m.ceilingKey(e); } 2317 public E higher(E e) { return m.higherKey(e); } 2318 public Comparator<? super E> comparator() { return m.comparator(); } 2319 public E first() { return m.firstKey(); } 2320 public E last() { return m.lastKey(); } 2321 public E pollFirst() { 2322 Map.Entry<E,Object> e = m.pollFirstEntry(); 2323 return (e == null) ? null : e.getKey(); 2324 } 2325 public E pollLast() { 2326 Map.Entry<E,Object> e = m.pollLastEntry(); 2327 return (e == null) ? null : e.getKey(); 2328 } 2329 public Iterator<E> iterator() { 2330 if (m instanceof ConcurrentSkipListMap) 2331 return ((ConcurrentSkipListMap<E,Object>)m).keyIterator(); 2332 else 2333 return ((ConcurrentSkipListMap.SubMap<E,Object>)m).keyIterator(); 2334 } 2335 public boolean equals(Object o) { 2336 if (o == this) 2337 return true; 2338 if (!(o instanceof Set)) 2339 return false; 2340 Collection<?> c = (Collection<?>) o; 2341 try { 2342 return containsAll(c) && c.containsAll(this); 2343 } catch (ClassCastException unused) { 2344 return false; 2345 } catch (NullPointerException unused) { 2346 return false; 2347 } 2348 } 2349 public Object[] toArray() { return toList(this).toArray(); } 2350 public <T> T[] toArray(T[] a) { return toList(this).toArray(a); } 2351 public Iterator<E> descendingIterator() { 2352 return descendingSet().iterator(); 2353 } 2354 public NavigableSet<E> subSet(E fromElement, 2355 boolean fromInclusive, 2356 E toElement, 2357 boolean toInclusive) { 2358 return new KeySet<E>(m.subMap(fromElement, fromInclusive, 2359 toElement, toInclusive)); 2360 } 2361 public NavigableSet<E> headSet(E toElement, boolean inclusive) { 2362 return new KeySet<E>(m.headMap(toElement, inclusive)); 2363 } 2364 public NavigableSet<E> tailSet(E fromElement, boolean inclusive) { 2365 return new KeySet<E>(m.tailMap(fromElement, inclusive)); 2366 } 2367 public NavigableSet<E> subSet(E fromElement, E toElement) { 2368 return subSet(fromElement, true, toElement, false); 2369 } 2370 public NavigableSet<E> headSet(E toElement) { 2371 return headSet(toElement, false); 2372 } 2373 public NavigableSet<E> tailSet(E fromElement) { 2374 return tailSet(fromElement, true); 2375 } 2376 public NavigableSet<E> descendingSet() { 2377 return new KeySet(m.descendingMap()); 2378 } 2379 } 2380 2381 static final class Values<E> extends AbstractCollection<E> { 2382 private final ConcurrentNavigableMap<Object, E> m; 2383 Values(ConcurrentNavigableMap<Object, E> map) { 2384 m = map; 2385 } 2386 public Iterator<E> iterator() { 2387 if (m instanceof ConcurrentSkipListMap) 2388 return ((ConcurrentSkipListMap<Object,E>)m).valueIterator(); 2389 else 2390 return ((SubMap<Object,E>)m).valueIterator(); 2391 } 2392 public boolean isEmpty() { 2393 return m.isEmpty(); 2394 } 2395 public int size() { 2396 return m.size(); 2397 } 2398 public boolean contains(Object o) { 2399 return m.containsValue(o); 2400 } 2401 public void clear() { 2402 m.clear(); 2403 } 2404 public Object[] toArray() { return toList(this).toArray(); } 2405 public <T> T[] toArray(T[] a) { return toList(this).toArray(a); } 2406 } 2407 2408 static final class EntrySet<K1,V1> extends AbstractSet<Map.Entry<K1,V1>> { 2409 private final ConcurrentNavigableMap<K1, V1> m; 2410 EntrySet(ConcurrentNavigableMap<K1, V1> map) { 2411 m = map; 2412 } 2413 2414 public Iterator<Map.Entry<K1,V1>> iterator() { 2415 if (m instanceof ConcurrentSkipListMap) 2416 return ((ConcurrentSkipListMap<K1,V1>)m).entryIterator(); 2417 else 2418 return ((SubMap<K1,V1>)m).entryIterator(); 2419 } 2420 2421 public boolean contains(Object o) { 2422 if (!(o instanceof Map.Entry)) 2423 return false; 2424 Map.Entry<K1,V1> e = (Map.Entry<K1,V1>)o; 2425 V1 v = m.get(e.getKey()); 2426 return v != null && v.equals(e.getValue()); 2427 } 2428 public boolean remove(Object o) { 2429 if (!(o instanceof Map.Entry)) 2430 return false; 2431 Map.Entry<K1,V1> e = (Map.Entry<K1,V1>)o; 2432 return m.remove(e.getKey(), 2433 e.getValue()); 2434 } 2435 public boolean isEmpty() { 2436 return m.isEmpty(); 2437 } 2438 public int size() { 2439 return m.size(); 2440 } 2441 public void clear() { 2442 m.clear(); 2443 } 2444 public boolean equals(Object o) { 2445 if (o == this) 2446 return true; 2447 if (!(o instanceof Set)) 2448 return false; 2449 Collection<?> c = (Collection<?>) o; 2450 try { 2451 return containsAll(c) && c.containsAll(this); 2452 } catch (ClassCastException unused) { 2453 return false; 2454 } catch (NullPointerException unused) { 2455 return false; 2456 } 2457 } 2458 public Object[] toArray() { return toList(this).toArray(); } 2459 public <T> T[] toArray(T[] a) { return toList(this).toArray(a); } 2460 } 2461 2462 /** 2463 * Submaps returned by {@link ConcurrentSkipListMap} submap operations 2464 * represent a subrange of mappings of their underlying 2465 * maps. Instances of this class support all methods of their 2466 * underlying maps, differing in that mappings outside their range are 2467 * ignored, and attempts to add mappings outside their ranges result 2468 * in {@link IllegalArgumentException}. Instances of this class are 2469 * constructed only using the <tt>subMap</tt>, <tt>headMap</tt>, and 2470 * <tt>tailMap</tt> methods of their underlying maps. 2471 * 2472 * @serial include 2473 */ 2474 static final class SubMap<K,V> extends AbstractMap<K,V> 2475 implements ConcurrentNavigableMap<K,V>, Cloneable, 2476 java.io.Serializable { 2477 private static final long serialVersionUID = -7647078645895051609L; 2478 2479 /** Underlying map */ 2480 private final ConcurrentSkipListMap<K,V> m; 2481 /** lower bound key, or null if from start */ 2482 private final K lo; 2483 /** upper bound key, or null if to end */ 2484 private final K hi; 2485 /** inclusion flag for lo */ 2486 private final boolean loInclusive; 2487 /** inclusion flag for hi */ 2488 private final boolean hiInclusive; 2489 /** direction */ 2490 private final boolean isDescending; 2491 2492 // Lazily initialized view holders 2493 private transient KeySet<K> keySetView; 2494 private transient Set<Map.Entry<K,V>> entrySetView; 2495 private transient Collection<V> valuesView; 2496 2497 /** 2498 * Creates a new submap, initializing all fields 2499 */ 2500 SubMap(ConcurrentSkipListMap<K,V> map, 2501 K fromKey, boolean fromInclusive, 2502 K toKey, boolean toInclusive, 2503 boolean isDescending) { 2504 if (fromKey != null && toKey != null && 2505 map.compare(fromKey, toKey) > 0) 2506 throw new IllegalArgumentException("inconsistent range"); 2507 this.m = map; 2508 this.lo = fromKey; 2509 this.hi = toKey; 2510 this.loInclusive = fromInclusive; 2511 this.hiInclusive = toInclusive; 2512 this.isDescending = isDescending; 2513 } 2514 2515 /* ---------------- Utilities -------------- */ 2516 2517 private boolean tooLow(K key) { 2518 if (lo != null) { 2519 int c = m.compare(key, lo); 2520 if (c < 0 || (c == 0 && !loInclusive)) 2521 return true; 2522 } 2523 return false; 2524 } 2525 2526 private boolean tooHigh(K key) { 2527 if (hi != null) { 2528 int c = m.compare(key, hi); 2529 if (c > 0 || (c == 0 && !hiInclusive)) 2530 return true; 2531 } 2532 return false; 2533 } 2534 2535 private boolean inBounds(K key) { 2536 return !tooLow(key) && !tooHigh(key); 2537 } 2538 2539 private void checkKeyBounds(K key) throws IllegalArgumentException { 2540 if (key == null) 2541 throw new NullPointerException(); 2542 if (!inBounds(key)) 2543 throw new IllegalArgumentException("key out of range"); 2544 } 2545 2546 /** 2547 * Returns true if node key is less than upper bound of range 2548 */ 2549 private boolean isBeforeEnd(ConcurrentSkipListMap.Node<K,V> n) { 2550 if (n == null) 2551 return false; 2552 if (hi == null) 2553 return true; 2554 K k = n.key; 2555 if (k == null) // pass by markers and headers 2556 return true; 2557 int c = m.compare(k, hi); 2558 if (c > 0 || (c == 0 && !hiInclusive)) 2559 return false; 2560 return true; 2561 } 2562 2563 /** 2564 * Returns lowest node. This node might not be in range, so 2565 * most usages need to check bounds 2566 */ 2567 private ConcurrentSkipListMap.Node<K,V> loNode() { 2568 if (lo == null) 2569 return m.findFirst(); 2570 else if (loInclusive) 2571 return m.findNear(lo, m.GT|m.EQ); 2572 else 2573 return m.findNear(lo, m.GT); 2574 } 2575 2576 /** 2577 * Returns highest node. This node might not be in range, so 2578 * most usages need to check bounds 2579 */ 2580 private ConcurrentSkipListMap.Node<K,V> hiNode() { 2581 if (hi == null) 2582 return m.findLast(); 2583 else if (hiInclusive) 2584 return m.findNear(hi, m.LT|m.EQ); 2585 else 2586 return m.findNear(hi, m.LT); 2587 } 2588 2589 /** 2590 * Returns lowest absolute key (ignoring directonality) 2591 */ 2592 private K lowestKey() { 2593 ConcurrentSkipListMap.Node<K,V> n = loNode(); 2594 if (isBeforeEnd(n)) 2595 return n.key; 2596 else 2597 throw new NoSuchElementException(); 2598 } 2599 2600 /** 2601 * Returns highest absolute key (ignoring directonality) 2602 */ 2603 private K highestKey() { 2604 ConcurrentSkipListMap.Node<K,V> n = hiNode(); 2605 if (n != null) { 2606 K last = n.key; 2607 if (inBounds(last)) 2608 return last; 2609 } 2610 throw new NoSuchElementException(); 2611 } 2612 2613 private Map.Entry<K,V> lowestEntry() { 2614 for (;;) { 2615 ConcurrentSkipListMap.Node<K,V> n = loNode(); 2616 if (!isBeforeEnd(n)) 2617 return null; 2618 Map.Entry<K,V> e = n.createSnapshot(); 2619 if (e != null) 2620 return e; 2621 } 2622 } 2623 2624 private Map.Entry<K,V> highestEntry() { 2625 for (;;) { 2626 ConcurrentSkipListMap.Node<K,V> n = hiNode(); 2627 if (n == null || !inBounds(n.key)) 2628 return null; 2629 Map.Entry<K,V> e = n.createSnapshot(); 2630 if (e != null) 2631 return e; 2632 } 2633 } 2634 2635 private Map.Entry<K,V> removeLowest() { 2636 for (;;) { 2637 Node<K,V> n = loNode(); 2638 if (n == null) 2639 return null; 2640 K k = n.key; 2641 if (!inBounds(k)) 2642 return null; 2643 V v = m.doRemove(k, null); 2644 if (v != null) 2645 return new AbstractMap.SimpleImmutableEntry<K,V>(k, v); 2646 } 2647 } 2648 2649 private Map.Entry<K,V> removeHighest() { 2650 for (;;) { 2651 Node<K,V> n = hiNode(); 2652 if (n == null) 2653 return null; 2654 K k = n.key; 2655 if (!inBounds(k)) 2656 return null; 2657 V v = m.doRemove(k, null); 2658 if (v != null) 2659 return new AbstractMap.SimpleImmutableEntry<K,V>(k, v); 2660 } 2661 } 2662 2663 /** 2664 * Submap version of ConcurrentSkipListMap.getNearEntry 2665 */ 2666 private Map.Entry<K,V> getNearEntry(K key, int rel) { 2667 if (isDescending) { // adjust relation for direction 2668 if ((rel & m.LT) == 0) 2669 rel |= m.LT; 2670 else 2671 rel &= ~m.LT; 2672 } 2673 if (tooLow(key)) 2674 return ((rel & m.LT) != 0) ? null : lowestEntry(); 2675 if (tooHigh(key)) 2676 return ((rel & m.LT) != 0) ? highestEntry() : null; 2677 for (;;) { 2678 Node<K,V> n = m.findNear(key, rel); 2679 if (n == null || !inBounds(n.key)) 2680 return null; 2681 K k = n.key; 2682 V v = n.getValidValue(); 2683 if (v != null) 2684 return new AbstractMap.SimpleImmutableEntry<K,V>(k, v); 2685 } 2686 } 2687 2688 // Almost the same as getNearEntry, except for keys 2689 private K getNearKey(K key, int rel) { 2690 if (isDescending) { // adjust relation for direction 2691 if ((rel & m.LT) == 0) 2692 rel |= m.LT; 2693 else 2694 rel &= ~m.LT; 2695 } 2696 if (tooLow(key)) { 2697 if ((rel & m.LT) == 0) { 2698 ConcurrentSkipListMap.Node<K,V> n = loNode(); 2699 if (isBeforeEnd(n)) 2700 return n.key; 2701 } 2702 return null; 2703 } 2704 if (tooHigh(key)) { 2705 if ((rel & m.LT) != 0) { 2706 ConcurrentSkipListMap.Node<K,V> n = hiNode(); 2707 if (n != null) { 2708 K last = n.key; 2709 if (inBounds(last)) 2710 return last; 2711 } 2712 } 2713 return null; 2714 } 2715 for (;;) { 2716 Node<K,V> n = m.findNear(key, rel); 2717 if (n == null || !inBounds(n.key)) 2718 return null; 2719 K k = n.key; 2720 V v = n.getValidValue(); 2721 if (v != null) 2722 return k; 2723 } 2724 } 2725 2726 /* ---------------- Map API methods -------------- */ 2727 2728 public boolean containsKey(Object key) { 2729 if (key == null) throw new NullPointerException(); 2730 K k = (K)key; 2731 return inBounds(k) && m.containsKey(k); 2732 } 2733 2734 public V get(Object key) { 2735 if (key == null) throw new NullPointerException(); 2736 K k = (K)key; 2737 return ((!inBounds(k)) ? null : m.get(k)); 2738 } 2739 2740 public V put(K key, V value) { 2741 checkKeyBounds(key); 2742 return m.put(key, value); 2743 } 2744 2745 public V remove(Object key) { 2746 K k = (K)key; 2747 return (!inBounds(k)) ? null : m.remove(k); 2748 } 2749 2750 public int size() { 2751 long count = 0; 2752 for (ConcurrentSkipListMap.Node<K,V> n = loNode(); 2753 isBeforeEnd(n); 2754 n = n.next) { 2755 if (n.getValidValue() != null) 2756 ++count; 2757 } 2758 return count >= Integer.MAX_VALUE ? Integer.MAX_VALUE : (int)count; 2759 } 2760 2761 public boolean isEmpty() { 2762 return !isBeforeEnd(loNode()); 2763 } 2764 2765 public boolean containsValue(Object value) { 2766 if (value == null) 2767 throw new NullPointerException(); 2768 for (ConcurrentSkipListMap.Node<K,V> n = loNode(); 2769 isBeforeEnd(n); 2770 n = n.next) { 2771 V v = n.getValidValue(); 2772 if (v != null && value.equals(v)) 2773 return true; 2774 } 2775 return false; 2776 } 2777 2778 public void clear() { 2779 for (ConcurrentSkipListMap.Node<K,V> n = loNode(); 2780 isBeforeEnd(n); 2781 n = n.next) { 2782 if (n.getValidValue() != null) 2783 m.remove(n.key); 2784 } 2785 } 2786 2787 /* ---------------- ConcurrentMap API methods -------------- */ 2788 2789 public V putIfAbsent(K key, V value) { 2790 checkKeyBounds(key); 2791 return m.putIfAbsent(key, value); 2792 } 2793 2794 public boolean remove(Object key, Object value) { 2795 K k = (K)key; 2796 return inBounds(k) && m.remove(k, value); 2797 } 2798 2799 public boolean replace(K key, V oldValue, V newValue) { 2800 checkKeyBounds(key); 2801 return m.replace(key, oldValue, newValue); 2802 } 2803 2804 public V replace(K key, V value) { 2805 checkKeyBounds(key); 2806 return m.replace(key, value); 2807 } 2808 2809 /* ---------------- SortedMap API methods -------------- */ 2810 2811 public Comparator<? super K> comparator() { 2812 Comparator<? super K> cmp = m.comparator(); 2813 if (isDescending) 2814 return Collections.reverseOrder(cmp); 2815 else 2816 return cmp; 2817 } 2818 2819 /** 2820 * Utility to create submaps, where given bounds override 2821 * unbounded(null) ones and/or are checked against bounded ones. 2822 */ 2823 private SubMap<K,V> newSubMap(K fromKey, 2824 boolean fromInclusive, 2825 K toKey, 2826 boolean toInclusive) { 2827 if (isDescending) { // flip senses 2828 K tk = fromKey; 2829 fromKey = toKey; 2830 toKey = tk; 2831 boolean ti = fromInclusive; 2832 fromInclusive = toInclusive; 2833 toInclusive = ti; 2834 } 2835 if (lo != null) { 2836 if (fromKey == null) { 2837 fromKey = lo; 2838 fromInclusive = loInclusive; 2839 } 2840 else { 2841 int c = m.compare(fromKey, lo); 2842 if (c < 0 || (c == 0 && !loInclusive && fromInclusive)) 2843 throw new IllegalArgumentException("key out of range"); 2844 } 2845 } 2846 if (hi != null) { 2847 if (toKey == null) { 2848 toKey = hi; 2849 toInclusive = hiInclusive; 2850 } 2851 else { 2852 int c = m.compare(toKey, hi); 2853 if (c > 0 || (c == 0 && !hiInclusive && toInclusive)) 2854 throw new IllegalArgumentException("key out of range"); 2855 } 2856 } 2857 return new SubMap<K,V>(m, fromKey, fromInclusive, 2858 toKey, toInclusive, isDescending); 2859 } 2860 2861 public SubMap<K,V> subMap(K fromKey, 2862 boolean fromInclusive, 2863 K toKey, 2864 boolean toInclusive) { 2865 if (fromKey == null || toKey == null) 2866 throw new NullPointerException(); 2867 return newSubMap(fromKey, fromInclusive, toKey, toInclusive); 2868 } 2869 2870 public SubMap<K,V> headMap(K toKey, 2871 boolean inclusive) { 2872 if (toKey == null) 2873 throw new NullPointerException(); 2874 return newSubMap(null, false, toKey, inclusive); 2875 } 2876 2877 public SubMap<K,V> tailMap(K fromKey, 2878 boolean inclusive) { 2879 if (fromKey == null) 2880 throw new NullPointerException(); 2881 return newSubMap(fromKey, inclusive, null, false); 2882 } 2883 2884 public SubMap<K,V> subMap(K fromKey, K toKey) { 2885 return subMap(fromKey, true, toKey, false); 2886 } 2887 2888 public SubMap<K,V> headMap(K toKey) { 2889 return headMap(toKey, false); 2890 } 2891 2892 public SubMap<K,V> tailMap(K fromKey) { 2893 return tailMap(fromKey, true); 2894 } 2895 2896 public SubMap<K,V> descendingMap() { 2897 return new SubMap<K,V>(m, lo, loInclusive, 2898 hi, hiInclusive, !isDescending); 2899 } 2900 2901 /* ---------------- Relational methods -------------- */ 2902 2903 public Map.Entry<K,V> ceilingEntry(K key) { 2904 return getNearEntry(key, (m.GT|m.EQ)); 2905 } 2906 2907 public K ceilingKey(K key) { 2908 return getNearKey(key, (m.GT|m.EQ)); 2909 } 2910 2911 public Map.Entry<K,V> lowerEntry(K key) { 2912 return getNearEntry(key, (m.LT)); 2913 } 2914 2915 public K lowerKey(K key) { 2916 return getNearKey(key, (m.LT)); 2917 } 2918 2919 public Map.Entry<K,V> floorEntry(K key) { 2920 return getNearEntry(key, (m.LT|m.EQ)); 2921 } 2922 2923 public K floorKey(K key) { 2924 return getNearKey(key, (m.LT|m.EQ)); 2925 } 2926 2927 public Map.Entry<K,V> higherEntry(K key) { 2928 return getNearEntry(key, (m.GT)); 2929 } 2930 2931 public K higherKey(K key) { 2932 return getNearKey(key, (m.GT)); 2933 } 2934 2935 public K firstKey() { 2936 return isDescending ? highestKey() : lowestKey(); 2937 } 2938 2939 public K lastKey() { 2940 return isDescending ? lowestKey() : highestKey(); 2941 } 2942 2943 public Map.Entry<K,V> firstEntry() { 2944 return isDescending ? highestEntry() : lowestEntry(); 2945 } 2946 2947 public Map.Entry<K,V> lastEntry() { 2948 return isDescending ? lowestEntry() : highestEntry(); 2949 } 2950 2951 public Map.Entry<K,V> pollFirstEntry() { 2952 return isDescending ? removeHighest() : removeLowest(); 2953 } 2954 2955 public Map.Entry<K,V> pollLastEntry() { 2956 return isDescending ? removeLowest() : removeHighest(); 2957 } 2958 2959 /* ---------------- Submap Views -------------- */ 2960 2961 public NavigableSet<K> keySet() { 2962 KeySet<K> ks = keySetView; 2963 return (ks != null) ? ks : (keySetView = new KeySet(this)); 2964 } 2965 2966 public NavigableSet<K> navigableKeySet() { 2967 KeySet<K> ks = keySetView; 2968 return (ks != null) ? ks : (keySetView = new KeySet(this)); 2969 } 2970 2971 public Collection<V> values() { 2972 Collection<V> vs = valuesView; 2973 return (vs != null) ? vs : (valuesView = new Values(this)); 2974 } 2975 2976 public Set<Map.Entry<K,V>> entrySet() { 2977 Set<Map.Entry<K,V>> es = entrySetView; 2978 return (es != null) ? es : (entrySetView = new EntrySet(this)); 2979 } 2980 2981 public NavigableSet<K> descendingKeySet() { 2982 return descendingMap().navigableKeySet(); 2983 } 2984 2985 Iterator<K> keyIterator() { 2986 return new SubMapKeyIterator(); 2987 } 2988 2989 Iterator<V> valueIterator() { 2990 return new SubMapValueIterator(); 2991 } 2992 2993 Iterator<Map.Entry<K,V>> entryIterator() { 2994 return new SubMapEntryIterator(); 2995 } 2996 2997 /** 2998 * Variant of main Iter class to traverse through submaps. 2999 */ 3000 abstract class SubMapIter<T> implements Iterator<T> { 3001 /** the last node returned by next() */ 3002 Node<K,V> lastReturned; 3003 /** the next node to return from next(); */ 3004 Node<K,V> next; 3005 /** Cache of next value field to maintain weak consistency */ 3006 V nextValue; 3007 3008 SubMapIter() { 3009 for (;;) { 3010 next = isDescending ? hiNode() : loNode(); 3011 if (next == null) 3012 break; 3013 Object x = next.value; 3014 if (x != null && x != next) { 3015 if (! inBounds(next.key)) 3016 next = null; 3017 else 3018 nextValue = (V) x; 3019 break; 3020 } 3021 } 3022 } 3023 3024 public final boolean hasNext() { 3025 return next != null; 3026 } 3027 3028 final void advance() { 3029 if (next == null) 3030 throw new NoSuchElementException(); 3031 lastReturned = next; 3032 if (isDescending) 3033 descend(); 3034 else 3035 ascend(); 3036 } 3037 3038 private void ascend() { 3039 for (;;) { 3040 next = next.next; 3041 if (next == null) 3042 break; 3043 Object x = next.value; 3044 if (x != null && x != next) { 3045 if (tooHigh(next.key)) 3046 next = null; 3047 else 3048 nextValue = (V) x; 3049 break; 3050 } 3051 } 3052 } 3053 3054 private void descend() { 3055 for (;;) { 3056 next = m.findNear(lastReturned.key, LT); 3057 if (next == null) 3058 break; 3059 Object x = next.value; 3060 if (x != null && x != next) { 3061 if (tooLow(next.key)) 3062 next = null; 3063 else 3064 nextValue = (V) x; 3065 break; 3066 } 3067 } 3068 } 3069 3070 public void remove() { 3071 Node<K,V> l = lastReturned; 3072 if (l == null) 3073 throw new IllegalStateException(); 3074 m.remove(l.key); 3075 lastReturned = null; 3076 } 3077 3078 } 3079 3080 final class SubMapValueIterator extends SubMapIter<V> { 3081 public V next() { 3082 V v = nextValue; 3083 advance(); 3084 return v; 3085 } 3086 } 3087 3088 final class SubMapKeyIterator extends SubMapIter<K> { 3089 public K next() { 3090 Node<K,V> n = next; 3091 advance(); 3092 return n.key; 3093 } 3094 } 3095 3096 final class SubMapEntryIterator extends SubMapIter<Map.Entry<K,V>> { 3097 public Map.Entry<K,V> next() { 3098 Node<K,V> n = next; 3099 V v = nextValue; 3100 advance(); 3101 return new AbstractMap.SimpleImmutableEntry<K,V>(n.key, v); 3102 } 3103 } 3104 } 3105 3106 // Unsafe mechanics 3107 private static final sun.misc.Unsafe UNSAFE; 3108 private static final long headOffset; 3109 static { 3110 try { 3111 UNSAFE = sun.misc.Unsafe.getUnsafe(); 3112 Class k = ConcurrentSkipListMap.class; 3113 headOffset = UNSAFE.objectFieldOffset 3114 (k.getDeclaredField("head")); 3115 } catch (Exception e) { 3116 throw new Error(e); 3117 } 3118 } 3119 }
下面从ConcurrentSkipListMap的添加,删除,获取这3个方面对它进行分析。
1. 添加
下面以put(K key, V value)为例,对ConcurrentSkipListMap的添加方法进行说明。
public V put(K key, V value) { if (value == null) throw new NullPointerException(); return doPut(key, value, false); }
实际上,put()是通过doPut()将key-value键值对添加到ConcurrentSkipListMap中的。
doPut()的源码如下:
private V doPut(K kkey, V value, boolean onlyIfAbsent) { Comparable<? super K> key = comparable(kkey); for (;;) { // 找到key的前继节点 Node<K,V> b = findPredecessor(key); // 设置n为“key的前继节点的后继节点”,即n应该是“插入节点”的“后继节点” Node<K,V> n = b.next; for (;;) { if (n != null) { Node<K,V> f = n.next; // 如果两次获得的b.next不是相同的Node,就跳转到”外层for循环“,重新获得b和n后再遍历。 if (n != b.next) break; // v是“n的值” Object v = n.value; // 当n的值为null(意味着其它线程删除了n);此时删除b的下一个节点,然后跳转到”外层for循环“,重新获得b和n后再遍历。 if (v == null) { // n is deleted n.helpDelete(b, f); break; } // 如果其它线程删除了b;则跳转到”外层for循环“,重新获得b和n后再遍历。 if (v == n || b.value == null) // b is deleted break; // 比较key和n.key int c = key.compareTo(n.key); if (c > 0) { b = n; n = f; continue; } if (c == 0) { if (onlyIfAbsent || n.casValue(v, value)) return (V)v; else break; // restart if lost race to replace value } // else c < 0; fall through } // 新建节点(对应是“要插入的键值对”) Node<K,V> z = new Node<K,V>(kkey, value, n); // 设置“b的后继节点”为z if (!b.casNext(n, z)) break; // 多线程情况下,break才可能发生(其它线程对b进行了操作) // 随机获取一个level // 然后在“第1层”到“第level层”的链表中都插入新建节点 int level = randomLevel(); if (level > 0) insertIndex(z, level); return null; } } }
说明:doPut() 的作用就是将键值对添加到“跳表”中。
要想搞清doPut(),首先要弄清楚它的主干部分 —— 我们先单纯的只考虑“单线程的情况下,将key-value添加到跳表中”,即忽略“多线程相关的内容”。它的流程如下:
第1步:找到“插入位置”。
即,找到“key的前继节点(b)”和“key的后继节点(n)”;key是要插入节点的键。
第2步:新建并插入节点。
即,新建节点z(key对应的节点),并将新节点z插入到“跳表”中(设置“b的后继节点为z”,“z的后继节点为n”)。
第3步:更新跳表。
即,随机获取一个level,然后在“跳表”的第1层~第level层之间,每一层都插入节点z;在第level层之上就不再插入节点了。若level数值大于“跳表的层次”,则新建一层。
主干部分“对应的精简后的doPut()的代码”如下(仅供参考):
private V doPut(K kkey, V value, boolean onlyIfAbsent) { Comparable<? super K> key = comparable(kkey); for (;;) { // 找到key的前继节点 Node<K,V> b = findPredecessor(key); // 设置n为key的后继节点 Node<K,V> n = b.next; for (;;) { // 新建节点(对应是“要被插入的键值对”) Node<K,V> z = new Node<K,V>(kkey, value, n); // 设置“b的后继节点”为z b.casNext(n, z); // 随机获取一个level // 然后在“第1层”到“第level层”的链表中都插入新建节点 int level = randomLevel(); if (level > 0) insertIndex(z, level); return null; } } }
理清主干之后,剩余的工作就相对简单了。主要是上面几步的对应算法的具体实现,以及多线程相关情况的处理!
2. 删除
下面以remove(Object key)为例,对ConcurrentSkipListMap的删除方法进行说明。
public V remove(Object key) { return doRemove(key, null); }
实际上,remove()是通过doRemove()将ConcurrentSkipListMap中的key对应的键值对删除的。
doRemove()的源码如下:
final V doRemove(Object okey, Object value) { Comparable<? super K> key = comparable(okey); for (;;) { // 找到“key的前继节点” Node<K,V> b = findPredecessor(key); // 设置n为“b的后继节点”(即若key存在于“跳表中”,n就是key对应的节点) Node<K,V> n = b.next; for (;;) { if (n == null) return null; // f是“当前节点n的后继节点” Node<K,V> f = n.next; // 如果两次读取到的“b的后继节点”不同(其它线程操作了该跳表),则返回到“外层for循环”重新遍历。 if (n != b.next) // inconsistent read break; // 如果“当前节点n的值”变为null(其它线程操作了该跳表),则返回到“外层for循环”重新遍历。 Object v = n.value; if (v == null) { // n is deleted n.helpDelete(b, f); break; } // 如果“前继节点b”被删除(其它线程操作了该跳表),则返回到“外层for循环”重新遍历。 if (v == n || b.value == null) // b is deleted break; int c = key.compareTo(n.key); if (c < 0) return null; if (c > 0) { b = n; n = f; continue; } // 以下是c=0的情况 if (value != null && !value.equals(v)) return null; // 设置“当前节点n”的值为null if (!n.casValue(v, null)) break; // 设置“b的后继节点”为f if (!n.appendMarker(f) || !b.casNext(n, f)) findNode(key); // Retry via findNode else { // 清除“跳表”中每一层的key节点 findPredecessor(key); // Clean index // 如果“表头的右索引为空”,则将“跳表的层次”-1。 if (head.right == null) tryReduceLevel(); } return (V)v; } } }
说明:doRemove()的作用是删除跳表中的节点。
和doPut()一样,我们重点看doRemove()的主干部分,了解主干部分之后,其余部分就非常容易理解了。下面是“单线程的情况下,删除跳表中键值对的步骤”:
第1步:找到“被删除节点的位置”。
即,找到“key的前继节点(b)”,“key所对应的节点(n)”,“n的后继节点f”;key是要删除节点的键。
第2步:删除节点。
即,将“key所对应的节点n”从跳表中移除 -- 将“b的后继节点”设为“f”!
第3步:更新跳表。
即,遍历跳表,删除每一层的“key节点”(如果存在的话)。如果删除“key节点”之后,跳表的层次需要-1;则执行相应的操作!
主干部分“对应的精简后的doRemove()的代码”如下(仅供参考):
final V doRemove(Object okey, Object value) { Comparable<? super K> key = comparable(okey); for (;;) { // 找到“key的前继节点” Node<K,V> b = findPredecessor(key); // 设置n为“b的后继节点”(即若key存在于“跳表中”,n就是key对应的节点) Node<K,V> n = b.next; for (;;) { // f是“当前节点n的后继节点” Node<K,V> f = n.next; // 设置“当前节点n”的值为null n.casValue(v, null); // 设置“b的后继节点”为f b.casNext(n, f); // 清除“跳表”中每一层的key节点 findPredecessor(key); // 如果“表头的右索引为空”,则将“跳表的层次”-1。 if (head.right == null) tryReduceLevel(); return (V)v; } } }
3. 获取
下面以get(Object key)为例,对ConcurrentSkipListMap的获取方法进行说明。
public V get(Object key) { return doGet(key); }
doGet的源码如下:
private V doGet(Object okey) { Comparable<? super K> key = comparable(okey); for (;;) { // 找到“key对应的节点” Node<K,V> n = findNode(key); if (n == null) return null; Object v = n.value; if (v != null) return (V)v; } }
说明:doGet()是通过findNode()找到并返回节点的。
private Node<K,V> findNode(Comparable<? super K> key) { for (;;) { // 找到key的前继节点 Node<K,V> b = findPredecessor(key); // 设置n为“b的后继节点”(即若key存在于“跳表中”,n就是key对应的节点) Node<K,V> n = b.next; for (;;) { // 如果“n为null”,则跳转中不存在key对应的节点,直接返回null。 if (n == null) return null; Node<K,V> f = n.next; // 如果两次读取到的“b的后继节点”不同(其它线程操作了该跳表),则返回到“外层for循环”重新遍历。 if (n != b.next) // inconsistent read break; Object v = n.value; // 如果“当前节点n的值”变为null(其它线程操作了该跳表),则返回到“外层for循环”重新遍历。 if (v == null) { // n is deleted n.helpDelete(b, f); break; } if (v == n || b.value == null) // b is deleted break; // 若n是当前节点,则返回n。 int c = key.compareTo(n.key); if (c == 0) return n; // 若“节点n的key”小于“key”,则说明跳表中不存在key对应的节点,返回null if (c < 0) return null; // 若“节点n的key”大于“key”,则更新b和n,继续查找。 b = n; n = f; } } }
说明:findNode(key)的作用是在返回跳表中key对应的节点;存在则返回节点,不存在则返回null。
先弄清函数的主干部分,即抛开“多线程相关内容”,单纯的考虑单线程情况下,从跳表获取节点的算法。
第1步:找到“被删除节点的位置”。
根据findPredecessor()定位key所在的层次以及找到key的前继节点(b),然后找到b的后继节点n。
第2步:根据“key的前继节点(b)”和“key的前继节点的后继节点(n)”来定位“key对应的节点”。
具体是通过比较“n的键值”和“key”的大小。如果相等,则n就是所要查找的键。
ConcurrentSkipListMap示例
import java.util.*; import java.util.concurrent.*; /* * ConcurrentSkipListMap是“线程安全”的哈希表,而TreeMap是非线程安全的。 * * 下面是“多个线程同时操作并且遍历map”的示例 * (01) 当map是ConcurrentSkipListMap对象时,程序能正常运行。 * (02) 当map是TreeMap对象时,程序会产生ConcurrentModificationException异常。 * * @author skywang */ public class ConcurrentSkipListMapDemo1 { // TODO: map是TreeMap对象时,程序会出错。 //private static Map<String, String> map = new TreeMap<String, String>(); private static Map<String, String> map = new ConcurrentSkipListMap<String, String>(); public static void main(String[] args) { // 同时启动两个线程对map进行操作! new MyThread("a").start(); new MyThread("b").start(); } private static void printAll() { String key, value; Iterator iter = map.entrySet().iterator(); while(iter.hasNext()) { Map.Entry entry = (Map.Entry)iter.next(); key = (String)entry.getKey(); value = (String)entry.getValue(); System.out.print("("+key+", "+value+"), "); } System.out.println(); } private static class MyThread extends Thread { MyThread(String name) { super(name); } @Override public void run() { int i = 0; while (i++ < 6) { // “线程名” + "序号" String val = Thread.currentThread().getName()+i; map.put(val, "0"); // 通过“Iterator”遍历map。 printAll(); } } } }
(某一次)运行结果:
(a1, 0), (a1, 0), (b1, 0), (b1, 0), (a1, 0), (b1, 0), (b2, 0), (a1, 0), (a1, 0), (a2, 0), (a2, 0), (b1, 0), (b1, 0), (b2, 0), (b2, 0), (b3, 0), (b3, 0), (a1, 0), (a2, 0), (a3, 0), (a1, 0), (b1, 0), (a2, 0), (b2, 0), (a3, 0), (b3, 0), (b1, 0), (b4, 0), (b2, 0), (a1, 0), (b3, 0), (a2, 0), (b4, 0), (a3, 0), (a1, 0), (a4, 0), (a2, 0), (b1, 0), (a3, 0), (b2, 0), (a4, 0), (b3, 0), (b1, 0), (b4, 0), (b2, 0), (b5, 0), (b3, 0), (a1, 0), (b4, 0), (a2, 0), (b5, 0), (a3, 0), (a1, 0), (a4, 0), (a2, 0), (a5, 0), (a3, 0), (b1, 0), (a4, 0), (b2, 0), (a5, 0), (b3, 0), (b1, 0), (b4, 0), (b2, 0), (b5, 0), (b3, 0), (b6, 0), (b4, 0), (a1, 0), (b5, 0), (a2, 0), (b6, 0), (a3, 0), (a4, 0), (a5, 0), (a6, 0), (b1, 0), (b2, 0), (b3, 0), (b4, 0), (b5, 0), (b6, 0),
结果说明:
示例程序中,启动两个线程(线程a和线程b)分别对ConcurrentSkipListMap进行操作。以线程a而言,它会先获取“线程名”+“序号”,然后将该字符串作为key,将“0”作为value,插入到ConcurrentSkipListMap中;接着,遍历并输出ConcurrentSkipListMap中的全部元素。 线程b的操作和线程a一样,只不过线程b的名字和线程a的名字不同。
当map是ConcurrentSkipListMap对象时,程序能正常运行。如果将map改为TreeMap时,程序会产生ConcurrentModificationException异常。
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