J.U.C并发框架源码阅读（十二）ConcurrentHashMap
基于版本jdk1.7.0_80
java.util.concurrent.ConcurrentHashMap
代码如下
/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * * * * * * * * * * * * * * * * * * * * */ /* * * * * * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.util.concurrent.locks.*; import java.util.*; import java.io.Serializable; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.ObjectStreamField; /** * A hash table supporting full concurrency of retrievals and * adjustable expected concurrency for updates. This class obeys the * same functional specification as {@link java.util.Hashtable}, and * includes versions of methods corresponding to each method of * <tt>Hashtable</tt>. However, even though all operations are * thread-safe, retrieval operations do <em>not</em> entail locking, * and there is <em>not</em> any support for locking the entire table * in a way that prevents all access. This class is fully * interoperable with <tt>Hashtable</tt> in programs that rely on its * thread safety but not on its synchronization details. * * <p> Retrieval operations (including <tt>get</tt>) generally do not * block, so may overlap with update operations (including * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results * of the most recently <em>completed</em> update operations holding * upon their onset. For aggregate operations such as <tt>putAll</tt> * and <tt>clear</tt>, concurrent retrievals may reflect insertion or * removal of only some entries. Similarly, Iterators and * Enumerations return elements reflecting the state of the hash table * at some point at or since the creation of the iterator/enumeration. * They do <em>not</em> throw {@link ConcurrentModificationException}. * However, iterators are designed to be used by only one thread at a time. * * <p> The allowed concurrency among update operations is guided by * the optional <tt>concurrencyLevel</tt> constructor argument * (default <tt>16</tt>), which is used as a hint for internal sizing. The * table is internally partitioned to try to permit the indicated * number of concurrent updates without contention. Because placement * in hash tables is essentially random, the actual concurrency will * vary. Ideally, you should choose a value to accommodate as many * threads as will ever concurrently modify the table. Using a * significantly higher value than you need can waste space and time, * and a significantly lower value can lead to thread contention. But * overestimates and underestimates within an order of magnitude do * not usually have much noticeable impact. A value of one is * appropriate when it is known that only one thread will modify and * all others will only read. Also, resizing this or any other kind of * hash table is a relatively slow operation, so, when possible, it is * a good idea to provide estimates of expected table sizes in * constructors. * * <p>This class and its views and iterators implement all of the * <em>optional</em> methods of the {@link Map} and {@link Iterator} * interfaces. * * <p> Like {@link Hashtable} but unlike {@link HashMap}, this class * does <em>not</em> allow <tt>null</tt> to be used as a key or value. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values */ public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable { private static final long serialVersionUID = 7249069246763182397L; /* * The basic strategy is to subdivide the table among Segments, * each of which itself is a concurrently readable hash table. To * reduce footprint, all but one segments are constructed only * when first needed (see ensureSegment). To maintain visibility * in the presence of lazy construction, accesses to segments as * well as elements of segment's table must use volatile access, * which is done via Unsafe within methods segmentAt etc * below. These provide the functionality of AtomicReferenceArrays * but reduce the levels of indirection. Additionally, * volatile-writes of table elements and entry "next" fields * within locked operations use the cheaper "lazySet" forms of * writes (via putOrderedObject) because these writes are always * followed by lock releases that maintain sequential consistency * of table updates. * * Historical note: The previous version of this class relied * heavily on "final" fields, which avoided some volatile reads at * the expense of a large initial footprint. Some remnants of * that design (including forced construction of segment 0) exist * to ensure serialization compatibility. */ /* ---------------- Constants -------------- */ /** * The default initial capacity for this table, * used when not otherwise specified in a constructor. */ static final int DEFAULT_INITIAL_CAPACITY = 16; /** * The default load factor for this table, used when not * otherwise specified in a constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * The default concurrency level for this table, used when not * otherwise specified in a constructor. */ static final int DEFAULT_CONCURRENCY_LEVEL = 16; /** * The maximum capacity, used if a higher value is implicitly * specified by either of the constructors with arguments. MUST * be a power of two <= 1<<30 to ensure that entries are indexable * using ints. */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * The minimum capacity for per-segment tables. Must be a power * of two, at least two to avoid immediate resizing on next use * after lazy construction. */ static final int MIN_SEGMENT_TABLE_CAPACITY = 2; /** * The maximum number of segments to allow; used to bound * constructor arguments. Must be power of two less than 1 << 24. */ static final int MAX_SEGMENTS = 1 << 16; // slightly conservative /** * Number of unsynchronized retries in size and containsValue * methods before resorting to locking. This is used to avoid * unbounded retries if tables undergo continuous modification * which would make it impossible to obtain an accurate result. */ static final int RETRIES_BEFORE_LOCK = 2; /* ---------------- Fields -------------- */ /** * holds values which can't be initialized until after VM is booted. */ private static class Holder { /** * Enable alternative hashing of String keys? * * <p>Unlike the other hash map implementations we do not implement a * threshold for regulating whether alternative hashing is used for * String keys. Alternative hashing is either enabled for all instances * or disabled for all instances. */ static final boolean ALTERNATIVE_HASHING; static { // Use the "threshold" system property even though our threshold // behaviour is "ON" or "OFF". String altThreshold = java.security.AccessController.doPrivileged( new sun.security.action.GetPropertyAction( "jdk.map.althashing.threshold")); int threshold; try { threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : Integer.MAX_VALUE; // disable alternative hashing if -1 if (threshold == -1) { threshold = Integer.MAX_VALUE; } if (threshold < 0) { throw new IllegalArgumentException("value must be positive integer."); } } catch(IllegalArgumentException failed) { throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed); } ALTERNATIVE_HASHING = threshold <= MAXIMUM_CAPACITY; } } /** * A randomizing value associated with this instance that is applied to * hash code of keys to make hash collisions harder to find. */ private transient final int hashSeed = randomHashSeed(this); private static int randomHashSeed(ConcurrentHashMap instance) { if (sun.misc.VM.isBooted() && Holder.ALTERNATIVE_HASHING) { return sun.misc.Hashing.randomHashSeed(instance); } return 0; } /** * Mask value for indexing into segments. The upper bits of a * key's hash code are used to choose the segment. */ final int segmentMask; /** * Shift value for indexing within segments. */ final int segmentShift; /** * The segments, each of which is a specialized hash table. */ final Segment<K,V>[] segments; transient Set<K> keySet; transient Set<Map.Entry<K,V>> entrySet; transient Collection<V> values; /** * ConcurrentHashMap list entry. Note that this is never exported * out as a user-visible Map.Entry. */ static final class HashEntry<K,V> { final int hash; final K key; volatile V value; volatile HashEntry<K,V> next; HashEntry(int hash, K key, V value, HashEntry<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } /** * Sets next field with volatile write semantics. (See above * about use of putOrderedObject.) */ final void setNext(HashEntry<K,V> n) { UNSAFE.putOrderedObject(this, nextOffset, n); } // Unsafe mechanics static final sun.misc.Unsafe UNSAFE; static final long nextOffset; static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class k = HashEntry.class; nextOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("next")); } catch (Exception e) { throw new Error(e); } } } /** * Gets the ith element of given table (if nonnull) with volatile * read semantics. Note: This is manually integrated into a few * performance-sensitive methods to reduce call overhead. */ @SuppressWarnings("unchecked") static final <K,V> HashEntry<K,V> entryAt(HashEntry<K,V>[] tab, int i) { return (tab == null) ? null : (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)i << TSHIFT) + TBASE); } /** * Sets the ith element of given table, with volatile write * semantics. (See above about use of putOrderedObject.) */ static final <K,V> void setEntryAt(HashEntry<K,V>[] tab, int i, HashEntry<K,V> e) { UNSAFE.putOrderedObject(tab, ((long)i << TSHIFT) + TBASE, e); } /** * Applies a supplemental hash function to a given hashCode, which * defends against poor quality hash functions. This is critical * because ConcurrentHashMap uses power-of-two length hash tables, * that otherwise encounter collisions for hashCodes that do not * differ in lower or upper bits. */ private int hash(Object k) { int h = hashSeed; if ((0 != h) && (k instanceof String)) { return sun.misc.Hashing.stringHash32((String) k); } h ^= k.hashCode(); // Spread bits to regularize both segment and index locations, // using variant of single-word Wang/Jenkins hash. h += (h << 15) ^ 0xffffcd7d; h ^= (h >>> 10); h += (h << 3); h ^= (h >>> 6); h += (h << 2) + (h << 14); return h ^ (h >>> 16); } /** * Segments are specialized versions of hash tables. This * subclasses from ReentrantLock opportunistically, just to * simplify some locking and avoid separate construction. */ static final class Segment<K,V> extends ReentrantLock implements Serializable { /* * Segments maintain a table of entry lists that are always * kept in a consistent state, so can be read (via volatile * reads of segments and tables) without locking. This * requires replicating nodes when necessary during table * resizing, so the old lists can be traversed by readers * still using old version of table. * * This class defines only mutative methods requiring locking. * Except as noted, the methods of this class perform the * per-segment versions of ConcurrentHashMap methods. (Other * methods are integrated directly into ConcurrentHashMap * methods.) These mutative methods use a form of controlled * spinning on contention via methods scanAndLock and * scanAndLockForPut. These intersperse tryLocks with * traversals to locate nodes. The main benefit is to absorb * cache misses (which are very common for hash tables) while * obtaining locks so that traversal is faster once * acquired. We do not actually use the found nodes since they * must be re-acquired under lock anyway to ensure sequential * consistency of updates (and in any case may be undetectably * stale), but they will normally be much faster to re-locate. * Also, scanAndLockForPut speculatively creates a fresh node * to use in put if no node is found. */ private static final long serialVersionUID = 2249069246763182397L; /** * The maximum number of times to tryLock in a prescan before * possibly blocking on acquire in preparation for a locked * segment operation. On multiprocessors, using a bounded * number of retries maintains cache acquired while locating * nodes. */ static final int MAX_SCAN_RETRIES = Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1; /** * The per-segment table. Elements are accessed via * entryAt/setEntryAt providing volatile semantics. */ transient volatile HashEntry<K,V>[] table; /** * The number of elements. Accessed only either within locks * or among other volatile reads that maintain visibility. */ transient int count; /** * The total number of mutative operations in this segment. * Even though this may overflows 32 bits, it provides * sufficient accuracy for stability checks in CHM isEmpty() * and size() methods. Accessed only either within locks or * among other volatile reads that maintain visibility. */ transient int modCount; /** * The table is rehashed when its size exceeds this threshold. * (The value of this field is always <tt>(int)(capacity * * loadFactor)</tt>.) */ transient int threshold; /** * The load factor for the hash table. Even though this value * is same for all segments, it is replicated to avoid needing * links to outer object. * @serial */ final float loadFactor; Segment(float lf, int threshold, HashEntry<K,V>[] tab) { this.loadFactor = lf; this.threshold = threshold; this.table = tab; } final V put(K key, int hash, V value, boolean onlyIfAbsent) { HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> first = entryAt(tab, index); for (HashEntry<K,V> e = first;;) { if (e != null) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; if (!onlyIfAbsent) { e.value = value; ++modCount; } break; } e = e.next; } else { if (node != null) node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); int c = count + 1; if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node); else setEntryAt(tab, index, node); ++modCount; count = c; oldValue = null; break; } } } finally { unlock(); } return oldValue; } /** * Doubles size of table and repacks entries, also adding the * given node to new table */ @SuppressWarnings("unchecked") private void rehash(HashEntry<K,V> node) { /* * Reclassify nodes in each list to new table. Because we * are using power-of-two expansion, the elements from * each bin must either stay at same index, or move with a * power of two offset. We eliminate unnecessary node * creation by catching cases where old nodes can be * reused because their next fields won't change. * Statistically, at the default threshold, only about * one-sixth of them need cloning when a table * doubles. The nodes they replace will be garbage * collectable as soon as they are no longer referenced by * any reader thread that may be in the midst of * concurrently traversing table. Entry accesses use plain * array indexing because they are followed by volatile * table write. */ HashEntry<K,V>[] oldTable = table; int oldCapacity = oldTable.length; int newCapacity = oldCapacity << 1; threshold = (int)(newCapacity * loadFactor); HashEntry<K,V>[] newTable = (HashEntry<K,V>[]) new HashEntry[newCapacity]; int sizeMask = newCapacity - 1; for (int i = 0; i < oldCapacity ; i++) { HashEntry<K,V> e = oldTable[i]; if (e != null) { HashEntry<K,V> next = e.next; int idx = e.hash & sizeMask; if (next == null) // Single node on list newTable[idx] = e; else { // Reuse consecutive sequence at same slot HashEntry<K,V> lastRun = e; int lastIdx = idx; for (HashEntry<K,V> last = next; last != null; last = last.next) { int k = last.hash & sizeMask; if (k != lastIdx) { lastIdx = k; lastRun = last; } } newTable[lastIdx] = lastRun; // Clone remaining nodes for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { V v = p.value; int h = p.hash; int k = h & sizeMask; HashEntry<K,V> n = newTable[k]; newTable[k] = new HashEntry<K,V>(h, p.key, v, n); } } } } int nodeIndex = node.hash & sizeMask; // add the new node node.setNext(newTable[nodeIndex]); newTable[nodeIndex] = node; table = newTable; } /** * Scans for a node containing given key while trying to * acquire lock, creating and returning one if not found. Upon * return, guarantees that lock is held. UNlike in most * methods, calls to method equals are not screened: Since * traversal speed doesn't matter, we might as well help warm * up the associated code and accesses as well. * * @return a new node if key not found, else null */ private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) { HashEntry<K,V> first = entryForHash(this, hash); HashEntry<K,V> e = first; HashEntry<K,V> node = null; int retries = -1; // negative while locating node while (!tryLock()) { HashEntry<K,V> f; // to recheck first below if (retries < 0) { if (e == null) { if (node == null) // speculatively create node node = new HashEntry<K,V>(hash, key, value, null); retries = 0; } else if (key.equals(e.key)) retries = 0; else e = e.next; } else if (++retries > MAX_SCAN_RETRIES) { lock(); break; } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) { e = first = f; // re-traverse if entry changed retries = -1; } } return node; } /** * Scans for a node containing the given key while trying to * acquire lock for a remove or replace operation. Upon * return, guarantees that lock is held. Note that we must * lock even if the key is not found, to ensure sequential * consistency of updates. */ private void scanAndLock(Object key, int hash) { // similar to but simpler than scanAndLockForPut HashEntry<K,V> first = entryForHash(this, hash); HashEntry<K,V> e = first; int retries = -1; while (!tryLock()) { HashEntry<K,V> f; if (retries < 0) { if (e == null || key.equals(e.key)) retries = 0; else e = e.next; } else if (++retries > MAX_SCAN_RETRIES) { lock(); break; } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) { e = first = f; retries = -1; } } } /** * Remove; match on key only if value null, else match both. */ final V remove(Object key, int hash, Object value) { if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> e = entryAt(tab, index); HashEntry<K,V> pred = null; while (e != null) { K k; HashEntry<K,V> next = e.next; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { V v = e.value; if (value == null || value == v || value.equals(v)) { if (pred == null) setEntryAt(tab, index, next); else pred.setNext(next); ++modCount; --count; oldValue = v; } break; } pred = e; e = next; } } finally { unlock(); } return oldValue; } final boolean replace(K key, int hash, V oldValue, V newValue) { if (!tryLock()) scanAndLock(key, hash); boolean replaced = false; try { HashEntry<K,V> e; for (e = entryForHash(this, hash); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { if (oldValue.equals(e.value)) { e.value = newValue; ++modCount; replaced = true; } break; } } } finally { unlock(); } return replaced; } final V replace(K key, int hash, V value) { if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V> e; for (e = entryForHash(this, hash); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; e.value = value; ++modCount; break; } } } finally { unlock(); } return oldValue; } final void clear() { lock(); try { HashEntry<K,V>[] tab = table; for (int i = 0; i < tab.length ; i++) setEntryAt(tab, i, null); ++modCount; count = 0; } finally { unlock(); } } } // Accessing segments /** * Gets the jth element of given segment array (if nonnull) with * volatile element access semantics via Unsafe. (The null check * can trigger harmlessly only during deserialization.) Note: * because each element of segments array is set only once (using * fully ordered writes), some performance-sensitive methods rely * on this method only as a recheck upon null reads. */ @SuppressWarnings("unchecked") static final <K,V> Segment<K,V> segmentAt(Segment<K,V>[] ss, int j) { long u = (j << SSHIFT) + SBASE; return ss == null ? null : (Segment<K,V>) UNSAFE.getObjectVolatile(ss, u); } /** * Returns the segment for the given index, creating it and * recording in segment table (via CAS) if not already present. * * @param k the index * @return the segment */ @SuppressWarnings("unchecked") private Segment<K,V> ensureSegment(int k) { final Segment<K,V>[] ss = this.segments; long u = (k << SSHIFT) + SBASE; // raw offset Segment<K,V> seg; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { Segment<K,V> proto = ss[0]; // use segment 0 as prototype int cap = proto.table.length; float lf = proto.loadFactor; int threshold = (int)(cap * lf); HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap]; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { // recheck Segment<K,V> s = new Segment<K,V>(lf, threshold, tab); while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) break; } } } return seg; } // Hash-based segment and entry accesses /** * Get the segment for the given hash */ @SuppressWarnings("unchecked") private Segment<K,V> segmentForHash(int h) { long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; return (Segment<K,V>) UNSAFE.getObjectVolatile(segments, u); } /** * Gets the table entry for the given segment and hash */ @SuppressWarnings("unchecked") static final <K,V> HashEntry<K,V> entryForHash(Segment<K,V> seg, int h) { HashEntry<K,V>[] tab; return (seg == null || (tab = seg.table) == null) ? null : (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); } /* ---------------- Public operations -------------- */ /** * Creates a new, empty map with the specified initial * capacity, load factor and concurrency level. * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @param concurrencyLevel the estimated number of concurrently * updating threads. The implementation performs internal sizing * to try to accommodate this many threads. * @throws IllegalArgumentException if the initial capacity is * negative or the load factor or concurrencyLevel are * nonpositive. */ @SuppressWarnings("unchecked") public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) { if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) concurrencyLevel = MAX_SEGMENTS; // Find power-of-two sizes best matching arguments int sshift = 0; int ssize = 1; while (ssize < concurrencyLevel) { ++sshift; ssize <<= 1; } this.segmentShift = 32 - sshift; this.segmentMask = ssize - 1; if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; int c = initialCapacity / ssize; if (c * ssize < initialCapacity) ++c; int cap = MIN_SEGMENT_TABLE_CAPACITY; while (cap < c) cap <<= 1; // create segments and segments[0] Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]); Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] this.segments = ss; } /** * Creates a new, empty map with the specified initial capacity * and load factor and with the default concurrencyLevel (16). * * @param initialCapacity The implementation performs internal * sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @throws IllegalArgumentException if the initial capacity of * elements is negative or the load factor is nonpositive * * @since 1.6 */ public ConcurrentHashMap(int initialCapacity, float loadFactor) { this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with the specified initial capacity, * and with default load factor (0.75) and concurrencyLevel (16). * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @throws IllegalArgumentException if the initial capacity of * elements is negative. */ public ConcurrentHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with a default initial capacity (16), * load factor (0.75) and concurrencyLevel (16). */ public ConcurrentHashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new map with the same mappings as the given map. * The map is created with a capacity of 1.5 times the number * of mappings in the given map or 16 (whichever is greater), * and a default load factor (0.75) and concurrencyLevel (16). * * @param m the map */ public ConcurrentHashMap(Map<? extends K, ? extends V> m) { this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); putAll(m); } /** * Returns <tt>true</tt> if this map contains no key-value mappings. * * @return <tt>true</tt> if this map contains no key-value mappings */ public boolean isEmpty() { /* * Sum per-segment modCounts to avoid mis-reporting when * elements are concurrently added and removed in one segment * while checking another, in which case the table was never * actually empty at any point. (The sum ensures accuracy up * through at least 1<<31 per-segment modifications before * recheck.) Methods size() and containsValue() use similar * constructions for stability checks. */ long sum = 0L; final Segment<K,V>[] segments = this.segments; for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { if (seg.count != 0) return false; sum += seg.modCount; } } if (sum != 0L) { // recheck unless no modifications for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { if (seg.count != 0) return false; sum -= seg.modCount; } } if (sum != 0L) return false; } return true; } /** * Returns the number of key-value mappings in this map. If the * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns * <tt>Integer.MAX_VALUE</tt>. * * @return the number of key-value mappings in this map */ public int size() { // Try a few times to get accurate count. On failure due to // continuous async changes in table, resort to locking. final Segment<K,V>[] segments = this.segments; int size; boolean overflow; // true if size overflows 32 bits long sum; // sum of modCounts long last = 0L; // previous sum int retries = -1; // first iteration isn't retry try { for (;;) { if (retries++ == RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } sum = 0L; size = 0; overflow = false; for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { sum += seg.modCount; int c = seg.count; if (c < 0 || (size += c) < 0) overflow = true; } } if (sum == last) break; last = sum; } } finally { if (retries > RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) segmentAt(segments, j).unlock(); } } return overflow ? Integer.MAX_VALUE : size; } /** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code key.equals(k)}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @throws NullPointerException if the specified key is null */ public V get(Object key) { Segment<K,V> s; // manually integrate access methods to reduce overhead HashEntry<K,V>[] tab; int h = hash(key); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; } } return null; } /** * Tests if the specified object is a key in this table. * * @param key possible key * @return <tt>true</tt> if and only if the specified object * is a key in this table, as determined by the * <tt>equals</tt> method; <tt>false</tt> otherwise. * @throws NullPointerException if the specified key is null */ @SuppressWarnings("unchecked") public boolean containsKey(Object key) { Segment<K,V> s; // same as get() except no need for volatile value read HashEntry<K,V>[] tab; int h = hash(key); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return true; } } return false; } /** * Returns <tt>true</tt> if this map maps one or more keys to the * specified value. Note: This method requires a full internal * traversal of the hash table, and so is much slower than * method <tt>containsKey</tt>. * * @param value value whose presence in this map is to be tested * @return <tt>true</tt> if this map maps one or more keys to the * specified value * @throws NullPointerException if the specified value is null */ public boolean containsValue(Object value) { // Same idea as size() if (value == null) throw new NullPointerException(); final Segment<K,V>[] segments = this.segments; boolean found = false; long last = 0; int retries = -1; try { outer: for (;;) { if (retries++ == RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } long hashSum = 0L; int sum = 0; for (int j = 0; j < segments.length; ++j) { HashEntry<K,V>[] tab; Segment<K,V> seg = segmentAt(segments, j); if (seg != null && (tab = seg.table) != null) { for (int i = 0 ; i < tab.length; i++) { HashEntry<K,V> e; for (e = entryAt(tab, i); e != null; e = e.next) { V v = e.value; if (v != null && value.equals(v)) { found = true; break outer; } } } sum += seg.modCount; } } if (retries > 0 && sum == last) break; last = sum; } } finally { if (retries > RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) segmentAt(segments, j).unlock(); } } return found; } /** * Legacy method testing if some key maps into the specified value * in this table. This method is identical in functionality to * {@link #containsValue}, and exists solely to ensure * full compatibility with class {@link java.util.Hashtable}, * which supported this method prior to introduction of the * Java Collections framework. * @param value a value to search for * @return <tt>true</tt> if and only if some key maps to the * <tt>value</tt> argument in this table as * determined by the <tt>equals</tt> method; * <tt>false</tt> otherwise * @throws NullPointerException if the specified value is null */ public boolean contains(Object value) { return containsValue(value); } /** * Maps the specified key to the specified value in this table. * Neither the key nor the value can be null. * * <p> The value can be retrieved by calling the <tt>get</tt> method * with a key that is equal to the original key. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key or value is null */ @SuppressWarnings("unchecked") public V put(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); return s.put(key, hash, value, false); } /** * {@inheritDoc} * * @return the previous value associated with the specified key, * or <tt>null</tt> if there was no mapping for the key * @throws NullPointerException if the specified key or value is null */ @SuppressWarnings("unchecked") public V putIfAbsent(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment<K,V>)UNSAFE.getObject (segments, (j << SSHIFT) + SBASE)) == null) s = ensureSegment(j); return s.put(key, hash, value, true); } /** * Copies all of the mappings from the specified map to this one. * These mappings replace any mappings that this map had for any of the * keys currently in the specified map. * * @param m mappings to be stored in this map */ public void putAll(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue()); } /** * Removes the key (and its corresponding value) from this map. * This method does nothing if the key is not in the map. * * @param key the key that needs to be removed * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key is null */ public V remove(Object key) { int hash = hash(key); Segment<K,V> s = segmentForHash(hash); return s == null ? null : s.remove(key, hash, null); } /** * {@inheritDoc} * * @throws NullPointerException if the specified key is null */ public boolean remove(Object key, Object value) { int hash = hash(key); Segment<K,V> s; return value != null && (s = segmentForHash(hash)) != null && s.remove(key, hash, value) != null; } /** * {@inheritDoc} * * @throws NullPointerException if any of the arguments are null */ public boolean replace(K key, V oldValue, V newValue) { int hash = hash(key); if (oldValue == null || newValue == null) throw new NullPointerException(); Segment<K,V> s = segmentForHash(hash); return s != null && s.replace(key, hash, oldValue, newValue); } /** * {@inheritDoc} * * @return the previous value associated with the specified key, * or <tt>null</tt> if there was no mapping for the key * @throws NullPointerException if the specified key or value is null */ public V replace(K key, V value) { int hash = hash(key); if (value == null) throw new NullPointerException(); Segment<K,V> s = segmentForHash(hash); return s == null ? null : s.replace(key, hash, value); } /** * Removes all of the mappings from this map. */ public void clear() { final Segment<K,V>[] segments = this.segments; for (int j = 0; j < segments.length; ++j) { Segment<K,V> s = segmentAt(segments, j); if (s != null) s.clear(); } } /** * Returns a {@link Set} view of the keys contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. The set supports element * removal, which removes the corresponding mapping from this map, * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or * <tt>addAll</tt> operations. * * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator * that will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. */ public Set<K> keySet() { Set<K> ks = keySet; return (ks != null) ? ks : (keySet = new KeySet()); } /** * Returns a {@link Collection} view of the values contained in this map. * The collection is backed by the map, so changes to the map are * reflected in the collection, and vice-versa. The collection * supports element removal, which removes the corresponding * mapping from this map, via the <tt>Iterator.remove</tt>, * <tt>Collection.remove</tt>, <tt>removeAll</tt>, * <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not * support the <tt>add</tt> or <tt>addAll</tt> operations. * * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator * that will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. */ public Collection<V> values() { Collection<V> vs = values; return (vs != null) ? vs : (values = new Values()); } /** * Returns a {@link Set} view of the mappings contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. The set supports element * removal, which removes the corresponding mapping from the map, * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or * <tt>addAll</tt> operations. * * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator * that will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. */ public Set<Map.Entry<K,V>> entrySet() { Set<Map.Entry<K,V>> es = entrySet; return (es != null) ? es : (entrySet = new EntrySet()); } /** * Returns an enumeration of the keys in this table. * * @return an enumeration of the keys in this table * @see #keySet() */ public Enumeration<K> keys() { return new KeyIterator(); } /** * Returns an enumeration of the values in this table. * * @return an enumeration of the values in this table * @see #values() */ public Enumeration<V> elements() { return new ValueIterator(); } /* ---------------- Iterator Support -------------- */ abstract class HashIterator { int nextSegmentIndex; int nextTableIndex; HashEntry<K,V>[] currentTable; HashEntry<K, V> nextEntry; HashEntry<K, V> lastReturned; HashIterator() { nextSegmentIndex = segments.length - 1; nextTableIndex = -1; advance(); } /** * Set nextEntry to first node of next non-empty table * (in backwards order, to simplify checks). */ final void advance() { for (;;) { if (nextTableIndex >= 0) { if ((nextEntry = entryAt(currentTable, nextTableIndex--)) != null) break; } else if (nextSegmentIndex >= 0) { Segment<K,V> seg = segmentAt(segments, nextSegmentIndex--); if (seg != null && (currentTable = seg.table) != null) nextTableIndex = currentTable.length - 1; } else break; } } final HashEntry<K,V> nextEntry() { HashEntry<K,V> e = nextEntry; if (e == null) throw new NoSuchElementException(); lastReturned = e; // cannot assign until after null check if ((nextEntry = e.next) == null) advance(); return e; } public final boolean hasNext() { return nextEntry != null; } public final boolean hasMoreElements() { return nextEntry != null; } public final void remove() { if (lastReturned == null) throw new IllegalStateException(); ConcurrentHashMap.this.remove(lastReturned.key); lastReturned = null; } } final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K> { public final K next() { return super.nextEntry().key; } public final K nextElement() { return super.nextEntry().key; } } final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V> { public final V next() { return super.nextEntry().value; } public final V nextElement() { return super.nextEntry().value; } } /** * Custom Entry class used by EntryIterator.next(), that relays * setValue changes to the underlying map. */ final class WriteThroughEntry extends AbstractMap.SimpleEntry<K,V> { WriteThroughEntry(K k, V v) { super(k,v); } /** * Set our entry's value and write through to the map. The * value to return is somewhat arbitrary here. Since a * WriteThroughEntry does not necessarily track asynchronous * changes, the most recent "previous" value could be * different from what we return (or could even have been * removed in which case the put will re-establish). We do not * and cannot guarantee more. */ public V setValue(V value) { if (value == null) throw new NullPointerException(); V v = super.setValue(value); ConcurrentHashMap.this.put(getKey(), value); return v; } } final class EntryIterator extends HashIterator implements Iterator<Entry<K,V>> { public Map.Entry<K,V> next() { HashEntry<K,V> e = super.nextEntry(); return new WriteThroughEntry(e.key, e.value); } } final class KeySet extends AbstractSet<K> { public Iterator<K> iterator() { return new KeyIterator(); } public int size() { return ConcurrentHashMap.this.size(); } public boolean isEmpty() { return ConcurrentHashMap.this.isEmpty(); } public boolean contains(Object o) { return ConcurrentHashMap.this.containsKey(o); } public boolean remove(Object o) { return ConcurrentHashMap.this.remove(o) != null; } public void clear() { ConcurrentHashMap.this.clear(); } } final class Values extends AbstractCollection<V> { public Iterator<V> iterator() { return new ValueIterator(); } public int size() { return ConcurrentHashMap.this.size(); } public boolean isEmpty() { return ConcurrentHashMap.this.isEmpty(); } public boolean contains(Object o) { return ConcurrentHashMap.this.containsValue(o); } public void clear() { ConcurrentHashMap.this.clear(); } } final class EntrySet extends AbstractSet<Map.Entry<K,V>> { public Iterator<Map.Entry<K,V>> iterator() { return new EntryIterator(); } public boolean contains(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> e = (Map.Entry<?,?>)o; V v = ConcurrentHashMap.this.get(e.getKey()); return v != null && v.equals(e.getValue()); } public boolean remove(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> e = (Map.Entry<?,?>)o; return ConcurrentHashMap.this.remove(e.getKey(), e.getValue()); } public int size() { return ConcurrentHashMap.this.size(); } public boolean isEmpty() { return ConcurrentHashMap.this.isEmpty(); } public void clear() { ConcurrentHashMap.this.clear(); } } /* ---------------- Serialization Support -------------- */ /** * Save the state of the <tt>ConcurrentHashMap</tt> instance to a * stream (i.e., serialize it). * @param s the stream * @serialData * the key (Object) and value (Object) * for each key-value mapping, followed by a null pair. * The key-value mappings are emitted in no particular order. */ private void writeObject(java.io.ObjectOutputStream s) throws IOException { // force all segments for serialization compatibility for (int k = 0; k < segments.length; ++k) ensureSegment(k); s.defaultWriteObject(); final Segment<K,V>[] segments = this.segments; for (int k = 0; k < segments.length; ++k) { Segment<K,V> seg = segmentAt(segments, k); seg.lock(); try { HashEntry<K,V>[] tab = seg.table; for (int i = 0; i < tab.length; ++i) { HashEntry<K,V> e; for (e = entryAt(tab, i); e != null; e = e.next) { s.writeObject(e.key); s.writeObject(e.value); } } } finally { seg.unlock(); } } s.writeObject(null); s.writeObject(null); } /** * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a * stream (i.e., deserialize it). * @param s the stream */ @SuppressWarnings("unchecked") private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Don't call defaultReadObject() ObjectInputStream.GetField oisFields = s.readFields(); final Segment<K,V>[] oisSegments = (Segment<K,V>[])oisFields.get("segments", null); final int ssize = oisSegments.length; if (ssize < 1 || ssize > MAX_SEGMENTS || (ssize & (ssize-1)) != 0 ) // ssize not power of two throw new java.io.InvalidObjectException("Bad number of segments:" + ssize); int sshift = 0, ssizeTmp = ssize; while (ssizeTmp > 1) { ++sshift; ssizeTmp >>>= 1; } UNSAFE.putIntVolatile(this, SEGSHIFT_OFFSET, 32 - sshift); UNSAFE.putIntVolatile(this, SEGMASK_OFFSET, ssize - 1); UNSAFE.putObjectVolatile(this, SEGMENTS_OFFSET, oisSegments); // set hashMask UNSAFE.putIntVolatile(this, HASHSEED_OFFSET, randomHashSeed(this)); // Re-initialize segments to be minimally sized, and let grow. int cap = MIN_SEGMENT_TABLE_CAPACITY; final Segment<K,V>[] segments = this.segments; for (int k = 0; k < segments.length; ++k) { Segment<K,V> seg = segments[k]; if (seg != null) { seg.threshold = (int)(cap * seg.loadFactor); seg.table = (HashEntry<K,V>[]) new HashEntry[cap]; } } // Read the keys and values, and put the mappings in the table for (;;) { K key = (K) s.readObject(); V value = (V) s.readObject(); if (key == null) break; put(key, value); } } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE; private static final long SBASE; private static final int SSHIFT; private static final long TBASE; private static final int TSHIFT; private static final long HASHSEED_OFFSET; private static final long SEGSHIFT_OFFSET; private static final long SEGMASK_OFFSET; private static final long SEGMENTS_OFFSET; static { int ss, ts; try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class tc = HashEntry[].class; Class sc = Segment[].class; TBASE = UNSAFE.arrayBaseOffset(tc); SBASE = UNSAFE.arrayBaseOffset(sc); ts = UNSAFE.arrayIndexScale(tc); ss = UNSAFE.arrayIndexScale(sc); HASHSEED_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("hashSeed")); SEGSHIFT_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("segmentShift")); SEGMASK_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("segmentMask")); SEGMENTS_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("segments")); } catch (Exception e) { throw new Error(e); } if ((ss & (ss-1)) != 0 || (ts & (ts-1)) != 0) throw new Error("data type scale not a power of two"); SSHIFT = 31 - Integer.numberOfLeadingZeros(ss); TSHIFT = 31 - Integer.numberOfLeadingZeros(ts); } }
0. ConcurrentHashMap简介
线程安全的Map，支持有限个线程并发写，与无限个线程并发读，提供了比用synchronized实现的Hashtable更高的性能。
1. 接口分析
ConcurrentHashMap继承于AbstractMap抽象类
ConcurrentMap（主要提供了putIfAbsent这个方法）, Serializable接口
2. ConcurrentHashMap原理概述
使用了分段锁技术，把线程冲突打散到多个segment中，放弃了强一致性，用弱一致性换来了更高的性能（例如clear方法）。
更具体的说，与HashMap使用Entry数组直接存储数据不同，ConcurrentHashMap维护了一个Segment数组，每个Segment内部维护了一个Entry数组来存储元素，而Segment又继承于ReentrantLock，因此Segment可以较为容易的实现内部线程安全语义
3. ConcurrentHashMap.put方法解析
/** * Maps the specified key to the specified value in this table. * Neither the key nor the value can be null. * * <p> The value can be retrieved by calling the <tt>get</tt> method * with a key that is equal to the original key. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key or value is null */ @SuppressWarnings("unchecked") public V put(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key);//用更复杂的算法重新计算key的hashCode int j = (hash >>> segmentShift) & segmentMask;//用hashCoude的高位计算key会被定位到哪个segment if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment//用UNSAFE里的方法直接去segments数组里找到key对应的segment s = ensureSegment(j);//如果segment为null，将其初始化 return s.put(key, hash, value, false);//调用Segment.put方法插入键值对 } /** * Returns the segment for the given index, creating it and * recording in segment table (via CAS) if not already present. * * @param k the index * @return the segment */ @SuppressWarnings("unchecked") private Segment<K,V> ensureSegment(int k) { final Segment<K,V>[] ss = this.segments; long u = (k << SSHIFT) + SBASE; // raw offset Segment<K,V> seg; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { Segment<K,V> proto = ss[0]; // use segment 0 as prototype//用segments数组的第一个Segment作为模板 int cap = proto.table.length; float lf = proto.loadFactor; int threshold = (int)(cap * lf); HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap]; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { // recheck Segment<K,V> s = new Segment<K,V>(lf, threshold, tab); while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {//用cas方法将新建的segment安全的放到segment数组中 if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) break; } } } return seg; }
代码较为简单，用key的hashCode再次hash，然后计算出对应的segment的位置，然后调用Segment.put方法。如果对应的segment不存在，则调用ensureSegment方法新建一个segment并用CAS操作将其安全的写入到segment数组中。
4. Segment.put方法解析
final V put(K key, int hash, V value, boolean onlyIfAbsent) { HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value);//锁定当前segment V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash;//计算key在segment的Entry数组中对应的下标 HashEntry<K,V> first = entryAt(tab, index); for (HashEntry<K,V> e = first;;) {//定位到Entry链表了 if (e != null) {//尝试遍历Entry链表，查找key是否已经存在 K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; if (!onlyIfAbsent) {//如果key已经存在，根据onlyIfAbsent变量判断是否要更新value e.value = value; ++modCount; } break; } e = e.next;//向后遍历Entry链表 } else {//找了一圈，发现key不存在，需要新建Entry节点并插入 if (node != null)//采用头插法向Entry链表中插入节点 node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); int c = count + 1; if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node);//扩容 else setEntryAt(tab, index, node);//新插入的节点被设置为Entry链表的头结点 ++modCount; count = c;//更新计数器 oldValue = null; break; } } } finally { unlock();//释放锁 } return oldValue; } /** * Scans for a node containing given key while trying to * acquire lock, creating and returning one if not found. Upon * return, guarantees that lock is held. UNlike in most * methods, calls to method equals are not screened: Since * traversal speed doesn't matter, we might as well help warm * up the associated code and accesses as well. * * @return a new node if key not found, else null */ private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) { HashEntry<K,V> first = entryForHash(this, hash);//计算key会落在Segment内维护的Entry数组的哪个格子上 HashEntry<K,V> e = first; HashEntry<K,V> node = null; int retries = -1; // negative while locating node while (!tryLock()) {//如果没有锁住segment就自旋 HashEntry<K,V> f; // to recheck first below if (retries < 0) {//retries初始值是-1，在找到键值对应该被插入的位置后retries会变为0 if (e == null) {//key不存在，需要新建Node if (node == null) // speculatively create node node = new HashEntry<K,V>(hash, key, value, null); retries = 0;//找到键值对的存放位置了 } else if (key.equals(e.key))//key已存在 retries = 0; else e = e.next;//向后遍历Entry链表 } else if (++retries > MAX_SCAN_RETRIES) {//如果自旋若干次还是没有获得锁，调用lock方法走阻塞锁流程 lock(); break; } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) {//自旋过程中每两次自旋检查一下Entry链表的头结点，如果发生变化，需要重新定位插入key的位置 e = first = f; // re-traverse if entry changed retries = -1; } } return node;//此时当前Segment已经被当前线程锁定，可以安全操作。如果返回值node不为空，说明当前segment中key不存在，需要新建节点 }
先用乐观锁tryLock方法试着去锁Segment，自旋若干次后如果还没锁定，就用悲观锁lock方法去锁Segment，将Segment成功锁定之后，就可以安全的更新元素了
先找到Segment维护的Entry数组的对应的格子上的链表，然后遍历链表，如果key已经存在，则根据putIfAbsent变量决定是否要更新value，如果遍历了一遍发现key不存在，那么新建Entry节点，并用头插法将新节点写入到链表头部。
Segment内还有一个count计数器，put方法会维护这个计数器。
5. Segment.get方法解析
/** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code key.equals(k)}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @throws NullPointerException if the specified key is null */ public V get(Object key) { Segment<K,V> s; // manually integrate access methods to reduce overhead HashEntry<K,V>[] tab; int h = hash(key);//重新计算hashCode long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;//计算key对应的Segment在Segment数组中的偏移量 if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) {//拿到key对应的Segment for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE);//在Segment底层的Entry数组中，key所在的格子存储的链表的头结点 e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value;//遍历链表，寻找key是否存在 } } return null; }
逻辑不复杂，大意是用key的hashCode，先计算出对应于那个Segment，然后再去Segment里找对应的链表，然后遍历链表看key是否已经存在。
但是有一个问题，get方法是无锁的，在get方法遍历链表的同时，put方法同时也可能会插入元素并更新链表，因此get方法不一定会返回最新的结果，也就是说ConcurrentHashMap是弱一致的。这也是为了极致性能而做出的妥协。
5. Segment.size方法解析
/** * Returns the number of key-value mappings in this map. If the * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns * <tt>Integer.MAX_VALUE</tt>. * * @return the number of key-value mappings in this map */ public int size() { // Try a few times to get accurate count. On failure due to // continuous async changes in table, resort to locking. final Segment<K,V>[] segments = this.segments; int size; boolean overflow; // true if size overflows 32 bits long sum; // sum of modCounts long last = 0L; // previous sum int retries = -1; // first iteration isn't retry try { for (;;) { if (retries++ == RETRIES_BEFORE_LOCK) {//如果自旋若干次都没有一致性的结果，那么把所有segment全部锁上然后再计算size for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } sum = 0L; size = 0; overflow = false; for (int j = 0; j < segments.length; ++j) {//遍历所有Segment，取其count计数之和 Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { sum += seg.modCount; int c = seg.count; if (c < 0 || (size += c) < 0) overflow = true; } } if (sum == last)//如果连续两次遍历，Segment.modCount之和没有变化，也就是说这两次遍历之间ConcurrentHashMap没有经历任何修改，那么size方法可以安全的返回了 break; last = sum; } } finally { if (retries > RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) segmentAt(segments, j).unlock(); } } return overflow ? Integer.MAX_VALUE : size; }
很有意思的设计，先乐观的自旋若干次，如果连续两次自旋所有的Segment.modCount之和没有改变（也就是说这两次自旋之间ConcurrentHashMap没有被修改），直接返回Segment.size之和。
如果自旋若干次还是没有一致性的结果，就遍历所有Segment并上锁，这样就可以安全的统计Segment.size之和并返回了。
6. ConcurrentHashMap.clear方法解析
ConcurrentHashMap.clear() /** * Removes all of the mappings from this map. */ public void clear() { final Segment<K,V>[] segments = this.segments; for (int j = 0; j < segments.length; ++j) {//遍历所有Segment Segment<K,V> s = segmentAt(segments, j); if (s != null) s.clear();//调用Segment.clear方法 } } ConcurrentHashMap.Segment.clear() final void clear() { lock();//加锁 try { HashEntry<K,V>[] tab = table; for (int i = 0; i < tab.length ; i++) setEntryAt(tab, i, null);//释放所有链表，相当于清空Segment了 ++modCount; count = 0; } finally { unlock();//释放锁 } }
ConcurrentHashMap.clear方法没有加全局锁，遍历Segment并调用Segment.clear方法的时候，会加上Segment级别的锁
这样的结果是，在清除后面的Segment时，可能前面的Segment又被插入元素了，于是即使ConcurrentHashMap.clear执行结束，ConcurrentHashMap中可能还是不为空。
也就是说ConcurrentHashMap.clear方法是弱一致的。
当然，这还是为性能做出的妥协，毕竟如果加上全局锁，开销实在是太大了。
7. 总结
ConcurrentHashMap是改进版的HashMap，用锁分段技术，提供了有限并发的写入，与无限并发的读取。
但是出于对性能的妥协，ConcurrentHashMap是弱一致的，比如get方法不一定能返回最新的结果，clear方法执行完毕后ConcurrentHashMap内可能还有元素。因此使用的时候还是要加以注意。
