java源码解析---LinkedHashMap

package explain;

import java.util.function.Consumer;
import java.util.AbstractCollection;
import java.util.AbstractSet;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.io.IOException;

/**
 * <p>Hash table and linked list implementation of the <tt>Map</tt> interface,
 * with predictable iteration order.  This implementation differs from
 * <tt>HashMap</tt> in that it maintains a doubly-linked list running through
 * all of its entries.  This linked list defines the iteration ordering,
 * which is normally the order in which keys were inserted into the map
 * (<i>insertion-order</i>).  Note that insertion order is not affected
 * if a key is <i>re-inserted</i> into the map.  (A key <tt>k</tt> is
 * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when
 * <tt>m.containsKey(k)</tt> would return <tt>true</tt> immediately prior to
 * the invocation.)
 *  Map接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与HashMap的不同之处在于，
    后者维护着一个运行于所有条目的双重链接列表。
    此链接列表定义了迭代顺序，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。
    注意，如果在映射中重新插入键，则插入顺序不受影响。会按照第一次插入的顺序，再次插入到之前的位置
   （如果在调用m.put(k,v) 前 m.containsKey(k) 返回了true，则调用时会将键k重新插入到映射m中）
 * <p>This implementation spares its clients from the unspecified, generally
 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),
 * without incurring the increased cost associated with {@link TreeMap}.  It
 * can be used to produce a copy of a map that has the same order as the
 * original, regardless of the original map's implementation:
   此实现可以让客户避免未指定的、由 HashMap（及 Hashtable）所提供的通常为杂乱无章的排序工作，同时无需增加与 TreeMap 相关的成本。      
       使用它可以生成一个与原来顺序相同的映射副本，而与原映射的实现无关：
 * <pre>
 *     void foo(Map m) {
 *         Map copy = new LinkedHashMap(m);
 *         ...
 *     }
 * </pre>
 * This technique is particularly useful if a module takes a map on input,
 * copies it, and later returns results whose order is determined by that of
 * the copy.  (Clients generally appreciate having things returned in the same
 * order they were presented.)
 * 如果模块通过输入得到一个映射，复制这个映射，然后返回由此副本确定其顺序的结果，这种情况下这项技术特别有用。
 （客户通常期望返回的内容与其出现的顺序相同。） 原样返回
 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is
 * provided to create a linked hash map whose order of iteration is the order
 * in which its entries were last accessed, from least-recently accessed to
 * most-recently (<i>access-order</i>).  This kind of map is well-suited to
 * building LRU caches.  Invoking the {@code put}, {@code putIfAbsent},
 * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent},
 * {@code computeIfPresent}, or {@code merge} methods results
 * in an access to the corresponding entry (assuming it exists after the
 * invocation completes). The {@code replace} methods only result in an access
 * of the entry if the value is replaced.  The {@code putAll} method generates one
 * entry access for each mapping in the specified map, in the order that
 * key-value mappings are provided by the specified map's entry set iterator.
 * <i>No other methods generate entry accesses.</i>  In particular, operations
 * on collection-views do <i>not</i> affect the order of iteration of the
 * backing map.
 提供特殊的构造方法来创建链接哈希映射，该哈希映射的迭代顺序就是最后访问其条目的顺序，
 从近期访问最少到近期访问最多的顺序（访问顺序）。这种映射很适合构建 LRU 缓存。调用put或get 方法将会访问相应的条目（假定调用完成后它还存在）。
     putAll 方法以指定映射的条目集迭代器提供的键-值映射关系的顺序，为指定映射的每个映射关系生成一个条目访问。
 任何其他方法均不生成条目访问。特别是，collection 视图上的操作不 影响底层映射的迭代顺序
 *
 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to
 * impose a policy for removing stale mappings automatically when new mappings
 * are added to the map.
 可以重写 removeEldestEntry(Map.Entry) 方法来实施策略，以便在将新映射关系添加到映射时自动移除旧的映射关系。
 * <p>This class provides all of the optional <tt>Map</tt> operations, and
 * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time
 * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and
 * <tt>remove</tt>), assuming the hash function disperses elements
 * properly among the buckets.  Performance is likely to be just slightly
 * below that of <tt>HashMap</tt>, due to the added expense of maintaining the
 * linked list, with one exception: Iteration over the collection-views
 * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i>
 * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt>
 * is likely to be more expensive, requiring time proportional to its
 * <i>capacity</i>.
 *此类提供所有可选的 Map 操作，并且允许 null 元素。与 HashMap 一样，
  它可以为基本操作（add、contains和remove）提供稳定的性能，假定哈希函数将元素正确分布到桶中。
  由于增加了维护链接列表的开支，其性能很可能比 HashMap 稍逊一筹，不过这一点例外：LinkedHashMap 的 collection 视图迭代所需时间与映射的大小成比例。
      HashMap 迭代时间很可能开支较大，因为它所需要的时间与其容量 成比例。
 * <p>A linked hash map has two parameters that affect its performance:
 * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely
 * as for <tt>HashMap</tt>.  Note, however, that the penalty for choosing an
 * excessively high value for initial capacity is less severe for this class
 * than for <tt>HashMap</tt>, as iteration times for this class are unaffected
 * by capacity.
 *链接的哈希映射具有两个影响其性能的参数：初始容量和加载因子。它们的定义与 HashMap 极其相似。要注意，为初始容量选择非常高的值对此类的影响比对 HashMap 要小，
      因为此类的迭代时间不受容量的影响。
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked hash map concurrently, and at least
 * one of the threads modifies the map structurally, it <em>must</em> be
 * synchronized externally.  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.
 *
 * If no such object exists, the map should be "wrapped" using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>
 * 注意，此实现不是同步的。如果多个线程同时访问链接的哈希映射，而其中至少一个线程从结构上修改了该映射，则它必须 保持外部同步。
       这一般通过对自然封装该映射的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedMap 方法来“包装”该映射。最好在创建时完成这一操作，
       以防止对映射的意外的非同步访问：
 * A structural modification is any operation that adds or deletes one or more
 * mappings or, in the case of access-ordered linked hash maps, affects
 * iteration order.  In insertion-ordered linked hash maps, merely changing
 * the value associated with a key that is already contained in the map is not
 * a structural modification.  <strong>In access-ordered linked hash maps,
 * merely querying the map with <tt>get</tt> is a structural modification.
 * </strong>)
 结构修改是指添加或删除一个或多个映射关系，或者在按访问顺序链接的哈希映射中影响迭代顺序的任何操作。
 在按插入顺序链接的哈希映射中，仅更改与映射中已包含键关联的值不是结构修改。在按访问顺序链接的哈希映射中，仅利用 get 查询映射是结构修改。）1.6和1.8有区别
 *当accessOrder为true时，在迭代器方式遍历map时，不允许使用get操作，因为在迭代器模式中不允许修改集合的结构，一般情况下get操作是只读的，
      但是本模式下，get操作需要修改map的链表结构，以将最近访问的元素放置到链表末尾。
 
 * <p>The iterators returned by the <tt>iterator</tt> method of the collections
 * returned by all of this class's collection view methods are
 * <em>fail-fast</em>: if the map is structurally modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * <tt>remove</tt> method, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the future.
 *Collection（由此类的所有 collection 视图方法所返回）的 iterator 方法返回的迭代器都是快速失败 的：在迭代器创建之后，如果从结构上对映射进行修改，
     除非通过迭代器自身的 remove方法，其他任何时间任何方式的修改，迭代器都将抛出ConcurrentModificationException。
 因此，面对并发的修改，迭代器很快就会完全失败，
 而不冒将来不确定的时间任意发生不确定行为的风险。
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *注意，迭代器的快速失败行为无法得到保证，因为一般来说，不可能对是否出现不同步并发修改做出任何硬性保证。
  快速失败迭代器会尽最大努力抛出ConcurrentModificationException。因此，编写依赖于此异常的程序的方式是错误的，
  正确做法是：迭代器的快速失败行为应该仅用于检测程序错误

 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @implNote
 * The spliterators returned by the spliterator method of the collections
 * returned by all of this class's collection view methods are created from
 * the iterators of the corresponding collections.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Josh Bloch
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Map
 * @see     HashMap
 * @see     TreeMap
 * @see     Hashtable
 * @since   1.4
 */

public class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>
{

    private static final long serialVersionUID = 3801124242820219131L;

    /**
     * The head of the doubly linked list.
     * 双向链表的头
     */
    private transient Entry<K,V> header;

    /**
     * The iteration ordering method for this linked hash map: <tt>true</tt>
     * for access-order, <tt>false</tt> for insertion-order.
     * LinkedHashMap 迭代的排序方法 当为True时，表示按访问顺序排序
       当为false时，表示按插入顺序排序，默认是按插入顺序排序
     * @serial
     */
    private final boolean accessOrder;

    /**
     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
     * with the specified initial capacity and load factor.
     * 构造一个空的按插入顺序排序的LinkedHashMap,并指定初始化容量和负载因子，调用父类的构造器构造
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    public LinkedHashMap(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);
        accessOrder = false;
    }

    /**
     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
     * with the specified initial capacity and a default load factor (0.75).
     * 构造一个空的按插入顺序排序的LinkedHashMap,并指定初始化容量和默认的负载因子0.75，调用父类的构造器构造
     * @param  initialCapacity the initial capacity
     * @throws IllegalArgumentException if the initial capacity is negative
     */
    public LinkedHashMap(int initialCapacity) {
    super(initialCapacity);
        accessOrder = false;
    }

    /**
     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
     * with the default initial capacity (16) and load factor (0.75).
     构造一个空的按插入顺序排序的LinkedHashMap,并指定默认初始化容量16和默认的负载因子0.75，调用父类的构造器构造
     */
    public LinkedHashMap() {
    super();
        accessOrder = false;
    }

    /**
     * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with
     * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>
     * instance is created with a default load factor (0.75) and an initial
     * capacity sufficient to hold the mappings in the specified map.
     * 用指定的Map构造一个按插入顺序排序的LinkedHashMap，默认的负载因子0.75和足够容纳给定map的初始化容量
     * @param  m the map whose mappings are to be placed in this map
     * @throws NullPointerException if the specified map is null
     */
    public LinkedHashMap(Map<? extends K, ? extends V> m) {
        super(m);
        accessOrder = false;
    }

    /**
     * Constructs an empty <tt>LinkedHashMap</tt> instance with the
     * specified initial capacity, load factor and ordering mode.
     * 构造一个空的按插入顺序排序的LinkedHashMap，并指定初始化容量,负载因子和排序方式
     * @param  initialCapacity the initial capacity
     * @param  loadFactor      the load factor
     * @param  accessOrder     the ordering mode - <tt>true</tt> for
     *         access-order, <tt>false</tt> for insertion-order
     * @throws IllegalArgumentException if the initial capacity is negative
     *         or the load factor is nonpositive
     */
    public LinkedHashMap(int initialCapacity,
             float loadFactor,
                         boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
    }

    /**
     * Called by superclass constructors and pseudoconstructors (clone,
     * readObject) before any entries are inserted into the map.  Initializes
     * the chain.
       覆写父类的init方法，在父类中都是空方法，在任何键值对被放入map之前在父类的构造器和伪构造器中调用
       初始化双向链表，初始化一个空的节点，hashcode的值是-1，并不存放在哈希表中，作为双向循环链表的头部，
       循环的开始节点或者可以认为也是结束的标志
     */
    void init() {
        header = new Entry<K,V>(-1, null, null, null);
        header.before = header.after = header;
    }

    /**
     * Transfers all entries to new table array.  This method is called
     * by superclass resize.  It is overridden for performance, as it is
     * faster to iterate using our linked list.
       将所有的键值对复制到新的哈希表中，该方法在父类的扩容被调用
       该方法被重写是因为他的性能，因为我们用维护的链表遍历速度更快
     */
    void transfer(HashMap.Entry[] newTable) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e = header.after; e != header; e = e.after) {
            int index = indexFor(e.hash, newCapacity);
            e.next = newTable[index];
            newTable[index] = e;
        }
    }


    /**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value.
     *
     * @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
     */
    public boolean containsValue(Object value) {
        // Overridden to take advantage of faster iterator
        //重写是为了性能，链表的遍历速度更快
        if (value==null) {
            for (Entry e = header.after; e != header; e = e.after)
                if (e.value==null)
                    return true;
        } else {
            for (Entry e = header.after; e != header; e = e.after)
                if (value.equals(e.value))
                    return true;
        }
        return false;
    }

    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     * 根据指定的Map的Key返回value值，如果没有返回null，也可能是key对于的Value就是null
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     * 更正式点说，如果存在这样个map(key==null ? k==null : key.equals(k)
     * 返回value，否则返回null，最多只有一个这个的键值对
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     并不一定会返回null，如果返回null值，表示不存在指定key的map，或者可能是该key的Value就是null
     可以通过containsKey操作来判别
     */
    public V get(Object key) {
        Entry<K,V> e = (Entry<K,V>)getEntry(key);
        if (e == null)
            return null;
        e.recordAccess(this);
        return e.value;
    }

    /**
     * Removes all of the mappings from this map.
     * The map will be empty after this call returns.
       删除所有键值对
     */
    public void clear() {
        super.clear();
        header.before = header.after = header;
    }

    /**
     * LinkedHashMap entry.
       LinkedHashMap的键值对对象，继承了HashMap的属性hash，key，value，next，并且新增属性 before和after，前四个属性是为了维护哈希表，before和after属性是为了维护双向循环链表
     */
    private static class Entry<K,V> extends HashMap.Entry<K,V> {
        // These fields comprise the doubly linked list used for iteration.
        //用来对双向循环链表进行遍历
        Entry<K,V> before, after;

    Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
            super(hash, key, value, next);
        }

        /**
         * Removes this entry from the linked list.
           从双向链表中移除当前节点
         */
        private void remove() {
            before.after = after;
            after.before = before;
        }

        /**
         * Inserts this entry before the specified existing entry in the list.
           在已存在的双向链表中的某个节点之前，插入当前节点
         */
        private void addBefore(Entry<K,V> existingEntry) {
            after  = existingEntry;
            before = existingEntry.before;
            before.after = this;
            after.before = this;
        }

        /**
         * This method is invoked by the superclass whenever the value
         * of a pre-existing entry is read by Map.get or modified by Map.set.
           该方法在父类中通过Map.get获取或者通过Map.set修改已经存在的键值对
         * If the enclosing Map is access-ordered, it moves the entry
         * to the end of the list; otherwise, it does nothing.
           如果封装的Map是访问排序的，该方法会将当前的键值对移到链表的结尾，其他情况，什么也不做
         */
        void recordAccess(HashMap<K,V> m) {
            LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
            if (lm.accessOrder) {
                lm.modCount++;
                remove();
                addBefore(lm.header);
            }
        }
         //从链表中移除当前键值对
        void recordRemoval(HashMap<K,V> m) {
            remove();
        }
    }
    //实现迭代器，不多说
    private abstract class LinkedHashIterator<T> implements Iterator<T> {
    Entry<K,V> nextEntry    = header.after;
    Entry<K,V> lastReturned = null;

    /**
     * The modCount value that the iterator believes that the backing
     * List should have.  If this expectation is violated, the iterator
     * has detected concurrent modification.
     */
    int expectedModCount = modCount;

    public boolean hasNext() {
            return nextEntry != header;
    }

    public void remove() {
        if (lastReturned == null)
        throw new IllegalStateException();
        if (modCount != expectedModCount)
        throw new ConcurrentModificationException();

            LinkedHashMap.this.remove(lastReturned.key);
            lastReturned = null;
            expectedModCount = modCount;
    }

    Entry<K,V> nextEntry() {
        if (modCount != expectedModCount)
        throw new ConcurrentModificationException();
            if (nextEntry == header)
                throw new NoSuchElementException();

            Entry<K,V> e = lastReturned = nextEntry;
            nextEntry = e.after;
            return e;
    }
    }

    private class KeyIterator extends LinkedHashIterator<K> {
    public K next() { return nextEntry().getKey(); }
    }

    private class ValueIterator extends LinkedHashIterator<V> {
    public V next() { return nextEntry().value; }
    }

    private class EntryIterator extends LinkedHashIterator<Map.Entry<K,V>> {
    public Map.Entry<K,V> next() { return nextEntry(); }
    }

    // These Overrides alter the behavior of superclass view iterator() methods
    Iterator<K> newKeyIterator()   { return new KeyIterator();   }
    Iterator<V> newValueIterator() { return new ValueIterator(); }
    Iterator<Map.Entry<K,V>> newEntryIterator() { return new EntryIterator(); }

    /**
     * This override alters behavior of superclass put method. It causes newly
     * allocated entry to get inserted at the end of the linked list and
     * removes the eldest entry if appropriate.
       修改父类的put方法，最近放入的键值对，放在链表的尾部，如果条件允许删除最不经常使用的元素，也就是
       在访问排序的模式下，链表最前面的那一个元素
     */
    void addEntry(int hash, K key, V value, int bucketIndex) {
        createEntry(hash, key, value, bucketIndex);

        // Remove eldest entry if instructed, else grow capacity if appropriate
        //如果被允许，删除最不常用的键值对，否则，就进行扩容
        Entry<K,V> eldest = header.after;
        if (removeEldestEntry(eldest)) {
            removeEntryForKey(eldest.key);
        } else {
            if (size >= threshold)
                resize(2 * table.length);
        }
    }

    /**
     * This override differs from addEntry in that it doesn't resize the
     * table or remove the eldest entry.
       这重写与addEntry，因为不用扩容和删除最不常用的节点
     */
    void createEntry(int hash, K key, V value, int bucketIndex) {
        HashMap.Entry<K,V> old = table[bucketIndex];
    Entry<K,V> e = new Entry<K,V>(hash, key, value, old);
        table[bucketIndex] = e;
        //将新增的键值对放到链表的尾部
        e.addBefore(header);
        size++;
    }

    /**
     * Returns <tt>true</tt> if this map should remove its eldest entry.
     * This method is invoked by <tt>put</tt> and <tt>putAll</tt> after
     * inserting a new entry into the map.  It provides the implementor
     * with the opportunity to remove the eldest entry each time a new one
     * is added.  This is useful if the map represents a cache: it allows
     * the map to reduce memory consumption by deleting stale entries.
     * 该方法为了是先LRU（Least Recently Used）缓存
     * <p>Sample use: this override will allow the map to grow up to 100
     * entries and then delete the eldest entry each time a new entry is
     * added, maintaining a steady state of 100 entries.
     * <pre>
     *     private static final int MAX_ENTRIES = 100;
     *
     *     protected boolean removeEldestEntry(Map.Entry eldest) {
     *        return size() > MAX_ENTRIES;
     *     }
     * </pre>
     *
     * <p>This method typically does not modify the map in any way,
     * instead allowing the map to modify itself as directed by its
     * return value.  It <i>is</i> permitted for this method to modify
     * the map directly, but if it does so, it <i>must</i> return
     * <tt>false</tt> (indicating that the map should not attempt any
     * further modification).  The effects of returning <tt>true</tt>
     * after modifying the map from within this method are unspecified.
     *
     * <p>This implementation merely returns <tt>false</tt> (so that this
     * map acts like a normal map - the eldest element is never removed).
     *
     * @param    eldest The least recently inserted entry in the map, or if
     *           this is an access-ordered map, the least recently accessed
     *           entry.  This is the entry that will be removed it this
     *           method returns <tt>true</tt>.  If the map was empty prior
     *           to the <tt>put</tt> or <tt>putAll</tt> invocation resulting
     *           in this invocation, this will be the entry that was just
     *           inserted; in other words, if the map contains a single
     *           entry, the eldest entry is also the newest.
     * @return   <tt>true</tt> if the eldest entry should be removed
     *           from the map; <tt>false</tt> if it should be retained.
     */
    protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        return false;
    }
}

 参考链接 http://blog.csdn.net/ns_code/article/details/37867985         https://www.cnblogs.com/xiaoxi/p/6170590.html