概述

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.

Map接口的hash表+链表实现（有可预测的iterator顺序）；

LinkedHashMap是双向链表；

This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (<i>insertion-order</i>).

定义了iterator顺序，一般是 插入顺序；
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.)

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>).  

提供了一个 LinkedHashMap(int,float,boolean)构造器 创建一个linked hash map

This class provides all of the optional <tt>Map</tt> operations, and permits null elements.  

LinkedHashMap允许null值；

A linked hash map has two parameters that affect its performance:<i>initial capacity</i> and <i>load factor</i>. 

LinkedHashMap有2个影响性能的参数： initial capacity、load factor

Note that this implementation is not synchronized.
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.

If no such object exists, the map should be "wrapped" using the {@link Collections#synchronizedMap Collections.synchronizedMap} method.  

LinkedHashMap是线程非同步的；

如果多个线程并发访问LinkedHashMap，需要在外部同步；

 同步方式：Collections.synchronizedMap；

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}.  

iterator方法是fail-fast：如果在iterator的同时对结构进行修改，将会抛出ConcurrentModificationException；

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

        static class Entry<K,V> extends HashMap.Node<K,V> {
            LinkedHashMap.Entry<K,V> before, after;
            Entry(int hash, K key, V value, Node<K,V> next) {
                super(hash, key, value, next);
            }
        }

        /**
         * The head (eldest) of the doubly linked list.
         */
        transient LinkedHashMap.Entry<K,V> head;
        /**
         * The tail (youngest) of the doubly linked list.
         */
        transient LinkedHashMap.Entry<K,V> tail;

        // The iteration ordering method for this linked hash map: true for access-order, false for insertion-order.
        // 迭代排序：true->access-order、false->insertion-order
        final boolean accessOrder;

        // Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance with the default initial capacity (16) and load factor (0.75).
        public LinkedHashMap() {
            super();
            accessOrder = false;
        }

        public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
            super(initialCapacity, loadFactor);
            this.accessOrder = accessOrder;
        }

    }

LinkedHashMap的扩展

accessOrder

　　构造参数accessOrder为true，get时会将该元素move到队尾；

LinkedHashMap<String, String> linkedHashMap = new LinkedHashMap<>(10, 0.75F,true);
        linkedHashMap.put("a", "a");
        linkedHashMap.put("b", "b");
        linkedHashMap.put("c", "c");
        linkedHashMap.put("d", "d");
        System.out.println(linkedHashMap.toString()); // {a=a, b=b, c=c, d=d}

        linkedHashMap.get("c");

        System.out.println(linkedHashMap.toString());  // {a=a, b=b, d=d, c=c}

        linkedHashMap.entrySet().stream().forEach(e -> System.out.print(" "+e)); // a=a b=b d=d c=c

removeEldestEntry

　　LinkedHashMap每次插入新的元素，都会调用removeEldestEntry方法，如果返回true，移除最老的元素；

static class FIFOCache<K, V> extends LinkedHashMap<K, V>{
        private final int cacheSize;
        public FIFOCache(int cacheSize){
            this.cacheSize = cacheSize;
        }

        // 当Entry个数超过cacheSize时，删除最老的Entry
        @Override
        protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
            return size() > cacheSize;
        }
    }


FIFOCache<String, String> fifoCache = new FIFOCache<>(4);
        fifoCache.put("a", "a");
        fifoCache.put("b", "b");
        fifoCache.put("c", "c");
        fifoCache.put("d", "d");
        fifoCache.put("e", "e");
        System.out.print(fifoCache);        // {b=b, c=c, d=d, e=e}

链路

Put

// java.util.HashMap.put
    public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

    // java.util.HashMap.putVal
    final V putVal(int hash, K key, V value, boolean onlyIfAbsent, boolean evict) {
        Node<K,V>[] tab; Node<K,V> p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
            n = (tab = resize()).length;
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
            Node<K,V> e; K k;
            if (p.hash == hash &&
                    ((k = p.key) == key || (key != null && key.equals(k))))
                e = p;
            else if (p instanceof TreeNode)
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                        p.next = newNode(hash, key, value, null);
                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
                            treeifyBin(tab, hash);
                        break;
                    }
                    if (e.hash == hash &&
                            ((k = e.key) == key || (key != null && key.equals(k))))
                        break;
                    p = e;
                }
            }
            if (e != null) { // existing mapping for key
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

    // java.util.LinkedHashMap.afterNodeInsertion
    void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {                      // removeEldestEntry每次Put都要看下是否需要移除最老的元素
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }

get

// java.util.LinkedHashMap.get
    public V get(Object key) {
        HashMap.Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);                                                                 // 如果accessOrder=true -> 将访问的元素插入尾部
        return e.value;
    }

    // java.util.LinkedHashMap.afterNodeAccess
    void afterNodeAccess(HashMap.Node<K,V> e) { // move node to last
        LinkedHashMap.Entry<K,V> last;
        if (accessOrder && (last = tail) != e) {
            LinkedHashMap.Entry<K,V> p =
                    (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
            p.after = null;
            if (b == null)
                head = a;
            else
                b.after = a;
            if (a != null)
                a.before = b;
            else
                last = b;
            if (last == null)
                head = p;
            else {
                p.before = last;
                last.after = p;
            }
            tail = p;
            ++modCount;
        }
    }