LinkedHashMap

• LinkedHashMap继承自HashMap， 采用数组+双向链表实现。
• LinkedHashMap有序， HashMap有序。 LinkedHashMap的有序分为插入顺序和访问顺序， 默认采用插入顺序。
• LinkedHashMap依然采用HashMap的数组+单向链表存储数据， 双向链表只是用来保证顺序。
• LinkedHashMap并没有重写HashMap的底层数据结构和几个重要方法， 而是采用钩子技术， 在不影响HashMap本身方法的情况下， 维护一个额外的双向链表。
• LinkedHashMap 线程不安全。

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数据结构

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

LinkedHashMap在继承HashMap.Node的基础上， 新增了before和after两个字段，代表双向链表的前后两个指针。在LinkedHashMap新增、删除、更新、访问数据的时候，通过操作该双向链表而保证有序。

• 新增

 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;
    }

如上代码所示， HashMap的putVal方法中， 存在newNode、afterNodeAccess、afterNodeInsertion 3个重要钩子方法。
LinkedHashMap分别重写了上述3个方法：

1. newNode

Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
       LinkedHashMap.Entry<K,V> p =
           new LinkedHashMap.Entry<K,V>(hash, key, value, e);
       linkNodeLast(p);
       return p;
   }

LinkedHashMap重写了newNode方法，每次新增元素时， 调用linkNodeLast方法，将新增元素追加到双向链表的末尾：

private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
        LinkedHashMap.Entry<K,V> last = tail;
        tail = p;
        if (last == null)
            head = p;
        else {
            p.before = last;
            last.after = p;
        }
 }

2. afterNodeAccess

void afterNodeAccess(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;
        }
    }

LinkedHashMap每次更新数据或访问数据时， 调用afterNodeAccess方法，将当前元素从双向链表中先删除， 再追加志链表末尾。afterNodeAccess方法仅在采用访问顺序排序的的情况下被调用。

3. afterNodeInsertion

void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K,V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }

LinkedHashMap不产生作用。

• 删除

final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node<K,V>[] tab; Node<K,V> p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {
            Node<K,V> node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode<K,V>)p).getTreeNode(hash, key);
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

如上代码所示， HashMap的removeNode方法中， 存在afterNodeRemoval钩子方法，LinkedHashMap通过重写该方法， 在删除数据时，更新双向链表：

void afterNodeRemoval(Node<K,V> e) { // unlink
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
        p.before = p.after = null;
        if (b == null)
            head = a;
        else
            b.after = a;
        if (a == null)
            tail = b;
        else
            a.before = b;
    }

• 访问
  LinkedHashMap默认采用插入顺序。采用访问顺序时，通过afterNodeAccess钩子方法，保证有序。

• 遍历

abstract class LinkedHashIterator {
        LinkedHashMap.Entry<K,V> next;
        LinkedHashMap.Entry<K,V> current;
        int expectedModCount;

        LinkedHashIterator() {
            next = head;
            expectedModCount = modCount;
            current = null;
        }

        public final boolean hasNext() {
            return next != null;
        }

        final LinkedHashMap.Entry<K,V> nextNode() {
            LinkedHashMap.Entry<K,V> e = next;
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            current = e;
            next = e.after;
            return e;
        }

        public final void remove() {
            Node<K,V> p = current;
            if (p == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            current = null;
            K key = p.key;
            removeNode(hash(key), key, null, false, false);
            expectedModCount = modCount;
        }
    }

    final class LinkedKeyIterator extends LinkedHashIterator
        implements Iterator<K> {
        public final K next() { return nextNode().getKey(); }
    }

    final class LinkedValueIterator extends LinkedHashIterator
        implements Iterator<V> {
        public final V next() { return nextNode().value; }
    }

    final class LinkedEntryIterator extends LinkedHashIterator
        implements Iterator<Map.Entry<K,V>> {
        public final Map.Entry<K,V> next() { return nextNode(); }
    }

LinkedHashMap遍历时， 通过遍历上述的双向链表， 而不是和HashMap一样，遍历数组+单向链表。

• 扩容

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其他

• LinkedHashMap可实现LRU Cache。