JDK源码分析-LinkedHashMap

系列文章：

类申明

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

LinkedHashMap集成了HashMap，实现了Map接口

数据结构

LinkedHashMap的结构图如下：对于HashMap，LinkedHashMap多出了一个链表，而链表的顺序是按照数据插入的顺序排列的，所以当我们希望有顺序地去存储key-value时，就需要使用LinkedHashMap了。

LinkedHashMap的Entry源码如下：

    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);
        }
    }

Entry定义了前后指针，确保LinkedHashMap中数据能够双向遍历但是LinkedHashMap并没有重写put方法是直接调用了HashMap的put方法，那如何保证按照插入数据的顺序遍历LinkedHashMap呢？查看HashMap的put方法代码如下：

    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)
            //此处是关键
            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)
                //此处是关键
                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
            else {
                for (int binCount = 0; ; ++binCount) {
                    if ((e = p.next) == null) {
                         //此处是关键
                        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) {
                V oldValue = e.value;
                if (!onlyIfAbsent || oldValue == null)
                    e.value = value;
                
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        
        afterNodeInsertion(evict);
        return null;
    }

LinkedHashMap重写了newNode和newTreeNode方法，而构建顺序插入的链表结构就是通过这两个方法完成的：

   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;
    }

    TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {
        TreeNode<K,V> p = new TreeNode<K,V>(hash, key, value, next);
        linkNodeLast(p);
        return p;
    }

两个方法都调用了linkNodeLast，也就是插入新节点到链表的尾部：

    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;
        }
    }