Java LinkedHashMap 源码分析

Java

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Demo

@Test
public void testLinkedHashMap() {
    Map<String, Integer> map = new LinkedHashMap<String, Integer>() {
        @Override
        protected boolean removeEldestEntry(Map.Entry<String, Integer> eldest) {
            return size() > 3;
        }
    };
    map.put("1", 1);
    map.put("2", 2);
    map.put("3", 3);
    map.put("4", 4);
    for(Map.Entry<String, Integer> entry : map.entrySet()) {
        System.out.println(entry.getKey() + ": " + entry.getValue());
    }
}

输出

2: 2
3: 3
4: 4

迭代输出能够保持插入顺序。

分析

内部结构

LinkedHashMap继承自HashMap，内部额外维护了一个Entry的双向链表，用于记录访问和插入顺序。官方注释：

Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap 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 (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map.

需要注意：如果某个key已经存在，再次put不会改变插入的顺序。

LinkedHashMap.Entry

LinkedHashMap.Entry只是比HashMap.Node多了两个指针而已，LinkedHashMap.Entry直接就是双向链表的元素了。

/**
 * HashMap.Node subclass for normal LinkedHashMap entries.
 */
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);
    }
}
/**
 * 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;

重要函数

HashMap中定义的3个函数：

// Callbacks to allow LinkedHashMap post-actions
void afterNodeAccess(Node<K,V> p) { }
void afterNodeInsertion(boolean evict) { }
void afterNodeRemoval(Node<K,V> p) { }

LinkedHashMap继承于HashMap，重写了这3个函数。这3个函数分别为在访问节点、插入节点、删除节点时做一些事情。

在对LinkedHashMap进行put操作时，执行的是HashMap的put方法，多态调用LinkedHashMap的上述3个函数。

HashMap 的 put 方法

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;
}

LinkedHashMap 的 get 方法

public V get(Object key) {
    Node<K,V> e;
    if ((e = getNode(hash(key), key)) == null)
        return null;
    if (accessOrder)
        afterNodeAccess(e);
    return e.value;
}

可以看到其核心为这3个函数。

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;
    }
}

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);
    }
}

afterNodeRemoval

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;
}

LinkedHashMap 和 HashMap 对比

LinkedHashMap 的应用场景？

• 需要记录插入顺序、访问顺序时
• 可以做缓存使用，因为可以重写removeEldestEntry，使map保持在一定的容量

LinkedHashMap 的 put 操作时间复杂度？

时间复杂度应该是o(n)，其中n为容量。因为put时在找到对应的value后，需要维护双向链表。

现在有一个疑问，LinkedHashMap存在有什么意义？既然要维护一个双向链表，就不可能做到HashMap o(1)的时间复杂度。LinkedHashMap和直接使用双向链表有什么区别？