Java集合--LinkedHashMap源碼分析

時間 2019-11-17

標籤 java 集合 linkedhashmap 源碼分析欄目 Java 简体版

原文原文鏈接

概述

分析一下 LinkedHashMap 源碼，看下內部結構，研究一下如何保證訪問順序。java

繼承關係

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

LinkedHashMap 繼承自 HashMap，是HashMap的一個子類。node

內部結構

/**
 * 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);
    }
}

private static final long serialVersionUID = 3801124242820219131L;

/**
 * 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: <tt>true</tt>
 * for access-order, <tt>false</tt> for insertion-order.
 *
 * @serial
 */
final boolean accessOrder;

Entry 類繼承自 HashMap.Node，增長了2個變量，before, after。來維護每一個Node前面一個對象和後面一個對象。head, tail 變量維護鏈表頭和尾。accessOrder 爲true表示按訪問順序來遍歷（訪問過的元素放到鏈表的尾部），爲false表示按插入順序遍歷。數組

其實主要的內部結構仍是和 HashMap 同樣的，只不過在HashMap的基礎上每一個Node都加上了先後2個對象的引用。它的大體結構是這樣。網絡

圖片來自網絡app

遍歷

遍歷一個LinkedHashMap，默認按照插入順序來顯示。結合這篇文章 HashIterator 的介紹，看下 LinkedHashMap 是怎麼遍歷的。this

public Set<Map.Entry<K,V>> entrySet() {
    Set<Map.Entry<K,V>> es;
    return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;
}

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

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

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

    ...
}

原理和HashMap差很少，可是 LinkedHashMap 的遍歷（看nextNode方法）是從 head 開始遍歷，直到 Entry 的 after 字段爲空。spa

訪問順序

get 方法.net

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;
}

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;
    }
}

get 方法和 HashMap 的基本同樣，可是多了一個 afterNodeAccess 方法，當 accessOrder 爲 true 的時候執行這個方法，將訪問過的node放到鏈表的最後。code

LinkedHashMap linkedHashMap = new LinkedHashMap(16, 0.75f , true);

也就是像這樣初始化一個 LinkedHashMap 以後，每次訪問過一個對象以後這個對象就被放到了鏈表的最後。orm

put 方法

其實 LinkedHashMap 本身沒有實現 put 方法，仍是調用的是 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;
}

這裏有一個 newNode(hash, key, value, null)，LinkedHashMap 重寫了這個方法

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;
}

// link at the end of list
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;
    }
}

因爲Java多態的特性，put方法實際生成的是 LinkedHashMap.Entry 類，而且將這個node放到了整個鏈表的最後（linkNodeLast）。

remove 方法

LinkedHashMap 也是調用的 HashMap 的 remove 方法

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;
}

出方法前有一個 afterNodeRemoval(node) 方法，LinkedHashMap 重寫了這個方法

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;
}

由於內部有一個鏈表，因此刪除的時候也要維護一下鏈表關係。