LinkedHashMap源碼分析

咱們都知道HashMap是無序的Map，TreeMap是有序的Map。而LinkedHashMap繼承於HashMap，也是一個有序的Map，這彷佛違背了Hash的理論。

咱們來看這麼一個例子

public class LinkedHashMapTest {
    public static void main(String[] args) {
        Map<String,String> test = new HashMap<>();
        test.put("化學","93");
        test.put("數學","98");
        test.put("生物","92");
        test.put("英語","97");
        test.put("物理","94");
        test.put("歷史","96");
        test.put("語文","99");
        test.put("地理","95");
        test.entrySet().stream().forEach(entry -> System.out.println(entry.getKey() + ":" +
                entry.getValue()));
        System.out.println("----------------");
        Map<String,String> test1 = new LinkedHashMap<>();
        test1.put("化學","93");
        test1.put("數學","98");
        test1.put("生物","92");
        test1.put("英語","97");
        test1.put("物理","94");
        test1.put("歷史","96");
        test1.put("語文","99");
        test1.put("地理","95");
        test1.entrySet().stream().forEach(entry -> System.out.println(entry.getKey() + ":" +
                entry.getValue()));

    }
}

運行結果

物理:94
生物:92
歷史:96
化學:93
數學:98
語文:99
英語:97
地理:95
----------------
化學:93
數學:98
生物:92
英語:97
物理:94
歷史:96
語文:99
地理:95

咱們來看一下LinkedHashMap的源碼，首先它是繼承於HashMap

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

而後它的節點是一個雙向鏈表

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);
    }
}

不只節點是雙向鏈表，它自己的數據結構跟HashMap最大的不一樣，HashMap是由一個可擴容的動態數組來構架的，而LinkedHashMap倒是由雙向鏈表來構架主體數據結構的

在HashMap中

transient Node<K,V>[] table;

在LinkedHashMap中

transient LinkedHashMap.Entry<K,V> head;
transient LinkedHashMap.Entry<K,V> tail;

因爲繼承於HashMap，LinkedHashMap沒有重寫put()方法

在HashMap中

public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}

final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //若是表爲空，rehash全表
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
    //若是表的該hash位置爲空，新建節點
    if ((p = tab[i = (n - 1) & hash]) == null)
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //若是Hash衝突，且插入的key與節點key相同，將新節點p賦給e
        if (p.hash == hash &&
            ((k = p.key) == key || (key != null && key.equals(k))))
            e = p;
        //若是p爲紅黑樹節點，將紅黑樹節點p賦給e
        else if (p instanceof TreeNode)
            e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
        //不然即爲鏈表節點，進行相應處理
        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;
}

咱們在putVal()方法中能夠看到有一個newNode()的方法，則LinkedHashMap重寫了該方法newNode()

在LinkedHashMap中

Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
    //新增節點
    LinkedHashMap.Entry<K,V> p =
        new LinkedHashMap.Entry<K,V>(hash, key, value, e);
    //將新節點插入到雙向鏈表的尾部
    linkNodeLast(p);
    return p;
}

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;
    }
}

咱們在HashMap的put()方法中看到有兩個方法afterNodeAccess(e)，afterNodeInsertion(evict);

void afterNodeAccess(Node<K,V> p) { }
void afterNodeInsertion(boolean evict) { }

它們在HashMap中的方法體爲空。

而在LinkedHashMap中也重寫了這兩個方法

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;
    }
}

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);
    }
}

protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
    return false;
}