Java容器解析系列(13) WeakHashMap详解

关于WeakHashMap其实没有太多可说的,其与HashMap大致相同,区别就在于:

1. 对每个key的引用方式为弱引用;

   关于java4种引用方式,参考java Reference

   网上很多说 弱引用指向 Entry,这种说法是完全错误的

2. 在key被回收时,对应的value并没有回收,只有在调用WeakHashMap的方法时才会回收value;

具体请看下列源码解析:

/**
 * 数据结构原理几乎与HashMap一致;
 * WeakHashMap 不会阻止Entry.key被回收;
 * WeakHashMap中的每个key都是通过weakreference引用;
 * 
 * expuntgeStaleEntries():清除 key已被回收 的entry,进而entry.value也被回收
 * WeakHashMap中所有的public的增删改查方法都直接或间接调用了expuntgeStaleEntries()方法;
 *
 * @since 1.2
 */
public class WeakHashMap<K, V> extends AbstractMap<K, V> implements Map<K, V> {

    private static final int DEFAULT_INITIAL_CAPACITY = 16;
    private static final int MAXIMUM_CAPACITY = 1 << 30;
    private static final float DEFAULT_LOAD_FACTOR = 0.75f;
    Entry<K, V>[] table;
    private int size;
    private int threshold;
    private final float loadFactor;

    // 当key被回收时,其Entry(继承自WeakReference)对象会被添加到该队列中
    private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
    int modCount;
    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;
    transient boolean useAltHashing;
    transient final int hashSeed = sun.misc.Hashing.randomHashSeed(this);

    // 各种构造器,与HashMap没有太大区别,hash算法相关代码,省略......

    // 清除那些key已经被回收的entry
    // WeakHashMap 中所有的public的增删改查方法都直接或间接调用了expuntgeStaleEntries()方法
    // 如果在添加元素到 WeakHashMap 后,如果不再调用任何该 WeakHashMap 的方法,
    // 那么 Entry 不会被回收,进而 Entry.value 不会被回收
    private void expungeStaleEntries() {
        // weakreference被回收时,会被添加到其注册的referencequeue中
        for (Object x; (x = queue.poll()) != null;) {
            synchronized (queue) {
                @SuppressWarnings("unchecked")
                Entry<K, V> e = (Entry<K, V>) x;
                int i = indexFor(e.hash, table.length);

                Entry<K, V> prev = table[i];
                Entry<K, V> p = prev;
                while (p != null) {
                    Entry<K, V> next = p.next;
                    if (p == e) {
                        if (prev == e)
                            table[i] = next;
                        else
                            prev.next = next;
                        // 既然key被清除了,这里把value也清除掉(帮助GC)
                        // 其实这里也可以不清除value,因为这里将entry从链表中移除了,等于entry被删除了,虚拟机在之后的某个时间会将entry回收掉
                        e.value = null; // Help GC
                        size--;
                        break;
                    }
                    prev = p;
                    p = next;
                }
            }
        }
    }

    private Entry<K, V>[] getTable() {
        expungeStaleEntries();
        return table;
    }

    public int size() {
        if (size == 0)
            return 0;
        expungeStaleEntries();
        return size;
    }

    public V get(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K, V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K, V> e = tab[index];
        while (e != null) {
            if (e.hash == h && eq(k, e.get()))
                return e.value;
            e = e.next;
        }
        return null;
    }

    public boolean containsKey(Object key) {
        return getEntry(key) != null;
    }

    Entry<K, V> getEntry(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K, V>[] tab = getTable();
        int index = indexFor(h, tab.length);
        Entry<K, V> e = tab[index];
        while (e != null && !(e.hash == h && eq(k, e.get())))
            e = e.next;
        return e;
    }

    public V put(K key, V value) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K, V>[] tab = getTable();// getTable() 调用了 expungeStaleEntries()
        int i = indexFor(h, tab.length);

        for (Entry<K, V> e = tab[i]; e != null; e = e.next) {
            if (h == e.hash && eq(k, e.get())) {
                V oldValue = e.value;
                if (value != oldValue)
                    e.value = value;
                return oldValue;
            }
        }
        modCount++;
        Entry<K, V> e = tab[i];
        tab[i] = new Entry<>(k, value, queue, h, e);
        if (++size >= threshold)
            resize(tab.length * 2);
        return null;
    }

    // 与HashMap极度相似的代码,省略......

    public V remove(Object key) {
        Object k = maskNull(key);
        int h = hash(k);
        Entry<K, V>[] tab = getTable();
        int i = indexFor(h, tab.length);
        Entry<K, V> prev = tab[i];
        Entry<K, V> e = prev;

        while (e != null) {
            Entry<K, V> next = e.next;
            if (h == e.hash && eq(k, e.get())) {
                modCount++;
                size--;
                if (prev == e)
                    tab[i] = next;
                else
                    prev.next = next;
                return e.value;
            }
            prev = e;
            e = next;
        }

        return null;
    }

    public void clear() {
        // 清空队列中的所有weakreference
        while (queue.poll() != null)
            ;

        modCount++;
        // 清空所有Entry
        Arrays.fill(table, null);
        size = 0;

        while (queue.poll() != null)
            ;
    }

    // Entry为WeakReference的子类,作为弱引用指向key
    private static class Entry<K, V> extends WeakReference<Object> implements Map.Entry<K, V> {
        V value;
        int hash;
        Entry<K, V> next;

        Entry(Object key, V value, ReferenceQueue<Object> queue, int hash, Entry<K, V> next) {
            super(key, queue);// 这里就确定了该 WeakReference 指向 key (referent 变量值为 key)
            this.value = value;// 对 value 的引用为强引用,所以必须手动回收(通过 expungeStaleEntries())
            this.hash = hash;
            this.next = next;
        }

        @SuppressWarnings("unchecked")
        public K getKey() {
            return (K) WeakHashMap.unmaskNull(get());
        }

        public V getValue() {
            return value;
        }

        public V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        public boolean equals(Object o) {
            if (!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            K k1 = getKey();
            Object k2 = e.getKey();
            if (k1 == k2 || (k1 != null && k1.equals(k2))) {
                V v1 = getValue();
                Object v2 = e.getValue();
                if (v1 == v2 || (v1 != null && v1.equals(v2)))
                    return true;
            }
            return false;
        }

        public int hashCode() {
            K k = getKey();
            V v = getValue();
            return ((k == null ? 0 : k.hashCode()) ^ (v == null ? 0 : v.hashCode()));
        }

        public String toString() {
            return getKey() + "=" + getValue();
        }
    }

    // 各种迭代器,遍历方式,省略......

}