Java WeakHashMap 源码解析

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$ java -version
java version "1.7.0_71"
Java(TM) SE Runtime Environment (build 1.7.0_71-b14)
Java HotSpot(TM) 64-Bit Server VM (build 24.71-b01, mixed mode)

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Object o = new Object();

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    //referent 为引用指向的对象
    Reference(T referent) {
        this(referent, null);
    }
    //ReferenceQueue对象，可以简单理解为一个队列
    //GC 在检测到appropriate reachability changes之后，
    //会把引用对象本身添加到这个queue中，便于清理引用对象本身
    Reference(T referent, ReferenceQueue<? super T> queue) {
        this.referent = referent;
        this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
    }

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Object o = new Object();
SoftReference<Object> sr = new SoftReference<>(o , queue);

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ReferenceQueue<? super T> queue;
Reference next;

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   /**
     * The entries in this hash table extend WeakReference, using its main ref
     * field as the key.
     */
    private static class Entry<K,V> extends WeakReference<Object> implements Map.Entry<K,V> {
        V value;
        int hash;
        Entry<K,V> next;

        /**
         * Creates new entry.
         */
        Entry(Object key, V value,
              ReferenceQueue<Object> queue,
              int hash, Entry<K,V> next) {
            //这里把key传给了父类WeakReference，说明key为弱引用（没有显式的 this.key = key）
            //所有如果key只有通过弱引用访问时，key会被 GC 清理掉
            //同时该key所代表的Entry会进入queue中，等待被处理
            //还可以看到value为强引用（有显式的 this.value = value ），但这并不影响
            //后面可以看到WeakHashMap.expungeStaleEntries方法是如何清理value的
            super(key, queue);
            this.value = value;
            this.hash  = hash;
            this.next  = next;
        }

        @SuppressWarnings("unchecked")
        //在获取key时需要unmaskNull，因为对于null的key，是用WeakHashMap的内部成员属性来表示的
        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();
        }
    }

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    /**
     * Reference queue for cleared WeakEntries
     */
    // 所有Entry在构造时都传入该queue
    private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

    /**
     * Expunges stale entries from the table.
     */
    private void expungeStaleEntries() {
        for (Object x; (x = queue.poll()) != null; ) {
            synchronized (queue) {
                // e 为要清理的对象
                @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 循环遍历冲突链
                while (p != null) {
                    Entry<K,V> next = p.next;
                    if (p == e) {
                        if (prev == e)
                            table[i] = next;
                        else
                            prev.next = next;
                        // Must not null out e.next;
                        // stale entries may be in use by a HashIterator
                        // 可以看到这里把value赋值为null，来帮助 GC 回收强引用的value
                        e.value = null; // Help GC
                        size--;
                        break;
                    }
                    prev = p;
                    p = next;
                }
            }
        }
    }

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import java.util.WeakHashMap;

class KeyHolder {
    @Override
    protected void finalize() throws Throwable {
        System.out.println("I am over from key");
        super.finalize();
    }
}
class ValueHolder {
    @Override
    protected void finalize() throws Throwable {
        System.out.println("I am over from value");
        super.finalize();
    }
}

public class RefTest {
    public static void main(String[] args) {
        WeakHashMap<KeyHolder, ValueHolder> weakMap = new WeakHashMap<KeyHolder, ValueHolder>();

        KeyHolder kh = new KeyHolder();    
        ValueHolder vh = new ValueHolder();

        weakMap.put(kh, vh);

        while (true) {
            for (KeyHolder key : weakMap.keySet()) {
                System.out.println(key + " : " + weakMap.get(key));
            }
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("here...");
            //这里把kh设为null，这样一来就只有弱引用指向kh指向的对象
            kh = null;
            System.gc();
        }
    }
}

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KeyHolder@a15670a : ValueHolder@20e1ed5b
here...
I am over from key   //输出这句话说明，该key对应的Entry已经被 GC 清理
here...
here...
here...
...
...
...