利用ReferenceQueue兑现自动清理的cache

利用ReferenceQueue实现自动清理的cache
理解StrongReference，SoftReference， WeakReference的区别（http://san-yun.iteye.com/blog/1683558)文中解释了各种Reference。 Reference都有构造函数可以传入ReferenceQueue来监听GC对referent的处理,构造函数如下：

Reference(T referent, ReferenceQueue<? super T> queue) {this.referent = referent;this.queue = (queue == null) ? ReferenceQueue.NULL : queue;    }

下面是一段测试代码：

public class A {}ReferenceQueue queue = new ReferenceQueue();WeakReference ref = new WeakReference(new A(), queue);Assert.assertNotNull(ref.get());Object obj = null;obj = queue.poll();Assert.assertNull(obj);System.gc();Assert.assertNull(ref.get());obj = queue.poll();Assert.assertNotNull(obj);

分析，在GC运行时，检测到new A()生成的对象只有一个WeakReference引用着，所以决定回收它，首先clear WeakReference的referent，然后referent的状态为finalizable，同时或者一段时间后把WeakReference放入监听的ReferenceQueue中。

注意有时候最后的Assert.assertNotNull(obj);有时会失败，因为还没有来的及把WeakReference放入监听的ReferenceQueue中。


下面是一个可自动回收的map实现：

import java.lang.ref.ReferenceQueue;import java.lang.ref.WeakReference;import java.util.HashMap;import java.util.Map;public class WeakIdentityMap {private transient ReferenceQueue<Object> idKeys = new ReferenceQueue<Object>();private Map<WeakIdKey, Object> objHashcodeToPyId = new HashMap<WeakIdKey, Object>();@SuppressWarnings("element-type-mismatch")private void cleanup() {Object k;while ((k = idKeys.poll()) != null) {System.out.println("cleanup");objHashcodeToPyId.remove(k);}}private class WeakIdKey extends WeakReference<Object> {private final int hashcode;WeakIdKey(Object obj) {super(obj, idKeys);hashcode = System.identityHashCode(obj);}@Overridepublic int hashCode() {return hashcode;}@Overridepublic boolean equals(Object other) {Object obj = get();if (obj != null) {return obj == ((WeakIdKey) other).get();} else {return this == other;}}}// Used by test_jy_internalspublic int _internal_map_size() {return objHashcodeToPyId.size();}public void put(Object key, Object val) {cleanup();objHashcodeToPyId.put(new WeakIdKey(key), val);}public Object get(Object key) {cleanup();return objHashcodeToPyId.get(new WeakIdKey(key));}public void remove(Object key) {cleanup();objHashcodeToPyId.remove(new WeakIdKey(key));}public static void main(String[] args) throws InterruptedException {WeakIdentityMap map = new WeakIdentityMap();map.put("1", "a");map.put(new Object(), "b");map.put(new Object(), "2");System.gc();Thread.sleep(1000);map.put("1", "a");}}

参考：http://zhang-xzhi-xjtu.iteye.com/blog/413159

查看更多 下一篇