简述：

《Thinking in Java》第4版 P519 页 WeakHashMap一章读书笔记

WeakHashMap 用来保存WeakReference，这一结构云逊垃圾回收器自动清理键和值

在添加键和值的操作时，映射会自动使用WeakReference包装它们，

见jdk源代码，

public V put(K key, V value) { Object k = maskNull(key); int h = hash(k); Entry<K,V>[] tab = getTable(); 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;}

其中new Entry<>(k, value, queue, h, e)一行使用了ReferenceQueue

/** * Reference queue for cleared WeakEntries */ private final ReferenceQueue<Object> queue = new ReferenceQueue<>();

点入new Entry的构造函数，进入super顶层可以看到，

/** * Creates a new weak reference that refers to the given object and is * registered with the given queue. * * @param referent object the new weak reference will refer to * @param q the queue with which the reference is to be registered, * or <tt>null</tt> if registration is not required */ public WeakReference(T referent, ReferenceQueue<? super T> q) { super(referent, q); }

这里new Entry同时也构造出来了一个WeakRefence对象

测试:

package com.anialy.test.data_structure.map;import java.util.Iterator;import java.util.WeakHashMap;public class WeakHashMapTest { public static void main(String[] args) { WeakHashMap wmap = new WeakHashMap<String, Object>(); final int SIZE = 10; String[] str = new String[SIZE]; for (int i=0; i<SIZE; i++){ String key = Integer.toString(i); String value = Integer.toString(i); // 每隔3个保留一个引用 if(i % 3 == 0) str[i] = key; wmap.put(key, value); } System.gc(); Iterator iter = wmap.keySet().iterator(); while(iter.hasNext()){ System.out.println(wmap.get(iter.next())); } }}

可以预料到，部分由于String[] 保留了弱引用，所以输出都是间隔3的

总结

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