一、引言
Java中的引用类型由四种情况，强引用、软引用、弱引用、虚引用。关于这些的介绍可以参见鄙人另外一篇博文。 http://www.cnblogs.com/plxx/p/4217178.html
二、概述
WeakHashMap，在家族中和HashMap是同辈的。
public class WeakHashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
针对WeakHashMap --- An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use.当key不再使用的时候，就会被自动清除。不再使用是个什么状态？？？当GC root 枚举不到的时候，就是认为此时可以被GC（finalizable -->finalized -->reclaimed 参阅）。
同HashMap一样，WeakHashMap允许<key,value>都为null，初始大小为16，load factor为0.75，不是线程安全的。
Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key
in a WeakHashMap at some later time and be surprised that its entry has been removed.
一旦Key被清除，就不可以再被创建，查询，稍后entry就会被移除。
Each key object in a WeakHashMap is stored indirectly as the referent of a weak reference.
value objects do not strongly refer to their own keys, either directly or indirectly,
since that will prevent the keys from being discarded
其中的key存放的是weak reference.而value中存放的是strong reference,value不知直接或者间接地和key发生关联，否则会阻止key的清除。
三、查看源码
private void expungeStaleEntries() {
for (Object x; (x = queue.poll()) != null; ) {
synchronized (queue) { //将队列加锁控制
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) x;
int i = indexFor(e.hash, table.length);
Entry<K,V> prev = table[i]; //找到实体所在的桶号
Entry<K,V> p = prev;
//找到这个实体
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
e.value = null; // Help GC
size--;
break;
}
//移除该实体--不可恢复
prev = p;
p = next;
}
}
}
}
expunge - Stale - Entries 擦除 陈旧 实体
在resize(),size()，getTables()的相关操作中都会调用该方法。将不使用的对象统统擦除。