《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
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