Java编程WeakHashMap实例解析

2019-11-26 10:19:11

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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