Java编程WeakHashMap实例解析

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

以上就是本文关于java编程weakhashmap实例解析的全部内容，希望对大家有所帮助。感兴趣的朋友可以继续参阅本站其他相关专题，如有不足之处，欢迎留言指出。感谢朋友们对本站的支持！