Java中WeakHashMap类型使用示例

在Java集合中有一种特殊的Map类型——WeakHashMap，在这种Map中存放了键对象的弱引用，当一个键对象被垃圾回收器回收时，那么相应的值对象的引用会从Map中删除。WeakHashMap能够节省存储空间，可用来缓存那些非必须存在的数据。


下面程序段测试了垃圾回收器对WeakHashMap中值对象的回收过程，其中能够被3整除的键不会被垃圾回收器回收是由于其引用是强引用。

import java.util.*;
import java.lang.ref.*;


class Key {
        String id;

        public Key(String id) {
                this.id = id;
        }

        public String toString() {
                return id;
        }

        public int hashCode() {
                return id.hashCode();
        }

        public boolean equals(Object r) {
                return (r instanceof Key)
                        && id.equals(((Key)r).id );
        }

        public void finalize() {
                System.out.println("Finalizing Key "+id);
        }
}

class Value {
        String id;

        public Value(String id) {
                this.id = id;
        }

        public String toString() {
                return id;
        }

        public void finalize(){
                System.out.println("Finalizing Value "+id);
        }

}


public class MapCache {
        public static void main(String[] args) throws Exception {
                int size = 1000;

                if(args.length > 0) {
                        size = Integer.parseInt(args[0]);
                }

                Key[] keys = new Key[size];
                WeakHashMap<Key,Value> whm = new WeakHashMap<Key,Value>();

                for(int i=0;i<size;i++) {
                        Key k = new Key(Integer.toString(i));
                        Value v = new Value(Integer.toString(i));
                        if(i%3==0) {
                                keys[i]=k;
                        }

                        whm.put(k,v);
                }

                System.gc();

                Thread.sleep(8000);  //把处理器的时间让给垃圾回收器进行垃圾回收
        }
}

代码运行的结果：
Lab-Computer-0db2f6:JavaExercises labuser$ java MapCache
Finalizing Key 250
Finalizing Key 998
Finalizing Key 997
Finalizing Key 995
Finalizing Key 994
Finalizing Key 992
Finalizing Key 991
Finalizing Key 989
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评论

1 楼 assertivef 2014-03-12  

简单可测试的代码，言简意赅。