Java Object References in JVM -- Weak References

弱引用(Weak References)只具有弱引用的对象拥有更短暂的生命周期。在垃圾回收器线程扫描它所管辖的内存区域的过程中，一旦发现了只具有弱引用的对象，不管当前内存空间足够与否，一概回收。不过，由于垃圾回收期是一个优先级很低的线程，所以不一定能执行。

弱引用可以和一个引用队列(Reference Queue)联合使用，如果弱引用所引用的对象被垃圾回收，JVM就会把这个弱引用加入到与之关联的引用队列中。

import java.lang.ref.WeakReference;

import java.util.ArrayList;

import java.util.List;

 

/*

 * A sample for Detecting and locating memory leaks in Java

 * URL: http://neverfear.org/blog/view/150/Java_References

 * Author: doug@neverfear.org

 */

publicclass ClassWeak {

 

    publicstaticclass Referred {

        protectedvoid finalize() {

            System.out.println("Good bye cruel world");

        }

    }

 

    publicstaticvoid collect() throws InterruptedException {

        System.out.println("Suggesting collection");

        System.gc();

        System.out.println("Sleeping");

        Thread.sleep(5000);

    }

 

    publicstaticvoid main(String args[]) throws InterruptedException {

        System.out.println("Creating weak references");

 

        // This is now a weak reference.

        // The object will be collected only if no strong references.

        Referred strong = new Referred();

        WeakReference<Referred> weak = new WeakReference<Referred>(strong);

        System.out.println("weak="+weak);

        // Attempt to claim a suggested reference.

        ClassWeak.collect();

        System.out.println("weak="+weak);

        System.out.println("Removing reference");

        // The object may be collected.

        strong = null;

        weak = null;

        System.out.println("weak="+weak);

        ClassWeak.collect();

        System.out.println("weak="+weak);

        System.out.println("Done");

    }

 

}

Output:

Creating weak references

weak=java.lang.ref.WeakReference@a90653

Suggesting collection

Sleeping

weak=java.lang.ref.WeakReference@a90653

Removing reference

weak=null

Suggesting collection

Sleeping

Good bye cruel world

weak=null

Done