Java Object References in JVM -- Phantom References

虚引用(Phantom References)并不会决定对象的生命周期。如果一个对象仅持有虚引用，那么它就和没有任何引用一样，在任何时候都可能被垃圾回收器回收。

虚引用主要用来跟踪对象被垃圾回收器回收的活动。虚引用与软引用和弱引用的一个区别在于：虚引用必须和引用队列(Reference Queue)联合使用。当垃圾回收器准备回收一个对象时，如果发现它还有虚引用，就会在回收对象的内存之前，把这个虚引用加入到与之关联的引用队列中。

import java.lang.ref.PhantomReference;

import java.lang.ref.Reference;

import java.lang.ref.ReferenceQueue;

import java.util.HashMap;

import java.util.Map;

 

/*

 * A sample for Detecting and locating memory leaks in Java

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

 * Author: doug@neverfear.org

 */

publicclass ClassPhantom {

 

    publicstaticclass Referred {

        // Note that if there is a finalize() method PhantomReference's don't get appended to a ReferenceQueue

      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 phantom references");

 

        // The reference itself will be appended to the dead queue for clean up.

        ReferenceQueue dead = new ReferenceQueue();

 

        // This map is just a sample we might use to locate resources we need to clean up.

        Map<Reference,String> cleanUpMap = new HashMap<Reference,String>();

 

        // This is now a phantom reference.

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

        Referred strong = new Referred();

 

        PhantomReference<Referred> phantom = new PhantomReference(strong, dead);

        cleanUpMap.put(phantom, "You need to clean up some resources, such as me!");

 

        strong = null;

 

        // The object may now be collected

        ClassPhantom.collect();

 

        // Check for

        Reference reference = dead.poll();

        if (reference != null) {

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

            System.out.println("cleanUpMap.size="+cleanUpMap.size());

            System.out.println("reference="+cleanUpMap.remove(reference));

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

        }

        System.out.println("Done");

    }

 

}

Output:

Creating phantom references

Suggesting collection

Sleeping

Good bye cruel world

Done

 

A ReferenceQueue instance can be supplied as an argument to SofyReference. WeakReference or PhantomReference. When an object is collected the reference instance itself will be enqueued to the supplied ReferenceQueue. This allows you to perform clean up operations on the object. This is useful if you are implementing any container classes that you want to contain a Soft, Weak or Phantom reference and some associated data because you get notified via the ReferenceQueue which Reference was just collected.

import java.util.Map;

import java.util.WeakHashMap;

 

/*

 * A sample for Detecting and locating memory leaks in Java

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

 * Author: doug@neverfear.org

 */

publicclass ClassWeakHashMap {

 

    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();

        Map<Referred,String> metadata = new WeakHashMap<Referred,String>();

        metadata.put(strong, "WeakHashMap's make my world go around");

 

        // Attempt to claim a suggested reference.

        ClassWeakHashMap.collect();

        System.out.println("Still has metadata entry? " + (metadata.size() == 1));

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

        // The object may be collected.

        strong = null;

        ClassWeakHashMap.collect();

 

        System.out.println("Still has metadata entry? " + (metadata.size() == 1));

 

        System.out.println("Done");

    }

 

}

Output:

Creating weak references

Suggesting collection

Sleeping

Still has metadata entry? true

Removing reference

Suggesting collection

Sleeping

Good bye cruel world

Still has metadata entry? false

Done