Java中软引用、弱引用和虚引用的使用方法示例
在Java引用中，使用最多的无疑是强引用类型，对于其余三种级别的引用类型用得比较少。它们使用的方式与强引用也有些区别。下面使用一个例子来介绍一下其余三种引用（包括软引用、弱引用和虚引用）的使用方法。

import java.lang.ref.*;import java.util.*;class Grocery {        private static final int SIZE=10000;        private double[] d = new double[SIZE];        private String id;        public Grocery(String id) {                this.id = id;        }        public String toString(){                return id;        }        public void finalize(){                System.out.println("Finalizing ... "+id);        }}public class References {        private static ReferenceQueue<Grocery> rq = new ReferenceQueue<Grocery>();        public static void checkQueue(){                Reference<? extends Grocery> inq = rq.poll();                if(inq!=null){                        System.out.println("In queue : "+inq.get());                }        }        public static void main(String[] args){                final int size = 10;                Set<SoftReference<Grocery>> sa = new HashSet<SoftReference<Grocery>>();                for(int i=0;i<size;i++){                        SoftReference<Grocery> ref =                                new SoftReference<Grocery>(new Grocery("Soft "+i),rq);                        System.out.println("Just created: "+ref.get());                        sa.add(ref);                }                System.gc();                checkQueue();                Set<WeakReference<Grocery>> wa = new HashSet<WeakReference<Grocery>>();                for(int i=0;i<size;i++){                        WeakReference<Grocery> ref =                                        new WeakReference<Grocery>(new Grocery("Weak "+i),rq);                        System.out.println("Just created: "+ref.get());                        wa.add(ref);                }                System.gc();                checkQueue();                Set<PhantomReference<Grocery>> pa = new HashSet<PhantomReference<Grocery>>();                for(int i=0;i<size;i++) {                        PhantomReference<Grocery> ref =                                new PhantomReference<Grocery>(new Grocery("Phantom "+i),rq);                        System.out.println("Just created: "+ref.get());                        pa.add(ref);                }                System.gc();                checkQueue();        }}

在上面的程序中，References类依次创建了10个软引用、10个弱引用和10个虚引用，它们各自引用一个Grocery对象。下面是运行结果：

Lab-Computer-0db2f6:JavaExercises labuser$ javac References.java
Lab-Computer-0db2f6:JavaExercises labuser$ java References
Just created: Soft 0
Just created: Soft 1
Just created: Soft 2
Just created: Soft 3
Just created: Soft 4
Just created: Soft 5
Just created: Soft 6
Just created: Soft 7
Just created: Soft 8
Just created: Soft 9
Just created: Weak 0
Just created: Weak 1
Just created: Weak 2
Just created: Weak 3
Just created: Weak 4
Just created: Weak 5
Just created: Weak 6
Just created: Weak 7
Just created: Weak 8
Just created: Weak 9
Just created: null
Just created: null
Just created: null
Just created: null
Just created: null
Just created: null
Just created: null
Just created: null
Just created: null
Just created: null
Finalizing ... Weak 9
Finalizing ... Weak 8
Finalizing ... Weak 7
Finalizing ... Weak 6
Finalizing ... Weak 5
Finalizing ... Weak 4
Finalizing ... Weak 3
Finalizing ... Weak 2
Finalizing ... Weak 1
Finalizing ... Weak 0
Finalizing ... Phantom 9
Finalizing ... Phantom 8
Finalizing ... Phantom 7
Finalizing ... Phantom 6
Finalizing ... Phantom 5
Finalizing ... Phantom 4
Finalizing ... Phantom 3
Finalizing ... Phantom 2
Finalizing ... Phantom 1
Finalizing ... Phantom 0


从程序运行结果可以看出，虚引用形同虚设，它所引用的对象随时可能被垃圾回收器回收，具有弱引用的对象拥有稍微长一点的生命周期，当垃圾回收器执行回收操作时，有可能被垃圾回收器回收，具有软引用的对象拥有更长的生命周期，但在Java虚拟机认为内存不足的情况下，也是会被垃圾回收器回收的。