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