java软引用、弱引用和虚引用

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。

Java代码  

1. import java.lang.ref.*;  
2. import java.util.*;  
3.   
4. class Grocery {  
5.         private static final int SIZE=10000;  
6.         private double[] d = new double[SIZE];  
7.         private String id;  
8.   
9.         public Grocery(String id) {  
10.                 this.id = id;  
11.         }  
12.   
13.         public String toString(){  
14.                 return id;  
15.         }  
16.   
17.         public void finalize(){  
18.                 System.out.println("Finalizing ... "+id);  
19.         }  
20.   
21. }  
22.   
23. public class References {  
24.         private static ReferenceQueue<Grocery> rq = new ReferenceQueue<Grocery>();  
25.   
26.         public static void checkQueue(){  
27.                 Reference<? extends Grocery> inq = rq.poll();  
28.                 if(inq!=null){  
29.                         System.out.println("In queue : "+inq.get());  
30.                 }  
31.         }  
32.   
33.         public static void main(String[] args){  
34.                 final int size = 10;  
35.                 Set<SoftReference<Grocery>> sa = new HashSet<SoftReference<Grocery>>();  
36.   
37.                 for(int i=0;i<size;i++){  
38.                         SoftReference<Grocery> ref =  
39.                                 new SoftReference<Grocery>(new Grocery("Soft "+i),rq);  
40.                         System.out.println("Just created: "+ref.get());  
41.                         sa.add(ref);  
42.                 }  
43.   
44.                 System.gc();  
45.                 checkQueue();  
46.   
47.                 Set<WeakReference<Grocery>> wa = new HashSet<WeakReference<Grocery>>();  
48.   
49.                 for(int i=0;i<size;i++){  
50.                         WeakReference<Grocery> ref =  
51.                                         new WeakReference<Grocery>(new Grocery("Weak "+i),rq);  
52.                         System.out.println("Just created: "+ref.get());  
53.                         wa.add(ref);  
54.                 }  
55.   
56.                 System.gc();  
57.                 checkQueue();  
58.   
59.                 Set<PhantomReference<Grocery>> pa = new HashSet<PhantomReference<Grocery>>();  
60.   
61.                 for(int i=0;i<size;i++) {  
62.                         PhantomReference<Grocery> ref =  
63.                                 new PhantomReference<Grocery>(new Grocery("Phantom "+i),rq);  
64.                         System.out.println("Just created: "+ref.get());  
65.                         pa.add(ref);  
66.                 }  
67.   
68.                 System.gc();  
69.                 checkQueue();  
70.         }  
71. }  

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。