WeakReference与SoftReference

WeakReference与SoftReference都可以用来保存对象的实例引用，这两个类与垃圾回收有关。

WeakReference是弱引用，其中保存的对象实例可以被GC回收掉。这个类通常用于在某处保存对象引用，而又不干扰该对象被GC回收，通常用于Debug、内存监视工具等程序中。因为这类程序一般要求即要观察到对象，又不能影响该对象正常的GC过程。

最近在JDK的Proxy类的实现代码中也发现了Weakrefrence的应用，Proxy会把动态生成的Class实例暂存于一个由Weakrefrence构成的Map中作为Cache。

SoftReference是强引用，它保存的对象实例，除非 JVM即将OutOfMemory，否则不会被GC回收。这个特性使得它特别适合设计对象Cache。对于Cache，我们希望被缓存的对象最好始终常驻内存，但是如果JVM内存吃紧，为了不发生OutOfMemoryError导致系统崩溃，必要的时候也允许JVM回收Cache的内存，待后续合适的时机再把数据重新Load到Cache中。这样可以系统设计得更具弹性。

WeakReference的一个测试程序：

Java代码

1. import java.lang.ref.WeakReference;  
2. public class WeakReferenceTest {  
3. /**
4.      * @param args
5.      */
6. public static void main(String[] args) {  
7.         A a = new A();  
8.         a.str = "Hello, reference";  
9.         WeakReference<A> weak = new WeakReference<A>(a);  
10.         a = null;  
11. int i = 0;  
12. while (weak.get() != null) {  
13.             System.out.println(String.format("Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i));  
14. if (i % 10 == 0) {  
15.                 System.gc();  
16.                 System.out.println("System.gc() was invoked!");  
17.             }  
18. try {  
19.                 Thread.sleep(500);  
20.             } catch (InterruptedException e) {  
21.             }  
22.         }  
23.         System.out.println("object a was cleared by JVM!");  
24.     }  
25. } 

Java代码 

1. import java.lang.ref.WeakReference; 
2. public class WeakReferenceTest { 
3. /**
4.      * @param args
5.      */
6. public static void main(String[] args) { 
7.         A a = new A(); 
8.         a.str = "Hello, reference"; 
9.         WeakReference<A> weak = new WeakReference<A>(a); 
10.         a = null; 
11. int i = 0; 
12. while (weak.get() != null) { 
13.             System.out.println(String.format("Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i)); 
14. if (i % 10 == 0) { 
15.                 System.gc(); 
16.                 System.out.println("System.gc() was invoked!"); 
17.             } 
18. try { 
19.                 Thread.sleep(500); 
20.             } catch (InterruptedException e) { 
21.             } 
22.         } 
23.         System.out.println("object a was cleared by JVM!"); 
24.     } 
25. } 

程序运行结果：

Get str from object of WeakReference: Hello, reference, count: 1

1. Get str from object of WeakReference: Hello, reference, count: 2
2. Get str from object of WeakReference: Hello, reference, count: 3
3. Get str from object of WeakReference: Hello, reference, count: 4
4. Get str from object of WeakReference: Hello, reference, count: 5
5. Get str from object of WeakReference: Hello, reference, count: 6
6. Get str from object of WeakReference: Hello, reference, count: 7
7. Get str from object of WeakReference: Hello, reference, count: 8
8. Get str from object of WeakReference: Hello, reference, count: 9
9. Get str from object of WeakReference: Hello, reference, count: 10
10. System.gc() was invoked!  
11. object a was cleared by JVM! 

Java代码 

1. Get str from object of WeakReference: Hello, reference, count: 1
2. Get str from object of WeakReference: Hello, reference, count: 2
3. Get str from object of WeakReference: Hello, reference, count: 3
4. Get str from object of WeakReference: Hello, reference, count: 4
5. Get str from object of WeakReference: Hello, reference, count: 5
6. Get str from object of WeakReference: Hello, reference, count: 6
7. Get str from object of WeakReference: Hello, reference, count: 7
8. Get str from object of WeakReference: Hello, reference, count: 8
9. Get str from object of WeakReference: Hello, reference, count: 9
10. Get str from object of WeakReference: Hello, reference, count: 10
11. System.gc() was invoked! 
12. object a was cleared by JVM! 

SoftReference的一个测试程序：

Java代码

1. import java.lang.ref.SoftReference;  
2. public class SoftReferenceTest {  
3. /**
4.      * @param args
5.      */
6. public static void main(String[] args) {  
7.         A a = new A();  
8.         a.str = "Hello, reference";  
9.         SoftReference<A> sr = new SoftReference<A>(a);  
10.         a = null;  
11. int i = 0;  
12. while (sr.get() != null) {  
13.             System.out.println(String.format("Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i));  
14. if (i % 10 == 0) {  
15.                 System.gc();  
16.                 System.out.println("System.gc() was invoked!");  
17.             }  
18. try {  
19.                 Thread.sleep(500);  
20.             } catch (InterruptedException e) {  
21.             }  
22.         }  
23.         System.out.println("object a was cleared by JVM!");  
24.     }  
25. } 

Java代码 

1. import java.lang.ref.SoftReference; 
2. public class SoftReferenceTest { 
3. /**
4.      * @param args
5.      */
6. public static void main(String[] args) { 
7.         A a = new A(); 
8.         a.str = "Hello, reference"; 
9.         SoftReference<A> sr = new SoftReference<A>(a); 
10.         a = null; 
11. int i = 0; 
12. while (sr.get() != null) { 
13.             System.out.println(String.format("Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i)); 
14. if (i % 10 == 0) { 
15.                 System.gc(); 
16.                 System.out.println("System.gc() was invoked!"); 
17.             } 
18. try { 
19.                 Thread.sleep(500); 
20.             } catch (InterruptedException e) { 
21.             } 
22.         } 
23.         System.out.println("object a was cleared by JVM!"); 
24.     } 
25. } 

程序运行结果：

Java代码

1. Get str from object of SoftReference: Hello, reference, count: 1
2. Get str from object of SoftReference: Hello, reference, count: 2
3. Get str from object of SoftReference: Hello, reference, count: 3
4. Get str from object of SoftReference: Hello, reference, count: 4
5. Get str from object of SoftReference: Hello, reference, count: 5
6. Get str from object of SoftReference: Hello, reference, count: 6
7. Get str from object of SoftReference: Hello, reference, count: 7
8. Get str from object of SoftReference: Hello, reference, count: 8
9. Get str from object of SoftReference: Hello, reference, count: 9
10. Get str from object of SoftReference: Hello, reference, count: 10
11. System.gc() was invoked!  
12. Get str from object of SoftReference: Hello, reference, count: 11
13. Get str from object of SoftReference: Hello, reference, count: 12
14. Get str from object of SoftReference: Hello, reference, count: 13
15. Get str from object of SoftReference: Hello, reference, count: 14
16. Get str from object of SoftReference: Hello, reference, count: 15
17. Get str from object of SoftReference: Hello, reference, count: 16
18. Get str from object of SoftReference: Hello, reference, count: 17
19. Get str from object of SoftReference: Hello, reference, count: 18
20. Get str from object of SoftReference: Hello, reference, count: 19
21. Get str from object of SoftReference: Hello, reference, count: 20
22. System.gc() was invoked!  
23. Get str from object of SoftReference: Hello, reference, count: 21
24. Get str from object of SoftReference: Hello, reference, count: 22
25. Get str from object of SoftReference: Hello, reference, count: 23
26. Get str from object of SoftReference: Hello, reference, count: 24
27. Get str from object of SoftReference: Hello, reference, count: 25
28. Get str from object of SoftReference: Hello, reference, count: 26
29. Get str from object of SoftReference: Hello, reference, count: 27
30. Get str from object of SoftReference: Hello, reference, count: 28

自己的标注 写道

上面的打印结果会一直持续下去。 因为soft.get()一直不会为空
SoftReference 比WeakReference生命力更强，当JVM的内存不吃紧时，即使引用的对象被置为空了，Soft还可以保留对该对象的引用，此时的JVM内存池实际上还保有原来对象，只有当内存吃紧的情况下JVM才会清除Soft的引用对象，并且会在未来重新加载该引用的对象。
而WeakReference则当清理内存池时会自动清理掉引用的对象。