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
2. 
3. public class
4. 
5. /**
6. * @param args
7. */
8. public static void
9. new
10. "Hello, reference";
11. new
12. null;
13. int i = 0;
14. while (weak.get() != null) {
15. "Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i));
16. if (i % 10 == 0) {
17. System.gc();
18. "System.gc() was invoked!");
19. }
20. try
21. 500);
22. catch
23. 
24. }
25. }
26. "object a was cleared by JVM!");
27. }
28. 
29. }

 程序运行结果：

 

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
2. 
3. public class
4. 
5. /**
6. * @param args
7. */
8. public static void
9. new
10. "Hello, reference";
11. new
12. null;
13. int i = 0;
14. while (sr.get() != null) {
15. "Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i));
16. if (i % 10 == 0) {
17. System.gc();
18. "System.gc() was invoked!");
19. }
20. try
21. 500);
22. catch
23. 
24. }
25. }
26. "object a was cleared by JVM!");
27. }
28. 
29. }

 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