WeakReference与SoftReference
装载自：http://flyneil.iteye.com/blog/1345177
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
- import java.lang.ref.WeakReference;
- public class WeakReferenceTest {
- /**
- * @param args
- */
- public static void main(String[] args) {
- A a = new A();
- a.str = "Hello, reference";
- WeakReference weak = new WeakReference(a);
- a = null;
- int i = 0;
- while (weak.get() != null) {
- System.out.println(String.format("Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i));
- if (i % 10 == 0) {
- System.gc();
- System.out.println("System.gc() was invoked!");
- }
- try {
- Thread.sleep(500);
- } catch (InterruptedException e) {
- }
- }
- System.out.println("object a was cleared by JVM!");
- }
- }
import java.lang.ref.WeakReference;
public class WeakReferenceTest {
/**
* @param args
*/
public static void main(String[] args) {
A a = new A();
a.str = "Hello, reference";
WeakReference weak = new WeakReference(a);
a = null;
int i = 0;
while (weak.get() != null) {
System.out.println(String.format("Get str from object of WeakReference: %s, count: %d", weak.get().str, ++i));
if (i % 10 == 0) {
System.gc();
System.out.println("System.gc() was invoked!");
}
try {
Thread.sleep(500);
} catch (InterruptedException e) {
}
}
System.out.println("object a was cleared by JVM!");
}
}
程序运行结果：
Java代码
- Get str from object of WeakReference: Hello, reference, count: 1
- Get str from object of WeakReference: Hello, reference, count: 2
- Get str from object of WeakReference: Hello, reference, count: 3
- Get str from object of WeakReference: Hello, reference, count: 4
- Get str from object of WeakReference: Hello, reference, count: 5
- Get str from object of WeakReference: Hello, reference, count: 6
- Get str from object of WeakReference: Hello, reference, count: 7
- Get str from object of WeakReference: Hello, reference, count: 8
- Get str from object of WeakReference: Hello, reference, count: 9
- Get str from object of WeakReference: Hello, reference, count: 10
- System.gc() was invoked!
- object a was cleared by JVM!
Get str from object of WeakReference: Hello, reference, count: 1
Get str from object of WeakReference: Hello, reference, count: 2
Get str from object of WeakReference: Hello, reference, count: 3
Get str from object of WeakReference: Hello, reference, count: 4
Get str from object of WeakReference: Hello, reference, count: 5
Get str from object of WeakReference: Hello, reference, count: 6
Get str from object of WeakReference: Hello, reference, count: 7
Get str from object of WeakReference: Hello, reference, count: 8
Get str from object of WeakReference: Hello, reference, count: 9
Get str from object of WeakReference: Hello, reference, count: 10
System.gc() was invoked!
object a was cleared by JVM!
SoftReference的一个测试程序：
Java代码
- import java.lang.ref.SoftReference;
- public class SoftReferenceTest {
- /**
- * @param args
- */
- public static void main(String[] args) {
- A a = new A();
- a.str = "Hello, reference";
- SoftReference sr = new SoftReference(a);
- a = null;
- int i = 0;
- while (sr.get() != null) {
- System.out.println(String.format("Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i));
- if (i % 10 == 0) {
- System.gc();
- System.out.println("System.gc() was invoked!");
- }
- try {
- Thread.sleep(500);
- } catch (InterruptedException e) {
- }
- }
- System.out.println("object a was cleared by JVM!");
- }
- }
import java.lang.ref.SoftReference;
public class SoftReferenceTest {
/**
* @param args
*/
public static void main(String[] args) {
A a = new A();
a.str = "Hello, reference";
SoftReference sr = new SoftReference(a);
a = null;
int i = 0;
while (sr.get() != null) {
System.out.println(String.format("Get str from object of SoftReference: %s, count: %d", sr.get().str, ++i));
if (i % 10 == 0) {
System.gc();
System.out.println("System.gc() was invoked!");
}
try {
Thread.sleep(500);
} catch (InterruptedException e) {
}
}
System.out.println("object a was cleared by JVM!");
}
}程序运行结果：
Java代码
- Get str from object of SoftReference: Hello, reference, count: 1
- Get str from object of SoftReference: Hello, reference, count: 2
- Get str from object of SoftReference: Hello, reference, count: 3
- Get str from object of SoftReference: Hello, reference, count: 4
- Get str from object of SoftReference: Hello, reference, count: 5
- Get str from object of SoftReference: Hello, reference, count: 6
- Get str from object of SoftReference: Hello, reference, count: 7
- Get str from object of SoftReference: Hello, reference, count: 8
- Get str from object of SoftReference: Hello, reference, count: 9
- Get str from object of SoftReference: Hello, reference, count: 10
- System.gc() was invoked!
- Get str from object of SoftReference: Hello, reference, count: 11
- Get str from object of SoftReference: Hello, reference, count: 12
- Get str from object of SoftReference: Hello, reference, count: 13
- Get str from object of SoftReference: Hello, reference, count: 14
- Get str from object of SoftReference: Hello, reference, count: 15
- Get str from object of SoftReference: Hello, reference, count: 16
- Get str from object of SoftReference: Hello, reference, count: 17
- Get str from object of SoftReference: Hello, reference, count: 18
- Get str from object of SoftReference: Hello, reference, count: 19
- Get str from object of SoftReference: Hello, reference, count: 20
- System.gc() was invoked!
- Get str from object of SoftReference: Hello, reference, count: 21
- Get str from object of SoftReference: Hello, reference, count: 22
- Get str from object of SoftReference: Hello, reference, count: 23
- Get str from object of SoftReference: Hello, reference, count: 24
- Get str from object of SoftReference: Hello, reference, count: 25
- Get str from object of SoftReference: Hello, reference, count: 26
- Get str from object of SoftReference: Hello, reference, count: 27
- 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。
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。