WeakReference与SoftReference

• 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。

• 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：

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<A> weak = new WeakReference<A>(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：

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：

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<A> sr = new SoftReference<A>(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：
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