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：

 

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