Java弱引用WeakReference的理解与使用

WeakReference是弱引用，弱引用持有的对象实例是可以被垃圾回收器回收的。通常用于保存保存对象的引用，却又不会影响对象被GC回收。对于DEBUG和内存监视工具非常有用。一般也常与队列联合使用。下面简单的介绍下如何使用WeakReference

import java.lang.ref.WeakReference;

public class TestWeakReferece {

    public static void main(String[] args) {
        Object o = new Object();
        WeakReference<Object> weak = new WeakReference<Object>(o);
        o = null;
        int i = 0;
        while (weak.get() != null) {
            i++;
            System.out.println("Object is not null. count is " + i);
            if (i % 10 == 0) {
                System.gc();
                System.out.println("System.gc() was called!");
            }
            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {
            }
        }
        System.out.println("object o was cleared by JVM!");
    }

}

测试结果
Object is not null. count is 1
Object is not null. count is 2
Object is not null. count is 3
Object is not null. count is 4
Object is not null. count is 5
Object is not null. count is 6
Object is not null. count is 7
Object is not null. count is 8
Object is not null. count is 9
Object is not null. count is 10
System.gc() was called!
object o was cleared by JVM!

WeakReference的语法：

WeakReference<T> weakReference = new WeakReference<T>(referent);
当要获得weak reference引用的对象时, 首先需要判断它是否已经被回收:
weakReference.get();
如果此方法为空, 那么说明weakReference指向的对象已经被回收了。

小结：
1.Java中当一个对象仅被一个弱引用引用时，如果GC运行, 那么这个对象就会被回收。
2.弱引用的一个特点是它何时被回收是不可确定的；