虚引用、软引用和弱引用的使用示例

import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;

public class Test {

    public static void main(String [] args) throws InterruptedException {
//        testPlantomReference(); //虚引用
//        testWeakReference(); //弱引用
        testSoftReference(); //软引用
    }

    static void testSoftReference() throws InterruptedException {
        Object obj = new Object();
        ReferenceQueue queue = new ReferenceQueue();
        SoftReference reference = new SoftReference(obj, queue);
        System.out.println("obj:" + obj);
        System.out.println("queue:" + queue.poll());
        System.out.println("reference:" + reference);
        System.out.println("reference.get():" + reference.get());//不为null
        obj = null;
        //不要使用下一行代码，因为下一行代码会导致reference对象被GC回收，导致queue的poll()返回空
//        reference = null; //这句话会导致下面的queue.poll()返回空，即reference对象都被回收了！！！
        System.gc();
        Thread.sleep(2000);
        System.out.println("-----after gc-----------");
        System.out.println("obj:" + obj);
        System.out.println("queue:" + queue.poll());
        System.out.println("reference:" + reference);
        System.out.println("reference.get():" + reference.get());//不为null,只在内存不足时，obj对象才会被GC回收

    }

    static void testWeakReference() throws InterruptedException {
        Object obj = new Object();
        ReferenceQueue queue = new ReferenceQueue();
        WeakReference reference = new WeakReference(obj, queue);
        System.out.println("obj:" + obj);
        System.out.println("queue:" + queue.poll());
        System.out.println("reference:" + reference);
        System.out.println("reference.get():" + reference.get()); //不为null，其余和虚引用一样
        obj = null;
        //不要使用下一行代码，因为下一行代码会导致reference对象被GC回收，导致queue的poll()返回空
//        reference = null; //这句话会导致下面的queue.poll()返回空，即reference对象都被回收了！！！
        System.gc();
        Thread.sleep(2000);
        System.out.println("-----after gc-----------");
        System.out.println("obj:" + obj);
        System.out.println("queue:" + queue.poll());
        System.out.println("reference:" + reference);
        System.out.println("reference.get():" + reference.get());
    }

    static void testPlantomReference() throws InterruptedException {
        Object obj = new Object();
        ReferenceQueue queue = new ReferenceQueue();
        PhantomReference reference = new PhantomReference(obj, queue);
        System.out.println("obj:" + obj);
        System.out.println("queue:" + queue.poll());
        System.out.println("reference:" + reference);
        System.out.println("reference.get():" + reference.get()); //为null，其余和虚引用一样
        obj = null;
        //不要使用下一行代码，因为下一行代码会导致reference对象被GC回收，导致queue的poll()返回空
//        reference = null; //这句话会导致下面的queue.poll()返回空，即reference对象都被回收了！！！
        System.gc();
        Thread.sleep(2000);
        System.out.println("-----after gc-----------");
        System.out.println("obj:" + obj);
        System.out.println("queue:" + queue.poll());
        System.out.println("reference:" + reference);
        System.out.println("reference.get():" + reference.get());
    }
}