java中对象的强、软、弱、虚四种引用详解

/**
     * 强引用
     */
    public void testStrongReference(){
        //强引用
        Object obj=new Object();
    }

/**
     * 软引用
     */
    public void testSoftReference(){
        //强引用
        Object obj=new Object();
        //放入SoftReference
        SoftReference<Object> softReference = new SoftReference<Object>(obj);
        System.out.println( softReference.get());//java.lang.Object@39a054a5
    }

/**
     * 软引用（引用队列）
     */
    public void testSoftReferenceQueue(){
        //引用队列
        ReferenceQueue referenceQueue = new ReferenceQueue<>();
        //使用守护线程
        Thread thread= new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    int i=0;
                    SoftReference<Integer> softReference;
                    //如果对象被gc回收
                    while((softReference = (SoftReference) referenceQueue.remove()) != null) {
                        System.out.println("第"+i+"次回收："+softReference);
                        i++;
                    }
                }catch (Exception e){
                }
            }
        });
        thread.setDaemon(true);//设置为守护线程，监听对象变化
        thread.start();
        Map<Object, Object> map = new HashMap<>();
        Object obj=new Object();
        for(int i=0;i<2000; i++){
            //放入SoftReference转为软引用
            byte [] bytes=new byte[1024*1024];
            SoftReference<byte []> softReference = new SoftReference<byte []>(bytes,referenceQueue);
            //将软引用作为key
            map.put(softReference,obj);
        }
    }

/**
     * 弱引用
     */
    public void testWeakReference(){
        //强引用
        Object obj=new Object();
        //放入WeakReference
        WeakReference<Object> weakReference = new WeakReference<Object>(obj);
        System.out.println(weakReference.get());//java.lang.Object@39a054a5
    }

/**
     * 虚引用
     */
    public void testPhantomReference(){
        //强引用
        Object obj=new Object();
        //必须使用引用队列
        ReferenceQueue referenceQueue = new ReferenceQueue();
        //放入PhantomReference
        PhantomReference<Object> phantomReference = new PhantomReference<Object>(obj,referenceQueue);
        System.out.println(phantomReference.get());//null 因为虚引用无法获取对象实例，只是监听作用
        System.out.println(obj);//java.lang.Object@39a054a5
    }