Object object = new Object();

public class ReferenceDemo {
    public static void main(String[] args) throws IOException {
        //強(qiáng)引用不會(huì)被垃圾回收
        ReferenceDemo referenceDemo = new ReferenceDemo();
        //強(qiáng)引用轉(zhuǎn)變?yōu)闊o(wú)引用，無(wú)引用可被垃圾回收
        referenceDemo = null;
        //觸發(fā)垃圾回收
        System.gc();
        //阻塞主線程，等待垃圾回收線程執(zhí)行
        System.in.read();
    }

    //對(duì)象被回收之前調(diào)用
    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("-----finalize-----");
    }
}

public class ReferenceDemo {
    public static void main(String[] args) throws InterruptedException {
        //創(chuàng)建ReferenceDemo對(duì)象的軟引用
        SoftReference<ReferenceDemo> softReference = new SoftReference<>(new ReferenceDemo());
        //觸發(fā)垃圾回收
        System.gc();
        //阻塞主線程，等待垃圾回收線程執(zhí)行
        Thread.sleep(5000);
        //softReference.get()返回軟引用對(duì)象，如果對(duì)象已經(jīng)被垃圾回收，則返回null
        System.out.println(softReference.get());

        //創(chuàng)建25M的字節(jié)數(shù)組
        byte[] bytes = new byte[1024 * 1024 * 25];
        //內(nèi)存已經(jīng)不足，阻塞主線程，等待垃圾回收線程執(zhí)行
        Thread.sleep(5000);
        //重新輸出軟引用對(duì)象
        System.out.println(softReference.get());
    }
}

/*
 * 輸出結(jié)果：
 * com.buhe.demo.demos.reference.ReferenceDemo@76fb509a
 * null
 */

public class ReferenceDemo {
    public static void main(String[] args) throws InterruptedException {
        //創(chuàng)建ReferenceDemo的弱引用
        WeakReference<ReferenceDemo> weakReference = new WeakReference<>(new ReferenceDemo());
        //weakReference.get()返回弱引用對(duì)象，如果對(duì)象已經(jīng)被垃圾回收，則返回null
        System.out.println(weakReference.get());

        //觸發(fā)垃圾回收
        System.gc();
        //阻塞主線程，等待垃圾回收線程執(zhí)行
        Thread.sleep(3000);

        //重新輸出弱引用對(duì)象
        System.out.println(weakReference.get());
    }
}

/*
 * 輸出結(jié)果：
 * com.buhe.demo.demos.reference.ReferenceDemo@76fb509a
 * null
 */

public class ReferenceDemo {
    public static void main(String[] args) throws InterruptedException {
        //創(chuàng)建引用隊(duì)列
        ReferenceQueue<Object> referenceQueue = new ReferenceQueue();
        //創(chuàng)建ReferenceDemo的虛引用
        PhantomReference<ReferenceDemo> phantomReference = new PhantomReference<>(new ReferenceDemo(), referenceQueue);
        //phantomReference.get()總是返回null
        System.out.println("phantomReference.get():" + phantomReference.get());
        //輪詢此隊(duì)列，查看是否有可用的Reference對(duì)象，有則返回該對(duì)象，否則返回null
        System.out.println("referenceQueue.poll():" + referenceQueue.poll());

        //觸發(fā)垃圾回收
        System.gc();
        //阻塞主線程，等待垃圾回收線程執(zhí)行
        Thread.sleep(3000);
        System.out.println("------垃圾回收之后------");

        System.out.println("phantomReference.get():" + phantomReference.get());
        System.out.println("referenceQueue.poll():" + referenceQueue.poll());
    }
}

/*
 * 輸出結(jié)果：
 * phantomReference.get():null
 * referenceQueue.poll():null
 * ------垃圾回收之后------
 * phantomReference.get():null
 * referenceQueue.poll():java.lang.ref.PhantomReference@76fb509a
 */