Java的四种引用方式

public class Reference {

    public static void main(String[] args) {
        try {
            System.out.println("强引用");
            strongReference();
            System.out.println("=== === === ===");
            System.out.println("软引用");
            softReference();
            System.out.println("=== === === ===");
            System.out.println("弱引用");
            weakReference();
            System.out.println("=== === === ===");
            System.out.println("虚引用");
            phantomReference();
            System.out.println("=== === === ===");
            System.out.println("WeakHashMap");
            weakHashMap();
            System.out.println("=== === === ===");
            System.out.println("ReferenceQueue");
            referenceQueue();
            System.out.println("=== === === ===");
        } catch (InterruptedException e) {
        }
    }

    /** 强引用，JVM的默认实现 */
    public static void strongReference() throws InterruptedException {
        Object obj = new Object();
        Object strong = obj;
        obj = null;
        System.gc();
        Thread.sleep(1000);
        System.out.println(strong);
    }

    /**
     * SoftReference SoftReference 于 WeakReference 的特性基本一致， 最大的区别在于
     * SoftReference 会尽可能长的保留引用直到 JVM 内存不足时才会被回收(虚拟机保证)
     * */
    public static void softReference() throws InterruptedException {
        Object obj = new Object();
        SoftReference<Object> sr = new SoftReference<Object>(obj);
        obj = null;
        System.gc();
        Thread.sleep(1000);
        System.out.println(sr.get());
    }

    /**
     * WeakReference 弱引用( 当所引用的对象在 JVM 内不再有强引用时, GC 后weak reference 将会被自动回收)
     * */
    public static void weakReference() throws InterruptedException {
        Object obj = new Object();
        WeakReference<Object> wr = new WeakReference<Object>(obj);
        obj = null;
        System.gc();
        Thread.sleep(1000);
        System.out.println(wr.get());
    }

    /**
     * PhantomReference Phantom Reference(幽灵引用) 与 WeakReference 和 SoftReference
     * 有很大的不同, 因为它的 get() 方法永远返回 null
     * */
    public static void phantomReference() throws InterruptedException {
        Object obj = new Object();
        ReferenceQueue<Object> rq = new ReferenceQueue<Object>();
        PhantomReference<Object> pr = new PhantomReference<Object>(obj, rq);
        System.out.println(pr.get());
    }

    /**
     * 使用 WeakReference 作为 key， 一旦没有指向 key 的强引用, WeakHashMap 在 GC 后将自动删除相关的
     * entry
     */
    public static void weakHashMap() throws InterruptedException {
        Map<Object, Object> map = new WeakHashMap<Object, Object>();
        Object key = new Object();
        Object value = new Object();
        map.put(key, value);
        key = null;
        System.gc();
        Thread.sleep(1000);
        System.out.println(value);
        System.out.println(map.containsValue(value));
    }
    
    /**
     * ReferenceQueue测试方法
     */
    public static void referenceQueue() throws InterruptedException {
        Object obj = new Object();
        ReferenceQueue<Object> rq = new ReferenceQueue<Object>();
        WeakReference<Object> pr = new WeakReference<Object>(obj, rq);
        System.out.println(pr.isEnqueued());//由程序或垃圾回收器通知是否已将此引用对象加入队列。
        System.out.println(rq.poll());//轮询此队列，查看是否存在可用的引用对象。
        obj = null;
        System.gc();
        System.out.println(pr.isEnqueued());
    }
}