Java中的SoftReference，WeakReference和PhantomReference

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本文深入探讨了SoftReference, WeakReference和PhantomReference在Java内存管理中的应用，解释了它们如何帮助垃圾回收器更有效地管理内存。

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SoftReference，WeakReference和PhantomReference都是继承自Reference<T>抽象类，

在这个类中有一个static代码块，会给当前线程组和它的所有父亲都启动一个线程叫做ReferenceHandler，

ReferenceHandler会随时来检查是否有pending中是否有GC加入的已经清除的对象，然后将它们加入到Reference对于的ReferenceQueue中。

    /* High-priority thread to enqueue pending References
     */
    private static class ReferenceHandler extends Thread {

        ReferenceHandler(ThreadGroup g, String name) {
            super(g, name);
        }

        public void run() {
            for (;;) {

                Reference r;
                synchronized (lock) {
                    if (pending != null) {
                        r = pending;
                        Reference rn = r.next;
                        pending = (rn == r) ? null : rn;
                        r.next = r;
                    } else {
                        try {
                            lock.wait();
                        } catch (InterruptedException x) { }
                        continue;
                    }
                }

                // Fast path for cleaners
                if (r instanceof Cleaner) {
                    ((Cleaner)r).clean();
                    continue;
                }

                ReferenceQueue q = r.queue;
                if (q != ReferenceQueue.NULL) q.enqueue(r);
            }
        }
    }

    static {
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());
        Thread handler = new ReferenceHandler(tg, "Reference Handler");
        /* If there were a special system-only priority greater than
         * MAX_PRIORITY, it would be used here
         */
        handler.setPriority(Thread.MAX_PRIORITY);
        handler.setDaemon(true);
        handler.start();
    }

https://weblogs.java.net/blog/2006/05/04/understanding-weak-references

http://www.cnblogs.com/blogoflee/archive/2012/03/22/2411124.html