WeakReference

http://rockybalboa.blog.51cto.com/1010693/813161

若干年前看了Java的四种引用类型，只是简单知道了不同类型的作用，但对其实现原理一直未能想明白，本文尝试结合jdk，openjdk6的部分源码分析弱引用实现的原理，供大家参考，部分技术细节没有仔细研究，如有疑问欢迎留言讨论

 实例分析

我们以WeakHashMap的处理过程为例介绍一个weak reference的生命周期，首先我们调用WeakHashMap的put方法放入对象到Map中，WeakHashMap的Entry继承了WeakReference

 

private static class Entry<K,V> extends WeakReference<K> implements Map.Entry<K,V> {
        private V value;
        private final int hash;
        private Entry<K,V> next;

下面是put的部分代码

Entry<K,V> e = tab[i];
        tab[i] = new Entry<K,V>(k, value, queue, h, e);
        if (++size >= threshold)
            resize(tab.length * 2);
        return null;
    }

注意new Entry传递了一个reference queue到构造函数中，此构造函数最终会调用Reference的构造函数

Reference(T referent, ReferenceQueue<? super T> queue) {
    this.referent = referent;
    this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
    }

referent是我们之前传入的hashmap的key对象，queue的作用是用来读取referent被回收的weak reference，生产者是谁后续介绍，此时WeakHashMap中已经存在了一个对象，先将key对象的strong ref制空并尝试触发gc，比如使用System.gc()来显式的触发gc，然后调用WeakHashMap的size方法返回集合的个数，绝大多数情况下会是0，这个过程中发生了什么呢？

第一步，key没有可达的strong ref，仅仅存在一个weak reference的referent变量仍然指向了key，触发GC时，以openjdk6的parNew为例，jvm在young generation gc时会尝试获取Reference对象里的静态全局锁

 

/* Object used to synchronize with the garbage collector.  The collector
     * must acquire this lock at the beginning of each collection cycle.  It is
     * therefore critical that any code holding this lock complete as quickly
     * as possible, allocate no new objects, and avoid calling user code.
     */
    static private class Lock { };
    private static Lock lock = new Lock();

在openjdk6里的部分源代码,完整代码请参考instanceRefKlass.cpp文件

void instanceRefKlass::acquire_pending_list_lock(BasicLock *pending_list_basic_lock) {
  // we may enter this with pending exception set
  PRESERVE_EXCEPTION_MARK;  // exceptions are never thrown, needed for TRAPS argument
  Handle h_lock(THREAD, java_lang_ref_Reference::pending_list_lock());
  ObjectSynchronizer::fast_enter(h_lock, pending_list_basic_lock, false, THREAD);
  assert(ObjectSynchronizer::current_thread_holds_lock(
           JavaThread::current(), h_lock),
         "Locking should have succeeded");
  if (HAS_PENDING_EXCEPTION) CLEAR_PENDING_EXCEPTION;
}

 此处代码在parNew gc时执行，目的就是尝试获取全局锁，在gc完成后，jvm会将key被回收的weak reference组成一个queue并赋值到Reference的pending属性然后释放锁，参考方法：

void instanceRefKlass::release_and_notify_pending_list_lock(
  BasicLock *pending_list_basic_lock) {
  // we may enter this with pending exception set
  PRESERVE_EXCEPTION_MARK;  // exceptions are never thrown, needed for TRAPS argument
  //
  Handle h_lock(THREAD, java_lang_ref_Reference::pending_list_lock());
  assert(ObjectSynchronizer::current_thread_holds_lock(
           JavaThread::current(), h_lock),
         "Lock should be held");
  // Notify waiters on pending lists lock if there is any reference.
  if (java_lang_ref_Reference::pending_list() != NULL) {
    ObjectSynchronizer::notifyall(h_lock, THREAD);
  }
  ObjectSynchronizer::fast_exit(h_lock(), pending_list_basic_lock, THREAD);
  if (HAS_PENDING_EXCEPTION) CLEAR_PENDING_EXCEPTION;
}

在一次gc后，Reference对象的pending属性不再为空，让我们看看Reference的部分代码

首先是pending属性的说明：

/* List of References waiting to be enqueued.  The collector adds
 * References to this list, while the Reference-handler thread removes
 * them.  This list is protected by the above lock object.
 */
private static Reference pending = null;

接下来是Reference中的内部类ReferenceHandler，它继承了Thread，看看run方法的代码

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);
        }
    }
    }

一旦jvm notify了前面提到的锁，这个线程就被激活并开始执行，作用是将之前jvm赋值过来的pending对象中的WeakReference对象enqueue到指定的队列中，比如WeakHashMap内部定义的ReferenceQueue属性

此时map的queue中保存了referent已经被回收的WeakReference队列，也就是map的Entry对象，当调用size方法时，内部首先调用expungStaleEntries方法清除被回收掉的Entry，代码如下

 

private void expungeStaleEntries() {
    Entry<K,V> e;
        while ( (e = (Entry<K,V>) queue.poll()) != null) {
            int h = e.hash;
            int i = indexFor(h, table.length);

            Entry<K,V> prev = table[i];
            Entry<K,V> p = prev;
            while (p != null) {
                Entry<K,V> next = p.next;
                if (p == e) {
                    if (prev == e)
                        table[i] = next;
                    else
                        prev.next = next;
                    e.next = null;  // Help GC
                    e.value = null; //  "   "
                    size--;
                    break;
                }
                prev = p;
                p = next;
            }
        }
    }

ok，就这样map的废弃Entry被clear，size返回为0

经过简单的测试程序发现：

一次gc未必能完全回收所有的weak ref

weak对象也可能会出现在old generation