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
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
参考：
http://weblogs.java.net/blog/2006/05/04/understanding-weak-references
http://stackoverflow.com/questions/154724/when-would-you-use-a-weakhashmap-or-a-weakreference
本文出自 “RockyBalboa” 博客，请务必保留此出处http://rockybalboa.blog.51cto.com/1010693/813161
