weakreference實現原理分析

 前言

若干年前看了Java的四種引用類型，只是簡單知道了不同類型的作用，但對其實現原理一直未能想明白，本文嘗試結合jdk，openjdk6的部分源碼分析弱引用實現的原理，供大家參考，部分技術細節沒有仔細研究，如有疑問歡迎留言討論

 實例分析

我們以WeakHashMap的處理過程爲例介紹一個weak reference的生命週期，首先我們調用WeakHashMap的put方法放入對象到Map中，WeakHashMap的Entry繼承了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的部分代碼

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傳遞了一個reference queue到構造函數中，此構造函數最終會調用Reference的構造函數

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

referent是我們之前傳入的hashmap的key對象，queue的作用是用來讀取referent被回收的weak reference，生產者是誰後續介紹，此時WeakHashMap中已經存在了一個對象，先將key對象的strong ref制空並嘗試觸發gc，比如使用System.gc()來顯式的觸發gc，然後調用WeakHashMap的size方法返回集合的個數，絕大多數情況下會是0，這個過程中發生了什麼呢？

第一步，key沒有可達的strong ref，僅僅存在一個weak reference的referent變量仍然指向了key，觸發GC時，以openjdk6的parNew爲例，jvm在young generation gc時會嘗試獲取Reference對象裏的靜態全局鎖

 

/* 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裏的部分源代碼,完整代碼請參考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;
}

 此處代碼在parNew gc時執行，目的就是嘗試獲取全局鎖，在gc完成後，jvm會將key被回收的weak reference組成一個queue並賦值到Reference的pending屬性然後釋放鎖，參考方法：

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後，Reference對象的pending屬性不再爲空，讓我們看看Reference的部分代碼

首先是pending屬性的說明：

/* 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;

接下來是Reference中的內部類ReferenceHandler，它繼承了Thread，看看run方法的代碼

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了前面提到的鎖，這個線程就被激活並開始執行，作用是將之前jvm賦值過來的pending對象中的WeakReference對象enqueue到指定的隊列中，比如WeakHashMap內部定義的ReferenceQueue屬性

此時map的queue中保存了referent已經被回收的WeakReference隊列，也就是map的Entry對象，當調用size方法時，內部首先調用expungStaleEntries方法清除被回收掉的Entry，代碼如下

 

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，就這樣map的廢棄Entry被clear，size返回爲0

經過簡單的測試程序發現：

一次gc未必能完全回收所有的weak ref

weak對象也可能會出現在old generation

 

參考：

 

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