Javadocs 说“当一个键被丢弃时，它的条目被有效地从 map 中移除”。

但除非有另一个线程偶尔会删除这样的 Map.Entry条目，值对象不会被 map 强引用吗？但是因为没有这样的线程在运行，所以只有 get方法调用可以删除此类条目 - 一次一个。

我几乎总是使用 WeakHashMap<K, WeakReference<V>>是因为。他们为什么不将其设为默认行为 - 值也作为弱引用？

最佳答案

引用队列用于自动删除条目。

http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/ref/ReferenceQueue.html

Reference queues, to which registered reference objects are appended by the garbage collector after the appropriate reachability changes are detected.

基本上，弱引用是垃圾收集器的核心部分，因此当 GC 扫描发生时，未使用的引用会被发现并放入队列，然后可以根据这些队列的内容采取行动。

线程可以位于队列的 remove 上在需要完成清理或轮询队列时发出警报的方法。

"Java theory and practice: Plugging memory leaks with weak references"解释:

The implementation of WeakHashMap illustrates a common idiom with weak references -- that some internal object extends WeakReference.

...

WeakHashMap uses weak references for holding map keys, which allows the key objects to be garbage collected when they are no longer used by the application, and the get() implementation can tell a live mapping from a dead one by whether WeakReference.get() returns null. But this is only half of what is needed to keep a Map's memory consumption from increasing throughout the lifetime of the application; something must also be done to prune the dead entries from the Map after the key object has been collected. Otherwise, the Map would simply fill up with entries corresponding to dead keys. And while this would be invisible to the application, it could still cause the application to run out of memory because the Map.Entry and value objects would not be collected, even if the key is.

...

Reference queues are the garbage collector's primary means of feeding back information to the application about object lifecycle. Weak references have two constructors: one takes only the referent as an argument and the other also takes a reference queue. When a weak reference has been created with an associated reference queue and the referent becomes a candidate for GC, the reference object (not the referent) is enqueued on the reference queue after the reference is cleared. The application can then retrieve the reference from the reference queue and learn that the referent has been collected so it can perform associated cleanup activities, such as expunging the entries for objects that have fallen out of a weak collection. (Reference queues offer the same dequeuing modes as BlockingQueue -- polled, timed blocking, and untimed blocking.)

编辑:

即使有队列，弱映射仍然会泄漏。 Ephemerons试图解决弱键引用引用该键的强保持值的情况。它们无法在 Java 中实现。

Ephemerons solve a problem which is commonly found when trying to "attach" properties to objects by using a registry. When some property should be attached to an object, the property should (in terms of GC behavior) typically have the life-time that an instance variable of this object would have. However, this is complicated by having an external association between the object and its property such as:

property --------- registry --------- association --------- object

Here, the registry (a third party) will hold onto the association itself which would require manual removal from the registry (instead of automated garbage collection). While this problem can always be solved in any given concrete situation by using one of the various weak association types, choosing the 'right' kind of association depends on a variety of factors some of which can change dynamically.

Ephemerons solve this problem by defining that the 'contents' (value) of an ephemeron will be held strongly until the key is known to be garbage collected. From then on, the contents of the ephemeron will be held weakly. Therefore, the contents of an ephemeron can become eligible for garbage collection if and only if the key is garbage collectable which is the exact behavior which we would observe for an instance variable of the object.

关于java - WeakHashMap 和强引用值，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9166610/