Quick note: WeakReference

1 The usage of WeakReference

Grammar： WeakReference<ClassA> weakA = new WeakReference<ClassA>(instanceOfA);

Scene：      Weak references are useful for mappings that should have their entries removed automatically once they are not referenced any more (from outside).

                   在map中弱引用是有用的，当外部不再引用时，它的条目会被自动移除。（为什么呢？？？）

 

Reference: http://developer.android.com/reference/java/lang/ref/WeakReference.html

2 WeakReference & garbage collection

  Once the garbage collector decides that an object objis is weakly-reachable, the following happens:

• A set ref of references is determined. ref contains the following elements:
  • All weak references pointing to obj.
  • All weak references pointing to objects from which obj is either strongly or softly reachable.
• All references in ref are atomically cleared.
• All objects formerly being referenced by ref become eligible for finalization.
• At some future point, all references in ref will be enqueued with their corresponding reference queues, if any.

  一旦垃圾收集器决定一个obj是弱可及的，将会发生下面的事情：

  1）一系列引用的ref被决定。ref包含：

    a) 所有指向obj的弱引用。b) 所有指向obj的强可及或弱可及对象的弱引用。

  2）ref中的所有引用被自动清除。

  3）ref中此前被引用的对象在finalization阶段（我的理解是finialize方法执行时）被选定。

  4）在未来的某个时刻，在ref中的所有引用将被加入到它们对应的引用队列中，如果队列存在的话。（加入以后呢？？？）

3 WeakReference vs. SoftReference

  The difference between a SoftReference and a WeakReference is the point of time at which the decision is made to clear and enqueue the reference:

• A SoftReference should be cleared and enqueued as late as possible, that is, in case the VM is in danger of running out of memory.
• A WeakReference may be cleared and enqueued as soon as is known to be weakly-referenced.

区别在于清除和进队的时机：

  a）软引用会被尽量晚的清除和进队，比如在VM内存将要耗尽的时候。

  b）弱引用可能会在确定为弱引用时被清除和进队。