Java:线程等待对象时是否释放所有监视器？

Question

在一个线程可以等待一个对象之前，它必须获得该对象的监视器。然后释放监视器，线程一旦醒来就尝试重新获取它。

但是当线程调用 wait 时，其他监视器会发生什么情况？

考虑这个例子:

   Object a = // ...   Object b = // ...   synchronized(a)   {       synchronized(b)       {           b.wait();           // continue       }   }

当线程调用b.wait()时，是否会释放a和b上的锁>，还是只有 b？

Answer 1

只有 b。

此类问题的专制来源是 Java 语言规范。本例中的相关部分是 17.8 Wait Sets and Notification :

Let thread t be the thread executing the wait method on object m, and let n be the number of lock actions by t on m that have not been matched by unlock actions. One of the following actions occurs.

• [...]

• Otherwise, the following sequence occurs:

  1. Thread t is added to the wait set of object m, and performs n unlock actions on m.
  2. [...]