如何运行不同线程的方法访问变量？

Question

When run() method runs in new thread, it will have its own stack. How are the variables (like countdown in this case) from the main thread accessed by the run method in different stack?

final CountDownLatch countdown = new CountDownLatch(1);
for (int i = 0; i < 10; ++ i) {
   Thread racecar = new Thread() {    
      public void run()    {
         countdown.await(); //all threads waiting
         System.out.println("Vroom!");
      }
   };
   racecar.start();
}
System.out.println("Go");
countdown.countDown();

Answer 1

While each thread has its own stack, it shares the same heap with the rest of the program. The normal class and variable scoping applies in that case since the run() method is in an anonymous inner class of the outer class, so it has access to the members of its host class.

The countdown variable is not being pushed on the stack. If it was being passed as a parameter to the method then it would be pushed on the stack.

Answer 2

Variables on the stack are local to the thread. The stack and thread is support at the native level by the CPU and not a detail of Java.

However, in your example, countdown is copied to a field on the sub-class of Thread you created, so it is a field of the object, not a stack local variable. (In fact you don't appear to have any)

In byte code, it may use the stack, but once optimised, most likely it won't use the stack at all. Instead registers are used.

Answer 3

The anonymous inner object is given a reference to the enclosing class behind the scenes.

See here for an excellent Jon Skeet answer on the subject.