如何在Java中使用wait（）/ notify（）

Question

I know that there are a few threads open regarding this topic, but I'm just looking for a VERY ELEMENTARY example of how to use wait() and notify() in Java. By "VERY ELEMENTARY," I mean simply printing something out. Thanks.

EDIT: Here's what I have tried thus far and I get an IllegalMonitorStateException:

 public void waiting() { for(int i = 0; i < 10; i++) { if(i == 5) try { this.wait(); } catch (InterruptedException e) { } else System.out.println(i); } System.out.println("notify me now"); this.notify(); } 

Answer 1

wait and notify are used in synchronized block while using threads to suspend and resume where left off.

Wait immediately looses the lock, whereas Nofity will leave the lock only when the ending bracket is encountered.

public class Mythread implements Runnable{



public synchronized void goo(){

System.out.println("Before Wait");

wait();

System.out.println("After Wait");


}


public synchronized void foo(){

System.out.println("Before Notify");

notify();

System.out.println("After Notify");

}


public class Test{

public static  void main(String[] args){

Thread t = new Thread(new Mythread);

t.start();

 }
}

Answer 2

Your IllegalMonitorStateException is due to the fact that you must synchronize on the object before calling wait or notify. So

this.wait

needs to be

   synchronized(this) {
      this.wait();
    }

Your example won't run because you'll never get to the notify call... as soon as your thread hits wait, it will suspend and advance no further. For wait / notify to work, you have to have two threads. One thread suspends when the wait method is invoked, and eventually, the second thread calls synchronized(this) { this.notify() } to cause the first thread to wake up and continue executing below the wait call.

The synchronization is required because you would ordinarily check some condition before waiting, ie,

  synchronized(this) { 
      if(! this.isReady) {
         this.wait(); 
      }
   }

You need to synchronize to make sure no other thread changes the state of the isReady flag between the line where you check the variable and the line where you wait. So your notify code would

   synchronized(this) {
      isReady = true;
      this.notify();
   }

Now the order of the method calls doesn't matter. If you notify first, no thread will wake up, but that's ok, because you aren't going to sleep since isReady = true. If you go to sleep first, isReady = true does nothing, but the notify call wakes up the thread. Finally, the synchronization ensures that you don't check the variable in thread A, then have thread B set the variable and notify (doing nothing), then have thread A go to sleep and never wake up.

Hope that helps.

Answer 3

wait()和notify()用于同步线程：一个线程可以告诉wait() ，并且在收到notify()调用之前不会继续执行任何操作。

Answer 4

这些函数的基本思想是wait()挂起一个线程（让它进入休眠状态），而notify()使线程在它进入休眠状态时从它离开的地方获取。

Answer 5

Take a look at: this or just look up simple prodcuer consumer problem java on google. I am sure you will find something to suit your needs.

Answer 6

在oracle java站点的受保护块上查看此示例 - 它包含一个简单的生产者 - 消费者问题的工作示例。