Java多线程开发系列-线程间协作

wait(),notify()和notifyAll()

他们都是java.lang.Object的方法：

• wait(): Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.
• notify(): Wakes up a single thread that is waiting on this object's monitor.
• notifyAll(): Wakes up all threads that are waiting on this object's monitor.

wait()的使用方式：

synchronized( lockObject )
{ 
    while( ! condition )
    { 
        lockObject.wait();
    }
     
    //take the action here;
}

notify()的使用方式：

synchronized(lockObject) 
{
    //establish_the_condition;
 
    lockObject.notify();
     
    //any additional code if needed
}

都需要同步，但notify不用在循环里。

notifyAll()的使用方式：

synchronized(lockObject) 
{
    establish_the_condition;
 
    lockObject.notifyAll();
}

一般情况下使用notifyAll()，notify()较难掌控，容易出现想不到的问题。（牺牲性能，满足安全）

一个简单的例子，2对执行完5遍之后最终将有2个线程一直在waiting状态：

public class OutputThread1 {
  private static Object lock;

  public static void main(String[] args) {
    lock = new Object();

    Thread1 thread1 = new Thread1();
    Thread2 thread2 = new Thread2();
    Thread3 thread3 = new Thread3();
    Thread3 thread4 = new Thread3();

    thread1.start();
    thread2.start();
    thread3.start();
    thread4.start();

  }

  static class Thread1 extends Thread {
    @Override
    public void run() {
      synchronized (lock) {
        while (true) {
          try {
            lock.wait();
          } catch (InterruptedException e) {
          }
          System.out
              .println("线程" + Thread.currentThread().getName() + ":" + "wait");
        }
      }
    }
  }

  static class Thread2 extends Thread {
    @Override
    public void run() {
      synchronized (lock) {
        while (true) {
          try {
            lock.wait();
          } catch (InterruptedException e) {
          }
          System.out
              .println("线程" + Thread.currentThread().getName() + ":" + "wait");
        }
      }
    }
  }

  static class Thread3 extends Thread {
    @Override
    public void run() {
      for (int i = 0; i < 6; i++) {
        synchronized (lock) {
          try {
            Thread.sleep(100);
            lock.notifyAll();
          } catch (Exception e) {
          }
          System.out.println(
              "线程" + Thread.currentThread().getName() + ":" + "notifyAll");
        }
      }
    }
  }

}

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