Object中线程阻塞及唤醒方法的使用

Object中线程阻塞及唤醒方法使用

相关方法

挂起： wait()/wait(long timeout)/wait(long timeout, int nanos)

唤醒： notify()/notifyAll()

使用说明

• 调用对象的这些方法(wait/notify)，必须先持有对象的监视器（monitor）；否则会抛出IllegalMonitorStateException异常

  例1：

  public static void main(String[] args) {
    Thread th1 = new Thread(){
        @Override
        public void run() {
            try {
                this.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    };
  
    th1.start();
  }

  运行结果：

  Exception in thread "Thread-0" java.lang.IllegalMonitorStateException
  
    at java.lang.Object.wait(Native Method)
  
    at java.lang.Object.wait(Object.java:503)
  
    at com.falco.src.verify.ObjectVerification$1.run(ObjectVerification.java:11)

• 当调用对象的wait方法后，线程处于阻塞状态，对应的监视器（monitor）也被释放，其他线程可以获得此监视器来唤醒该线程。当线程重新被唤醒时，它会再次获取该监视器（monitor），然后被唤醒

唤醒方式

调用wait方法后，线程处于阻塞状态。可以通过以下四种唤醒方式：

1. 调用notify/notifyAll方法唤醒

   例二：

   public static void main(String[] args) {
        final Object bt = new Object();
        Thread th1 = new Thread() {
            @Override
            public void run() {
                try {
                    synchronized (bt) {
                        bt.wait();
                        System.out.println("the thread has been woke up.");
                    }
                } catch (InterruptedException e) {
                    System.out.println("the current thread has been interupted.");
                    e.printStackTrace();
                }
            }
        };
        th1.start();
   
        Thread th2 = new Thread() {
            @Override
            public void run() {
                synchronized (bt) {
                    bt.notify();
                    System.out.println("finish waking up.");
                }
            }
        };
        th2.start();
        System.out.println("wait the program exit.");
    }

   结果：

   wait the program exit.
   finish waking up.
   the thread has been woke up.

2. 超时自动唤醒，即：wait方法指定一个超时时间，达到这个超时时间且没有被唤醒的线程将自动唤醒

   例三：

    public static void main(String[] args) {
        final Object bt = new Object();
        Thread th1 = new Thread() {
            @Override
            public void run() {
                try {
                    synchronized (bt) {
                        bt.wait(10,1);
                        System.out.println("the thread has been woke up.");
                    }
                } catch (InterruptedException e) {
                    System.out.println("the current thread has been interupted.");
                    e.printStackTrace();
                }
            }
        };
        th1.start();
        System.out.println("wait the program exit.");
    }

   运行结果：

   wait the program exit.
   the thread has been woke up.

3. 中断唤醒，在其他线程中，调用阻塞线程的中断方法，从而唤醒阻塞的线程

   例四：

   public static void main(String[] args) {
        Thread th1 = new Thread() {
            @Override
            public void run() {
                try {
                    synchronized (this) {
                        this.wait();
                    }
                } catch (InterruptedException e) {
                    System.out.println("the current thread has been interupted.");
                    e.printStackTrace();
                }
            }
        };
   
        th1.start();
        System.out.println("before interrupting,the interrupt flag:"+th1.isInterrupted());
        th1.interrupt();
        System.out.println("after interrupting,the interrupt flag:"+th1.isInterrupted());
    }

   运行结果：

   before interrupting,the interrupt flag:false
   after interrupting,the interrupt flag:true
   the current thread has been interupted.
   java.lang.InterruptedException
    at java.lang.Object.wait(Native Method)
    at java.lang.Object.wait(Object.java:503)
    at com.falco.src.verify.ObjectVerification$1.run(ObjectVerification.java:11)

4. 假醒（spurious wakeup），除了以上三种方式外，处于阻塞中的线程还可能自动被唤醒

   由于假醒的存在，因此需要自行处理假醒的场景，通过下面的形式排出假醒，避免程序出错

   ...
   synchronized (bt) {
       while (condition) {//condition为不满足唤醒条件，即不满足唤醒条件时执行循环
        bt.wait();
       }
   }
   ...

   假醒相关内容就不在这里介绍了，如果有兴趣，请自行研究

   假醒wiki地址