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

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：

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.

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.

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)

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

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