java - 关于多线程notify的问题

问题描述

public class WaitTest { static class ThreadA extends Thread {public ThreadA(String name){ super(name);}@Overridepublic void run() { synchronized (this){ System.out.println(Thread.currentThread().getName()+' call notify()'); //notify();//notify之后 要等到这个代码块结束之后才会把锁让出去，当然如果在notify之后又有wait，那就会主动把锁让出去 try { System.out.println(Thread.currentThread().getName()+' wait'); //wait(); //Thread.sleep(10000); } catch (Exception e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName()+' after notify'); }} } public static void main(String[] args) throws InterruptedException {ThreadA t1 =new ThreadA('t1');synchronized (t1){ System.out.println(Thread.currentThread().getName()+' start t1'); t1.start(); System.out.println(Thread.currentThread().getName()+' wait'); t1.wait();////System.out.println(Thread.currentThread().getName()+' notify'); // t1.notify(); System.out.println(t1.getName()); System.out.println(Thread.currentThread().getName()+' continue'); //t1.notify();} }}

照理来说 t1.wait() 应该会阻塞主线程，并没有其他地方notify而去掉t1.start()之后，就能阻塞住了

这是什么道理？编译器优化？还是synchronized代码块内如果不对monitor进行操作，结束主动notify？？

问题解答

并不是优化其实，跟线程的执行有关的。在java doc中，public final synchronized void join(long millis)这个方法的注释上面写着一句话

<p> This implementation uses a loop of {@code this.wait} calls conditioned on {@code this.isAlive}. As a thread terminates the {@code this.notifyAll} method is invoked. It is recommended that applications not use {@code wait}, {@code notify}, or {@code notifyAll} on {@code Thread} instances.

看到加黑体，其实是线程结束之后调用的notifyAll导致wait苏醒的。并不是什么虚拟机优化导致的。希望能解答你的困惑