問題描述

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 要等到這個代碼塊結束之后才會把鎖讓出去，當然如果在notify之后又有wait，那就會主動把鎖讓出去 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();} }}

照理來說 t1.wait() 應該會阻塞主線程，并沒有其他地方notify而去掉t1.start()之后，就能阻塞住了

這是什么道理？編譯器優化？還是synchronized代碼塊內如果不對monitor進行操作，結束主動notify？？

問題解答

并不是優化其實，跟線程的執行有關的。在java doc中，public final synchronized void join(long millis)這個方法的注釋上面寫著一句話

<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.

看到加黑體，其實是線程結束之后調用的notifyAll導致wait蘇醒的。并不是什么虛擬機優化導致的。希望能解答你的困惑