問(wèn)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(gè)代碼塊結(jié)束之后才會(huì)把鎖讓出去，當(dāng)然如果在notify之后又有wait，那就會(huì)主動(dòng)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(lái)說(shuō) t1.wait() 應(yīng)該會(huì)阻塞主線程，并沒(méi)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(yōu)化？還是synchronized代碼塊內(nèi)如果不對(duì)monitor進(jìn)行操作，結(jié)束主動(dòng)notify？？

問(wèn)題解答

并不是優(yōu)化其實(shí)，跟線程的執(zhí)行有關(guān)的。在java doc中，public final synchronized void join(long millis)這個(gè)方法的注釋上面寫(xiě)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(shí)是線程結(jié)束之后調(diào)用的notifyAll導(dǎo)致wait蘇醒的。并不是什么虛擬機(jī)優(yōu)化導(dǎo)致的。希望能解答你的困惑