java - 關於多執行緒notify的問題

Question

public class WaitTest {

    static class ThreadA extends Thread {
        public ThreadA(String name){
            super(name);
        }

        @Override
        public 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();
        }
    }
}

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

這是什麼道理？編譯器優化？還是synchronized程式碼區塊內如果不對monitor進行操作，結束主動notify？ ？

Answer 1

並不是優化其實，跟執行緒的執行有關的。在java doc中，public final synchronized void join(long millis)這個方法的註解上面寫著一句話

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 inis injed inisgended that applications not use {@code wait}, {@code notify}, or {@code notifyAll} on {@code Thread} instances.

看到加黑體，其實是線程結束之後調用的notifyAll導致wait甦醒的。並不是什麼虛擬機器優化導致的。希望能解答你的困惑