Java同步等待和通知方法

Question

I'm a newbie to synchronization and trying to understand how wait and notify works.

Question:-

The programs executes 2 threads - T1 & T2 simultaneously but based on the output T1 runs first, executes the question(first) method, prints the question, sets the flag(true), runs the notify() method, executes the question(second) method & enters the wait method. Now T2 starts and executes.

1. Why does T2 doesn't start until T1 calls the wait method of the Object Chat.
2. When the question method is executed first time and notify() method is called, there is no wait() on the chat object(since t2 did not yet start). so, which thread listens to this notify method.

My codes:

package TestThread;

class Chat {
    boolean flag = false;

    public synchronized void Question(String msg) {

        System.out.println("Question method = " + flag);
        if (flag) {
            try {
                System.out.println("Question method wait start");
                wait();
                System.out.println("Question method wait finish");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println(msg);
        flag = true;
        notify();
    }

    public synchronized void Answer(String msg) {
        System.out.println("Answer method = " + flag);
        if (!flag) {
            try {
                System.out.println("Answer method wait start");
                wait();
                System.out.println("Answer method wait finish");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.println(msg);
        flag = false;
        notify();
    }
}

class T1 implements Runnable {
    Chat m;
    String[] s1 = { "Hi", "How are you ?", "I am also doing fine!" };

    public T1(Chat m1) {
        this.m = m1;
        new Thread(this, "Question").start();
    }

    public void run() {
        for (int i = 0; i < s1.length; i++) {
            m.Question(s1[i]);
        }
    }
}

class T2 implements Runnable {
    Chat m;
    String[] s2 = { "Hi", "I am good, what about you?", "Great!" };

    public T2(Chat m2) {
        this.m = m2;
        new Thread(this, "Answer").start();
    }

    public void run() {
        for (int i = 0; i < s2.length; i++) {
            m.Answer(s2[i]);
        }
    }
}

public class MultiThread {
    public static void main(String[] args) {
        Chat m = new Chat();
        new T1(m);
        new T2(m);            
    }
}

Result:-

Question method = false
Hi
Question method = true
Question method wait start
Answer method = true
Hi
Answer method = false
Answer method wait start
Question method wait finish
How are you ?
Question method = true
Question method wait start
Answer method wait finish
I am good, what about you?
Answer method = false
Answer method wait start
Question method wait finish
I am also doing fine!
Answer method wait finish
Great!

Answer 1

Why does T2 doesn't start until T1 calls the wait method of the Object Chat?

You've got a race condition. Either of the two threads will execute first. Since they both share a single Chat instance, whose methods are synchronized , one will block until the other has unlocked the monitor on the Chat instance.

When the question method is executed first time and notify() method is called, there is no wait() on the chat object(since t2 did not yet start). so, which thread listens to this notify method?

None do.

Answer 2

1.) Why does T2 doesn't start until T1 calls the wait method of the Object Chat?

Actually T2 starts before that but you don't notice because you don't have any logging statements to show up. Put a System.out inside run method of T2 and run your program a few times. You will see T2 starting before T1 calls wait method. Keep in mind T2 cannot enter the Chat object's Answer method since its lock is acquired by T1 before wait() is eventually called.

Another thing to keep in mind is that there is no guarantee of the order in which threads will be started. Just because T1.start appears before T2.start , does not mean T1 will always be started first. Thread scheduler makes that decision.

2.)When the question method is executed first time and notify() method is called, there is no wait() on the chat object(since t2 did not yet start). so, which thread listens to this notify method.

--If there has been no wait calls , then notify won't impact any threads.

Answer 3

Why does T2 doesn't start until T1 calls the wait method of the Object Chat.

You have a same object 'm' and passed it to different threads and there is method level synchronization , so the moment a 'call' enters the question() there will be lock on whole object till wait() is called Hence answer method will be blocked.

When the question method is executed first time and notify() method is called, there is no wait() on the chat object(since t2 did not yet start). so, which thread listens to this notify method.

notify() has no significance for the first time, synchronized method has to finish thats all. as said earlier answer() is not executed till the question() finishes / releases the lock (calling wait())