简单线程同步问题

Question

In below code, it doesn't make a difference in output if I synchronize the bar method or not - what am I doing wrong? I believe that synchronizing "bar" will make "ThreaOne" to be printed 10 times and then only printing "ThreadTwo" will start, but it is not the case. The ouput I get is as follows:

---

I am Test thread:Thread OneOneOneOne
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread OneOneOneOne
I am in main now
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread Two
I am Test thread:Thread Two
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread Two
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread Two
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread Two
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread Two
I am Test thread:Thread OneOneOneOne
I am Test thread:Thread Two
I am Test thread:Thread Two
I am Test thread:Thread Two
I am Test thread:Thread Two
I am in main now

and so on. Here is my code:

package com.rahul;

class ThreadTest implements Runnable{
    @Override
    public void run() {
        bar();
    }
    public synchronized void bar() {
        for(int i=0;i<10;i++){
            System.out.println("I am Test thread:"+Thread.currentThread().getName());
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

    }
}

public class Test{
    public static void main(String[] args) {
        Thread t1 = new Thread(new ThreadTest(),"Thread OneOneOneOne");
        Thread t2 = new Thread(new ThreadTest(),"Thread Two");
        t1.start();
        try{
            Thread.sleep(4000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        t2.start();
        while(true){
            System.out.println("I am in main now");
            try {
                t2.join();
                Thread.sleep(4000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

Answer 1

You have two instances of ThreadTest . They have nothing to do with each other; you're not calling the same bar()

If you do this instead:

ThreadTest tt = new ThreadTest();
Thread t1 = new Thread(tt,"Thread OneOneOneOne");
Thread t2 = new Thread(tt,"Thread Two");

Then the two threads are sharing an instance of ThreadTest and only one thread will be able to call bar() at a time on that single instance.

Answer 2

Declaring the method to be synchronized will not have the effect you expect. Each thread is synchronized on its own instance of ThreadTest , so the calls do not interact. You need to synchronized on a shared object for one thread to block another. For instance:

class ThreadTest implements Runnable{
    private static Object LOCK_OBJECT = new Object();
    @Override
    public void run() {
        bar();
    }
    public void bar() {
        synchronized (LOCK_OBJECT) {
            for(int i=0;i<10;i++){
                System.out.println("I am Test thread:"+Thread.currentThread().getName());
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }
}

Answer 3

The method you're synchronizing is being accessed by only one thread, since they are instance methods, so each thread is calling it's own bar method.

Maybe you want to declare the bar method as static .