Synchronization not working for below simple Java code

Question

I have started learning about multi-threading and synchronization in Java and decided to do some practical. I wrote a simple code wherein I have two synchronized methods whose class object is being shared by three threads, but when I run this code , synchronization doesn't seem to work, am I missing a point here? Any help is greatly appreciated.

Runner.java

public class Runner {

private int count;

public synchronized int getCount() {

    return count;

}

public synchronized void setCount(int count) {

    this.count = count;

}

}

ThreadOne.java

public class ThreadOne extends Thread{

Runner r;
int count;

public ThreadOne(Runner r , int count)
{
    this.r=r;
    this.count=count;
}

@Override
public void run()
{
    r.setCount(count);
    System.out.println("count is: "+r.getCount());

}

}

Similarly, I have ThreadTwo.java and ThreadThree.java classes and finally the main class:

MainRunner.java

public class MainRunner {

public static void main(String[] args)  {
    // TODO Auto-generated method stub

    Runner runner = new Runner();
    ThreadOne one = new ThreadOne(runner, 1);
    ThreadTwo two = new ThreadTwo(runner, 2);
    ThreadThree three = new ThreadThree(runner, 3);

    one.start();
    two.start();
    three.start();


}

}

And the Output I am getting is:

count is: 1
count is: 3
count is: 2

It doesn't look synchronized. I know I am missing something here, Please let me know. Thanks in advance

Synchronization doesn't establish order. And you're not synchronizing `run()` anyway. — shmosel, Nov 28 '18 at 19:08
@shmosel , but shouldn't it print 1,2,3 for the above code? If not, how can I achieve it with synchronization? — rojosa, Nov 28 '18 at 19:16
No, it shouldn't. If you want sequential execution, you don't need threads altogether. Synchronization is the wrong tool for the job. — shmosel, Nov 28 '18 at 19:19
Why is your `Runner` class named "Runner?" It doesn't run anything. Your code will be easier for others to read if you choose names that describe what things are/what things mean/what things do. — Solomon Slow, Nov 28 '18 at 19:50
You said, "Similarly, I have ThreadTwo.java and ThreadThree.java..." _How_ similar? Are those two classes, in fact, _identical_ to the ThreadOne? class. If they are identical, then why not just declare one class and create three separate instances of it? — Solomon Slow, Nov 28 '18 at 19:52
@SolomonSlow I'll take care of the naming next time for sure. Yes, I had three identical classes with some minor differences to start three threads that share the same object of the Runner class. My problem is solved but I'll certainly take care of the naming conventions, Thank you. :) — rojosa, Nov 29 '18 at 20:57

nyarian · Accepted Answer · 2018-11-28T19:20:25.310

Why doesn't? Serialization is established on modifying and reading the value, which means that any two threads are not able to appear inside getCount or setCount methods of the single Runner instance, but it doesn't make ANY guarantees on the order of execution, because it is all about scheduling. Here, ThreadThree can just start his execution first and set the value to 3 and print it after that, which will be followed by setting the value to 2 by ThreadTwo and printing it.

Synchronization not working for below simple Java code

1 Answers1