Why Synchronize?

2 minute read

We are going to write a program, a bit program, lots of stuff happening here and there, ok, big program. Now, lots of threads of course, loads and loads of them, we have to synchronize, yes we do, I think so, yeah, synchronize very important stuff, good idea, but, why?

Remember what we have been taught in school? That mutex thing? Mutual Exclusive? That 2 threads are trying to read and write some mutual value at the same time and create a big mess? So that we have to put that wierd synchronized keyword all over the place try to help it? Here is an example:

Support we are trying to generate some auto increasing IDs,

class AutoIncrementID {

    private static long id = 0L;

    static long gen() {
        long prev = id; // read

        try {
            Thread.sleep(200);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        id = prev + 1; // write
        return id;
    }
}

In this example, we read the value, we increase it by 1, and then we write it back. Not atomic at all. Notice that the sleep there is to make it more likely to be inconsistant while we run it with multiple threads.

If we call gen() within two threads, like this:

public static void main(String[] args) {
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                System.out.println(AutoIncrementID.gen());
            }
        });

        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                System.out.println(AutoIncrementID.gen());
            }
        });

        t1.start();
        t2.start();
    }

We will have duplicated IDs. To fix it, just add synchronized keyword to the gen().

So rule number 1, if there are read and write at the same time, meaning the operation is not atomic, then we need synchronization.

But, this is NOT the end of the story. Even if the operation is atomic, we sometimes still need to synchronize. Take a look at the following example:

public class StopThread {
    private static boolean stopRequested;

    public static void main(String[] args)
            throws InterruptedException {
        Thread backgroundThread = new Thread(new Runnable() {
            public void run() {
                int i = 0;
                while (!stopRequested)
                    i++;
            }
        });
        backgroundThread.start();
        TimeUnit.SECONDS.sleep(1);
        stopRequested = true;
    }
}

This program may never end. Why? Because the change to stopRequested in the main thread may not be seen in the background thread. What is the fix? Add volatile to stopRequested to make sure changes from one thread will properly propogates to other threads.

This is rule number 2, Synchronization is required for reliable communication between threads.

~The End~

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