Java synchronized method usage

Question

I am trying to grasp the concept of synchronized methods in Java, but encountered the behavior that seems confusing, at least to me; In the code:

public class parallelUpdate
{
    public static void main(String[] args)
    {
        Ob ob = new Ob();
        new Thread(ob).start();
        new Thread(ob).start();
    }
}

class Ob implements Runnable
{
    static int cnt = 0;

    private synchronized void inc()
    {
        cnt++;
    }

    @Override
    public void run()
    {
        for (int i = 0; i < 5; i++) {
            System.out.println(Thread.currentThread());
            inc();
            try {
                Thread.sleep(100);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

... incrementing is performed in parallel by both treads:

Thread[Thread-0,5,main]
Thread[Thread-1,5,main]
Thread[Thread-0,5,main]
Thread[Thread-1,5,main]
Thread[Thread-1,5,main]
Thread[Thread-0,5,main]
Thread[Thread-0,5,main]
Thread[Thread-1,5,main]

I expected that the use of synchronized for the inc() method would yield sequential incrementing, like:

Thread[Thread-0,5,main]
Thread[Thread-0,5,main]
Thread[Thread-0,5,main]
Thread[Thread-0,5,main]
Thread[Thread-0,5,main]
Thread[Thread-1,5,main]
Thread[Thread-1,5,main]
Thread[Thread-1,5,main]
Thread[Thread-1,5,main]
Thread[Thread-1,5,main]

What is wrong with my code?

Try making the `run()` method `synchronized` instead of the `inc()` method. — Jesper, Jul 21 '17 at 11:47
"I expected that the use of synchronized for the inc() method would yield sequential incrementing" why you expect it? You synchronized only `inc` method. It means that no two thread can execute it in parallel, but it doesn't mean that will be some specific order of execution. — talex, Jul 21 '17 at 11:53

score 0 · Answer 1 · answered Jul 21 '17 at 12:12

This happens because Thread Scheduler decides what thread to run and there is no particular order in which it picks a thread. You can set a priority on a thread, but again it's up to scheduler to pick it or not. You can ask scheduler to run a particular thread, but not force it. Synchronising a method guaranties that method is only run by one thread at a time. I would also advice to check Thread Executers API which provides cleaner way to create, maintain and destroy a thread. Here is a useful link on threads in java: http://winterbe.com/posts/2015/04/07/java8-concurrency-tutorial-thread-executor-examples/

score 0 · Accepted Answer · answered Jul 21 '17 at 12:19

Pleas find the correct Code:-

 public class parallelUpdate {
        public static void main(String[] args) {
            Ob ob = new Ob();
            new Thread(ob).start();
            new Thread(ob).start();

        }
    }


    class Ob implements Runnable {
        static int cnt = 0;
        private synchronized void inc() {
              for (int i = 0; i < 5; i++) {
                  System.out.println(Thread.currentThread());
                  cnt++;
                  try {
                      Thread.sleep(100);
                  } catch (InterruptedException e) {
                      e.printStackTrace();
                  }
              } 
        }
        @Override
        public void run() {
          inc();
        }
    }

Explanation against your code :-

you are making inc(); method as a synchronized but their is no synchronized for **for loop** so your two treads enters and perform the inc(); synchronized operation.

score 0 · Answer 3 · answered Jul 21 '17 at 12:22

If thread 2 start after thread 1 then a better solution is to use a join method, that means the program will want until thread 1 finish to start thread 2.

how wait for a thread to finish...

public static void main(String[] args) {
        Ob ob = new Ob();
        Thread myFirstThread = new Thread(ob);
        myFirstThread.start();

        try {
            myFirstThread.join();
        } catch (InterruptedException e) {

            e.printStackTrace();
        }   

        Thread mySecondThread = new Thread(ob);
        mySecondThread.start();

    }

Go to java documentation to clarify how syncrhonized methods works Synchronized Methos

First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads.

Java synchronized method usage

3 Answers3