Question

Ok, so can some one explain to me the gap in my knowledge here?

Initially the example below was trying to synchronize an instance method, but then realised that I spawn a new instance and therefore a lock wouldn’t happen.

So I decided to make a lock on a static method of the class in the hope that the thread would then run in order but still no luck. Can any explain the error of my ways? ( Bear with me there are probably better ways to do this its just getting the understanding right, I’m a PHP developer going into Java, I love it – but I’m only 2 days in 😉 )

So at this time the numbers print out in a random order.

Class 1

package learningjava;


public class LearningJava {
    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {

      ThreadCaller ob1 = new ThreadCaller("This is a test string 1");
      ThreadCaller ob2 = new ThreadCaller("This is a test string 2");
      ThreadCaller ob3 = new ThreadCaller("This is a test string 3");
      ThreadCaller ob4 = new ThreadCaller("This is a test string 4");
      ThreadCaller ob5 = new ThreadCaller("This is a test string 5");

      try {

           ob1.t.join();
           ob2.t.join();
           ob3.t.join();
           ob4.t.join();
           ob5.t.join();

      } catch (InterruptedException e) {

          System.out.println(e);

      }

    }    

}

Class 2

package learningjava;

public class ThreadCaller implements Runnable {

    private String message;
    public Thread t;

    public ThreadCaller(String text) {
        message = text;
        t = new Thread(this);
        t.start();
    }

    public static synchronized void echo(String message) {
        System.out.println(message);
    }


    public void run() {
            ThreadCaller.echo(this.message);
    }

}

Answer 1

Can any explain the error of my ways?

Basically, you’re expecting ordering which simply isn’t guaranteed. You’re calling start() on lots of threads in succession… there’s no guarantee about which one will actually start executing first. The fact that you’ve got a static synchronized method just means that only one thread will be executing that method at a time – it doesn’t guarantee anything about ordering.

Imagine you have a running track with a one-lane gate 100m down the track. You start the race – which runner will reach the gate first?

Fortunately, this isn’t usually a problem – if it’s worth starting multiple threads to do something, you usually don’t care about the order in which they execute. If you do, it’s time to re-examine your design.