Question

public class GetCurrentThread implements Runnable {
   Thread th;

   public GetCurrentThread(String threadName) {
      th = new Thread(this,threadName); //<----DOUBT 
      System.out.println("get threadname "+th.getName());
      th.start();
   }

  public void run() {
     System.out.println(th.getName()+" is starting.....");
     System.out.println("Current thread name : " + Thread.currentThread().getName());
  }

  public static void main(String args[]) {
     System.out.println("Current thread name : " + Thread.currentThread().getName());
     new GetCurrentThread("1st Thread");
     //new GetCurrentThread("2nd Thread");
   }
}

Can someone explain what the 2nd line of above code is doing?
My understanding for “th = new Thread(this,threadName)” is, it will create thread object with the name given; let say name “1st Thread”.Now, What “this” keyword is doing here?because when i remove this and try get name of the thread I got the name with no issues, but it never started run().can someone explain in simple terms rather than with one single line answer. i really appreciate everyone’s help.

Answer 1

th = new Thread(this, threadName); //<—-DOUBT

Your 2nd line is constructing a new Thread object with your GetCurrentThread class as the target (this) and a thread name of "threadName". Your this can be the target because it implements Runnable. Inside of the Thread class, when the thread is started, it calls Thread.run() which does:

public void run() {
    if (target != null) {
        target.run();
    }
}

Your GetCurrentThread is the target since you pass this in to the constructor. Really it should not be called GetCurrentThread since it isn’t a thread. Two lines after you have constructed your Thread, you start it running:

th.start();

The start() method does the actual work of creating a separate working native thread. The first thing the thread does is to call the run() method in your GetCurrentThread class.

As a comment, typically it is not recommended for a class to start() itself in its constructor. There are race conditions that are inherent with this that can cause problems. It is better to have a start() method on your GetCurrentThread

/** start the underlying thread */
public void start() {
   th.start();
}

Your main would look like:

public static void main(String args[]) {
   System.out.println("Current thread name : " + Thread.currentThread().getName());
   GetCurrentThread thread1 = new GetCurrentThread("1st Thread");
   thread1.start();
}