Which method calls run()?

Question

public class HelloRunnable implements Runnable {

public void run() {
    System.out.println("Hello from a thread!");
}

public static void main(String args[]) {
    (new Thread(new HelloRunnable())).start();
} } 

According to Java Doc

The Runnable interface defines a single method, run, meant to contain the code executed in the thread. The Runnable object is passed to the Thread constructor.

So, When we execute HelloRunnable, who calls the inside run method? In the Thread class, the start method looks like this:

public synchronized void start() {
     if (threadStatus != 0)
         throw new IllegalThreadStateException();
     group.add(this);
     start0();
     if (stopBeforeStart) {
         stop0(throwableFromStop);
     }
 }

From this code, we can see that the start method is not calling the run() method.

Answer 1

It is stated right in the documentation of start:

the Java Virtual Machine calls the run method of this thread

So, it is the native code in start0 of the JVM that takes care of calling run in the newly created thread. (This is not quite unexpected, as launching a thread is very OS-specific and cannot be implemented in pure Java.)

Note: start0 does not call run directly. Instead (on a high-level view, ignoring JVM-internal management), it instructs the operating system to create a new thread and let that thread execute run.

Just to clarify, here is a short description of the involved methods:

• start is the high-level function to start a new Thread.

• start0 is the native method which creates a new Thread from the operating system and is responsible to ensure that run is called.

• run is the method defined in your Runnable classes. This method is what will be executed in the new thread. A Thread object in Java itself has no idea about the user code it should execute. This is the responsibility of the associated Runnable object.

Thus, when you call Thread.start(), the run method of the Runnable will automatically be called.

Of course, you can always call the run method of a Runnable explicitly:

HelloRunnable hr = new HelloRunnable();
hr.run();

However, this will, of course, not be executed in a separate thread, but block the execution.