What's the difference between Thread start() a

Question

Say we have these two Runnables:

class R1 implements Runnable {
    public void run() { … }
    …
}

class R2 implements Runnable {
    public void run() { … }
    …
}

Then what's the difference between this:

public static void main() {
    R1 r1 = new R1();
    R2 r2 = new R2();

    r1.run();
    r2.run();
}

And this:

public static void main() {
    R1 r1 = new R1();
    R2 r2 = new R2();
    Thread t1 = new Thread(r1);
    Thread t2 = new Thread(r2);

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

Answer 1

If you directly call run() method, you are not using multi-threading feature since run() method is executed as part of caller thread.

If you call start() method on Thread, the Java Virtual Machine will call run() method and two threads will run concurrently - Current Thread (main() in your example) and Other Thread (Runnable r1 in your example).

Have a look at source code of start() method in Thread class

 /**
     * Causes this thread to begin execution; the Java Virtual Machine
     * calls the <code>run</code> method of this thread.
     * <p>
     * The result is that two threads are running concurrently: the
     * current thread (which returns from the call to the
     * <code>start</code> method) and the other thread (which executes its
     * <code>run</code> method).
     * <p>
     * It is never legal to start a thread more than once.
     * In particular, a thread may not be restarted once it has completed
     * execution.
     *
     * @exception  IllegalThreadStateException  if the thread was already
     *               started.
     * @see        #run()
     * @see        #stop()
     */
    public synchronized void start() {
        /**
         * This method is not invoked for the main method thread or "system"
         * group threads created/set up by the VM. Any new functionality added
         * to this method in the future may have to also be added to the VM.
         *
         * A zero status value corresponds to state "NEW".
         */
        if (threadStatus != 0)
            throw new IllegalThreadStateException();
        group.add(this);
        start0();
        if (stopBeforeStart) {
            stop0(throwableFromStop);
        }
    }

    private native void start0();

In above code, you can't see invocation to run() method.

private native void start0() is responsible for calling run() method. JVM executes this native method.

Answer 2

Thread.start() code registers the Thread with scheduler and the scheduler calls the run() method. Also, Thread is class while Runnable is an interface.