How Thread run starts?

Question

I was looking a small example on Threads.For creating Threads we can do in 2 ways either by implementing Runnable interface or by extending Thread.I used the 1st way

package test;

public class test implements Runnable{
    public static void main(String args[])
    {
        test t=new test();
        t.run();Thread th=Thread.currentThread();
        th.start();
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        System.out.println("hi");
    }
}

My doubt is when we are calling th.start(); then run() is called.I want to know how.I thought internally there start() may be calling run() so I looked in the documentation of Thread class

The following is the start() declaration in Thread class

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();

    /* Notify the group that this thread is about to be started
     * so that it can be added to the group's list of threads
     * and the group's unstarted count can be decremented. */
    group.add(this);

    boolean started = false;
    try {
        start0();
        started = true;
    } finally {
        try {
            if (!started) {
                group.threadStartFailed(this);
            }
        } catch (Throwable ignore) {
            /* do nothing. If start0 threw a Throwable then
              it will be passed up the call stack */
        }
    }
}

As you can see inside start(),run() is not called but when we are calling th.start() then automatically overriden run() is called.Can anybody please throw some light in this

Answer 1

The mechanism whereby the run method is invoked on a new thread is extralinguistic: it cannot be represented in terms of Java code. This is the crucial line in the start method:

    start0();

start0 is a native method whose invocation will:

• cause a new native thread-of-execution to be created;
• cause the run method to be invoked on that thread.

Answer 2

 Thread th=Thread.currentThread();
    th.start();// its call run method automatically

 if you call direct run method with class object than its treat as a normal method not thread method

start() method of Thread class is used to start a newly created thread. It performs following tasks: A new thread starts(with new callstack). The thread moves from New state to the Runnable state. When the thread gets a chance to execute, its target run() method will run.