CODEWITHSUNDEEP
javamultithreading
Android Killer
Android Killer

Reputation: 18489

Running Thread by calling start() and run(), what is the difference?

It may be a basic question, i was confused with this,

in one file i have like this :

public class MyThread extends Thread {
    @Override
    public void run() {
        //stuffs
    }
}

now in another file i have this :

public class Test {
    public static void main(String[] args) {
        Thread obj = new MyThread();

        //now cases where i got confused
        //case 1
        obj.start();   //it makes the run() method run
        //case 2
        obj.run();     //it is also making run() method run
    }
}

so in above what is the difference between two cases, is it case 1 is creating a new thread and case 2 is not creating a thread ? that's my guess...hope for better answer SO guys.Thanks

Upvotes: 2

Views: 955

Answers (7)

user207421
user207421

Reputation: 310907

start() starts the thread. run() just runs the code, in the current thread.

Upvotes: 1

Narendra Pathai
Narendra Pathai

Reputation: 41945

In one line directly calling run() is synchronous (your code will block until run() returns) and calling start() (your code will not wait for run to complete as it is called in other thread obj) is asynchronous.

When you directly use start() method, then the thread will run once using the Runnable instance you provided to it and then the thread will be unusable.

But to leverage the Thread Pooling and Scheduling capabilities that are inbuilt in Java, extending a Runnable or Callable is the way to go.

Upvotes: 1

Corbin
Corbin

Reputation: 33447

start() runs the code in run() in a new thread. Calling run() directly does not execute run() in a new thread, but rather the thread run() was called from.

If you call run() directly, you're not threading. Calling run() directly will block until whatever code in run() completes. start() creates a new thread, and since the code in run is running in that new thread, start() returns immediately. (Well, technically not immediately, but rather after it's done creating the new thread and kicking it off.)

Also, you should be implementing runnable, not extending thread.

Upvotes: 18

Nandkumar Tekale
Nandkumar Tekale

Reputation: 16158

  • run() method after calling start() : this is executed by your thread which is created by you, it is allocated processor to run independently.

  • run() method called by you : executed from your calling thread.

Upvotes: 2

T.J. Crowder
T.J. Crowder

Reputation: 1074335

Calling start starts the thread. It does the underlying work to create and launch the new thread, and then calls run on that new thread.

Calling run just calls the run method, on the current thread. You never call run directly, use start.

Upvotes: 2

Brian Dilley
Brian Dilley

Reputation: 4006

the simple answer to your question is this:

run(): runs the code in the run() method, blocking until it's complete

start(): returns immediately (without blocking) and another thread runs the code in the run() method

Upvotes: 2

Sashi Kant
Sashi Kant

Reputation: 13465

Calling start() will create a new execution thread, and then the run() will be executed in the newly created thread

where as directly calling run() will execute the code in the current thread

Upvotes: 2

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