Thread.start() vs Runnable.run()

Difference 1: Thread.start() execute the run() method in separate thread.

Thread start() method create new thread, which will call the run method internally. But when you execute 'run()' method of Runnable object directly, then it execute by the same thread that invokes run() method.

Example

Runnable runnable1 = .....;

Runnable runnable2 = .....;

When you call run method of Runnables runnable1, runnable2 directly from the main method, then these methods get executed by main thread.

App.java

package com.sample.app;

public class App {

	public static void main(String args[]) {
		Runnable runnable1 = () -> {
			Thread currentThread = Thread.currentThread();

			System.out.println("Runnable 1 is executed by : " + currentThread.getName());
		};

		Runnable runnable2 = () -> {
			Thread currentThread = Thread.currentThread();

			System.out.println("Runnable 2 is executed by : " + currentThread.getName());
		};

		runnable1.run();
		runnable2.run();

	}

}

Output

Runnable 1 is executed by : main
Runnable 2 is executed by : main

Passing Runnables to a thread

Thread thread1 = new Thread(runnable1);

Thread thread2 = new Thread(runnable2);

When you call thread1.start(), it starts a new thread and calls the run() method of runnable runnable1 within the new thread.

App.java

package com.sample.app;

public class App {

	public static void main(String args[]) {
		Runnable runnable1 = () -> {
			Thread currentThread = Thread.currentThread();

			System.out.println("Runnable 1 is executed by : " + currentThread.getName());
		};

		Runnable runnable2 = () -> {
			Thread currentThread = Thread.currentThread();

			System.out.println("Runnable 2 is executed by : " + currentThread.getName());
		};

		Thread thread1 = new Thread(runnable1);
		Thread thread2 = new Thread(runnable2);

		thread1.start();
		thread2.start();

	}

}

Output

Runnable 1 is executed by : Thread-0
Runnable 2 is executed by : Thread-1

Difference 2: run() method can be called any number of times, but start() method can be called atmost once.

If you call start() method of already started thread, then IllegalThreadStateException is thrown.

App.java

package com.sample.app;

public class App {

	public static void main(String args[]) {
		Runnable runnable1 = () -> {
			Thread currentThread = Thread.currentThread();

			System.out.println("Runnable 1 is executed by : " + currentThread.getName());
		};

		runnable1.run();
		runnable1.run();

		Thread thread1 = new Thread(runnable1);

		thread1.start();
		thread1.start();

	}

}

Output

Runnable 1 is executed by : main
Runnable 1 is executed by : main
Runnable 1 is executed by : Thread-0
Exception in thread "main" java.lang.IllegalThreadStateException
	at java.lang.Thread.start(Thread.java:708)
	at com.sample.app.App.main(App.java:18)

Difference 3: Most of the code of Thread start() method written in native language.

When you see the definition of start() method of Thread class, it looks like below.

public synchronized void start() {
		if (threadStatus != 0)
			throw new IllegalThreadStateException();

		group.add(this);

		boolean started = false;
		try {
			start0();
			started = true;
		} finally {
			try {
				if (!started) {
					group.threadStartFailed(this);
				}
			} catch (Throwable ignore) {

			}
		}
}

private native void start0();

As you see above definition, actual logic of thread starting (start0()) method is written in native language. Whereas definition of run method will be written in pure Java.

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