Java: Thread start() vs. run()

In Java, the difference between start() and run() in threading is crucial for creating and managing concurrent processes correctly.

start()

The start() method is used to begin the execution of a new thread. When you call start() on a thread, it initiates a new, separate call stack and invokes the run() method of the Thread object in that new thread. This approach is what enables true concurrent execution of code.

Here’s a quick example:

class MyThread extends Thread {
    public void run() {
        System.out.println("Thread is running in a separate call stack.");
    }

    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.start(); // Creates a new thread and calls run() in that new thread.
    }
}

Explanation:

• Calling start() creates a new thread, where the run() method’s code is executed independently.
• This is the correct way to start a new thread because it takes advantage of Java’s multithreading capabilities.

run()

The run() method, on the other hand, contains the code that defines what the thread will do when it is executed. However, if you call run() directly, it will not start a new thread; instead, it will execute the run() method in the current thread’s call stack, just like a regular method call.

Here’s what happens when you call run() directly:

class MyThread extends Thread {
    public void run() {
        System.out.println("Running within the main thread, not a separate thread.");
    }

    public static void main(String[] args) {
        MyThread t = new MyThread();
        t.run(); // Calls run() in the current (main) thread, not a new one.
    }
}

Explanation:

• Calling run() directly does not create a new thread; it simply executes the run() method in the calling (e.g., main) thread.
• This approach does not utilize Java’s threading capabilities and is generally incorrect if the intention is to create a concurrent process.

Key Differences

Aspect	start()	run()
Thread Creation	Creates a new thread and runs run() in it	Does not create a new thread; runs in the current thread
Purpose	Enables concurrent execution	Runs like a regular method
Use Case	Use start() to initiate a new thread	Rarely used alone, but run() contains the logic to be executed

Conclusion

In multithreading with Java, you should almost always use start() to begin a thread, as it allows run() to execute in a new thread. Directly calling run() is like calling any other method and will not result in parallel processing.