Thread start() vs. Runnable run(): What's the Difference in Java Multithreading?

Thread start() vs. Runnable run(): Unraveling the Threading Paradigm

In the realm of Java multithreaded programming, the Thread start() and Runnable run() methods play distinct roles in orchestrating thread execution. Comprehending their differences is crucial for designing effective concurrent applications.

Runnable run() vs. Thread start()

The Runnable interface defines the run() method, which encapsulates the code to be executed on a separate thread. However, invoking run() directly does not create a new thread. Instead, it executes the code within the current thread.

In contrast, invoking Thread start() sets a new thread in motion. It creates a thread object, passing the corresponding Runnable instance to its constructor, and starts its execution. The Runnable's run() method is subsequently invoked within the new thread.

Example: Single vs. Multithreaded Execution

Consider the following code:

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

In this example, r1.run() and r2.run() are called directly, which executes both Runnables in a single thread. There is no thread creation.

Now consider this modified version:

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();
}

In this case, Thread objects are created, encapsulating r1 and r2. When start() is called on these Thread objects, two separate threads are launched, and the Runnables' run() methods are executed within these threads.

Implications and Considerations

Using Thread start() over Runnable run() provides thread-level control. It enables thread creation, scheduling, and resource management. This control is crucial for coordinating multiple tasks concurrently and optimizing performance.

On the other hand, directly invoking run() does not create separate threads, but rather executes the code in the context of the current thread. This approach is suitable for simple, synchronous tasks that do not require thread-level concurrency.

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