Understanding the Difference Between start() and run() Methods in Java Threads

Java's multithreading capabilities are a cornerstone of its power and flexibility. Among the foundational concepts in Java's threading model are the start() and run() methods. These methods are often a source of confusion for beginners. In this article, we will delve deep into the distinction between these two methods, providing clarity for software engineers, full-stack developers, and other professionals in the developer realm.

The Core Difference Between start() and run()

The primary distinction between the start() and run() methods lies in how they operate concerning threads:

• start() Method: When the start() method is invoked, a new thread is spawned, and the code inside the run() method is executed within this new thread. This is the standard way to initiate a new thread in Java.
• run() Method: If you directly call the run() method, no new thread is created. Instead, the code inside the run() method is executed in the current thread, just like any other regular method call.

This difference is crucial. Invoking the run() method directly, in most cases, is considered a programming oversight. The intent is usually to call start() to initiate a new thread. Many static code analysis tools can detect this common error.

Practical Implications of Using start() vs. run()

Thread Creation and Execution

If you have a task that is time-intensive, always opt for the start() method. This ensures that your main thread remains responsive. Directly calling the run() method for such tasks will cause the main thread to be occupied, leading to potential performance issues.

Method Invocation Restrictions

Another notable difference is the restriction on method invocations. You cannot call the start() method more than once on a thread object. Doing so will result in an IllegalStateException. On the other hand, there's no such restriction with the run() method; you can call it multiple times.

Code Illustration: start() vs. run()

Here's a simple code snippet that demonstrates the difference:

public class StartVsRunCall {
    public static void main(String args[]) {
        // Creating two threads for start and run method call
        Thread startThread = new Thread(new Task("start"));
        Thread runThread = new Thread(new Task("run"));

        startThread.start(); // New thread execution
        runThread.run();     // Current thread execution
    }

    private static class Task implements Runnable {
        private String caller;

        public Task(String caller) {
            this.caller = caller;
        }

        @Override
        public void run() {
            System.out.println("Caller: " + caller + " executed by: " + Thread.currentThread().getName());
        }
    }
}

Output:

Caller: start executed by: Thread-0
Caller: run executed by: main

This output clearly shows that the start() method executes in a new thread (Thread-0), while the run() method executes in the main thread.

Best Practices for Using start() and run()

For developers, especially those in roles such as software engineers, full-stack developers, and frontend developers, understanding the intricacies of multithreading in Java is crucial. Here are some best practices to ensure effective and efficient use of the start() and run() methods:

1. Always Use start() for Multithreading

If your intention is to execute code concurrently in a separate thread, always use the start() method. This ensures that the task runs in parallel, leveraging the benefits of multithreading.

2. Use run() for Single Thread Execution

If you don't need parallel execution and want the task to run in the current thread, you can call the run() method directly. However, be cautious and ensure that this is the desired behavior.

3. Avoid Calling start() Multiple Times

As mentioned earlier, invoking start() more than once on a thread object will throw an IllegalStateException. Always ensure that you're not inadvertently calling start() on an already started thread.

4. Monitor Thread Health

For long-running applications, it's essential to monitor the health and status of threads. Tools like Java's built-in JMX (Java Management Extensions) can be invaluable in this regard.

5. Prioritize Thread Safety

When working with multiple threads, always ensure that shared resources are accessed in a thread-safe manner. Utilize synchronization mechanisms provided by Java, such as the synchronized keyword or ReentrantLock, to prevent race conditions.

Advanced Topics Related to Java Threads

For those looking to deepen their understanding of Java threads, here are some advanced topics worth exploring:

• Thread Pools: Learn about the Java Executor framework and how to efficiently manage a pool of worker threads.
• Thread Communication: Dive into inter-thread communication mechanisms like wait(), notify(), and notifyAll().
• ThreadLocal Variables: Understand how to use ThreadLocal to store thread-specific data.
• Deadlocks and Starvation: Grasp the concepts of deadlocks and starvation in multithreading and learn strategies to avoid them.

Conclusion

In essence, the only difference between the start() and run() methods in Java threads is the thread of execution. The start() method spawns a new thread, while the run() method executes in the current thread. It's essential to understand this distinction to effectively harness the power of Java's multithreading capabilities.