How to Resolve java.lang.IllegalThreadStateException in Java?

What causes java.lang.IllegalThreadStateException in Java and how can it be resolved?

The `java.lang.IllegalThreadStateException` is thrown in Java when a thread is not in an appropriate state to perform a certain action. This typically occurs when there are attempts to start a thread that has already been started or to use a thread that has already completed its execution.

// Example of triggering IllegalThreadStateException
class MyRunnable implements Runnable {
    public void run() {
        System.out.println("Running...");
    }
}

public class ThreadExample {
    public static void main(String[] args) {
        MyRunnable myRunnable = new MyRunnable();
        Thread thread = new Thread(myRunnable);
        thread.start(); // Starting the thread for the first time
        thread.start(); // This will throw IllegalThreadStateException
    }
}

Causes

• Attempting to start a thread that has already been started.
• Invoking methods on a thread that has been finished.
• Calling `join()` on a thread that is not alive.
• Improperly managing thread states, especially in complex thread management scenarios.

Solutions

• Ensure that each thread is started only once by checking its state before calling `start()`.
• Use `Thread.join()` only after the thread has completed execution to avoid illegal operations.
• If reusing a thread, create a new instance instead of trying to restart an existing one.
• Utilize `ThreadFactory` for creating new threads or a thread pool using `Executors`.

Mistake: Calling `start()` on the same thread instance multiple times.

Solution: Create a new thread instance for each separate execution.

Mistake: Not managing thread lifecycles properly, leading to invalid states.

Solution: Track the state of each thread and ensure proper transitions.

Mistake: Forgetting to check if a thread is alive before operations like `join()`.

Solution: Always verify the thread's state before invoking operations.