Can we start a Thread twice in Java

“Starting a thread twice” in the context of Java multithreading refers to the attempt to initiate the execution of a single thread more than once using the start() method. Java threads have a lifecycle and can’t be restarted after you call start() again once the thread has started or finished execution. As a result, you can only begin to one thread at a time.

What is Start a thread twice

• In Java, initiating a thread twice means attempting to initiate the same thread several times by using the Start() method.
• It’s not allowed in Java and is considered a criminal offence. It initiates thread execution by calling start() on the Thread object.
• Once a thread has started, you cannot start it again using start(). You will result in a java.lang if you attempt to do so. IllegalThreadStateException.
• As I have already said in previous replies, if you need to perform a single task within two distinct threads, it is essential that you create two other Thread objects and begin them on their own.
• If you want to perform identical logic in separate threads at once, each thread has a specific execution context, and you must create your own set of Thread objects.

Example:

public class FirstCode extends Thread {
    public void run() {
        System.out.println("Thread is running...");
    }
    
    public static void main(String[] args) {
        FirstCode thread1 = new FirstCode ();
        FirstCode thread2 = new FirstCode ();
        
        thread1.start();
        thread2.start();
    }
}

Advantages of starting a thread twice

• Starting a thread twice in Java generally does not offer any real advantages. It’s not recommended, and it may cause you to behave in unexpected ways.
• You cannot restart the thread by calling start() again when it starts using the start() method and enters the execution phase, or when it has completed its execution or been terminated. Doing so will result in a Java.lang.IllegalThreadStateException.
• If you have an activity that needs to be executed multiple times in parallel, the right approach is to create a series of instances of the thread class and start each instance independently.
• For each thread, a separate execution context should be provided. It’s not much of an advantage.
• Starting a thread several times might lead to code that doesn’t work well and won’t be remembered.

Disadvantages of starting a thread twice

There are many disadvantages to being able to run the second thread in Java, which can cause problems for your multithreaded application:

• IllegalThreadStateException: As mentioned earlier, attempting to start a thread already running results in java.lang. IllegalThreadStateException. This is an unbounded exception and means you are not following the basic rules of Java thread management.
• Unpredictable behaviour: Starting multiple threads without throwing an exception, for example, by creating a new thread object that corresponds to one of your running threads, can cause unpredictable behaviour. A thread must be in a consistent and predictable state in order to control its internal state.
• Concurrency issues: Running the same thread twice can lead to concurrency issues, including data races, synchronisation issues, and cross-thread interference. The result may be an application that does not match the results you are trying to achieve.
• Code confusion: This could make the code difficult to understand and maintain. Running the same thread repeatedly can confuse the developers reading the code, leading to maintenance problems or introducing bugs.
• Loss of parallelism: The proper way to use multithreading is to run your own parallel execution threads. If you run the same thread multiple times, you will not achieve the significant benefits of parallelism and may negatively affect your application’s performance.

In Java, the difference between Thread.start()+ and Thread.run()+ is as follows:

The most essential methods in the Java multithreading concept are start() and run(). There are several differences between the Thread.start(). and Thread.run() methods, as follows:

New thread creation: A new thread will be created when the program calls the start() method, and then a run is performed. But if we call the running method directly, there’s no new thread to create, so it behaves like a normal method call on the existing calling thread. And none of the multiple readings will take place. For example, let’s see how this works:

class MyThread extends Thread {
    public void run()
    {
        System.out.println("Current thread name: "+ Thread.currentThread().getName());
        System.out.println("run() method called");
    }
}

class FirstCode {
    public static void main(String[] args)
    {
        MyThread t = new MyThread();
        t.start();
    }
}

Output:

Current thread name: Thread-0
run() method called

When calling the start method of a thread class, you create a new thread with your default nameThread0 and call Run() to execute everything in it; as we’ve seen from this example. Instead of calling start()+ method, we want to call run()+ method:

class MyThread extends Thread {
    public void run()
    {
        System.out.println("Current thread name: "
                        + Thread.currentThread().getName());

        System.out.println("run() method called");
    }
}

class FirstCode{
    public static void main(String[] args)
    {
        MyThread t = new MyThread();
        t.run();
    }
}

Output:

Current thread name: main
run() method called

Conclusion

It is essential to start each thread only once to maintain a reliable, clean Multithreading process and ensure appropriate mechanisms for managing its lifecycle. The creation of a robust and efficient multithreaded application depends on correctly synchronising resources, appropriately managing their use, and carefully designing them.