如何在具有相同主体的线程内启动新线程？

Question

I want to start a thread within a thread with the same body. Once the new thread is created, I'd like to bypass the line starting up the new thread and run the rest of the code.

The following code is my failure implementation. I expect there is "return" in the output. In fact, only "start" is printed. How can I fix this?

Thanks in advance for the help!

public static void main(String[] args) throws InterruptedException{
    System.out.println("start");
    new Thread(myThread()).start();
    System.out.println("return");
    return;
}

private static Runnable myThread() throws InterruptedException{
    System.out.println("start");
    Thread.sleep(1000);
    new Thread(myThread()).start();
    System.out.println("return");
    return null;
}

Answer 1

First, to create a thead, you pass the constructor a runnable. What you did is trying to pass it the value that myThread() returns, instead of a method reference.

(DO NOT) try this: (it may crash your system as it spawns an infinite amount of threads)

public static void main(String[] args) throws InterruptedException{
    System.out.println("start");
    new Thread(() -> myThread()).start(); // Added () -> 
    System.out.println("return");
}

private static void myThread() throws InterruptedException{
    System.out.println("start");
    Thread.sleep(1000);
    new Thread(() -> myThread()).start(); // Added () -> 
    System.out.println("return");
}

I also made it return void since returning null is pointless at this point.

Then, as pointed out, you need to limit the amount of threads created. For example, if you want two threads:

private static final int numThreads = 0; // added

public static void main(String[] args) throws InterruptedException{
    System.out.println("start");
    new Thread(() -> myThread()).start();
    System.out.println("return");
}

private static void myThread() throws InterruptedException{
    System.out.println("start");
    Thread.sleep(1000);
    if (++numThreads < 2) // added
        new Thread(() -> myThread()).start();

    System.out.println("return");
}

Answer 2

Try this :

public static void main(String[] args) throws InterruptedException{
    System.out.println("start");
    new Thread(myThread()).start();
    System.out.println("return");
    return;
}

static boolean threadStared = false;

private static Runnable myThread() throws InterruptedException{
    System.out.println("start");
    Thread.sleep(1000);
    if(!threadStared){
        new Thread(myThread()).start();
        threadStared = true;
    }
    System.out.println("return");
    return null;
}

Answer 3

I want to start a thread within a thread with the same body. Once the new thread is created, I'd like to bypass the line starting up the new thread and run the rest of the code.

Something like this:

public class Test {
    public class Worker extends Runnable {
        private boolean launchedSubthread = false;

        public void run() {
            if (!launchedSubthread) {
                launchedSubthread = true;
                new Thread(this).start();
            }
            // Now do some stuff.
        }
    }

    public void main(String[] args) {
        new Thread(new Worker()).start();
    }
}

Notice that when we launch the second child thread in the Worker class, we are passing this as the Runnable . So the two child threads will be sharing a single Worker instance as their "body". (I assume that is what you are trying to do.)

In you want the two threads to read or update other variables of the Worker , then you must use volatile or synchronized appropriately. This does not apply to the way that I have used launchedSubthread . This is because there is a happens before between the start() call and the run() call on the newly started thread.

---

There were a couple of problems with your attempt.

1. myThread is misnamed. It is returning a Runnable not a Thread .

2. You weren't doing anything to prevent an infinite chain of creation of MyThread instances.

3. You weren't actually creating any threads. If you look carefully at myThread() , it is recursing (infinitely) before any threads can be created.

4. The myThread() call returns null . If that was actually passed to the Thread(Runnable) constructor, you would get an NPE.