Is it legal to call the start method twice on the same Thread?

Question

The following code leads to "java.lang.IllegalThreadStateException: Thread already started." the second time it is run through on the program.

updateUI.join();    

if (!updateUI.isAlive()) 
    updateUI.start();

This happens the second time updateUI.start() is called. I've stepped through it multiple times and the thread is called and completly runs to completion before hitting updateUI.start().

Calling updateUI.run() avoids the error but causes the thread to run in the UI thread (the calling thread, as mentioned in other posts on SO), which is not what I want.

Can a Thread be started only once? If so than what do I do if I want to run the thread again? This particular thread is doing some calculation in the background, if I don't do it in the thread than it's done in the UI thread and the user has an unreasonably long wait.

Answer 1

From the Java API Specification for the Thread.start method:

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

Furthermore:

Throws:
IllegalThreadStateException - if the thread was already started.

So yes, a Thread can only be started once.

If so than what do I do if I want to run the thread again?

If a Thread needs to be run more than once, then one should make an new instance of the Thread and call start on it.

Answer 2

It is as you said, a thread cannot be started more than once.

Straight from the horse's mouth: Java API Spec

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

If you need to re-run whatever is going on in your thread, you will have to create a new thread and run that.

Answer 3

The just-arrived answer covers why you shouldn't do what you're doing. Here are some options for solving your actual problem.

This particular thread is doing some calculation in the background, if I don't do it in the thread than it's done in the UI thread and the user has an unreasonably long wait.

Dump your own thread and use AsyncTask.

Or create a fresh thread when you need it.

Or set up your thread to operate off of a work queue (e.g., LinkedBlockingQueue) rather than restarting the thread.

Answer 4

Exactly right. From the documentation:

It is never legal to start a thread more than once. In particular, a thread may not be restarted once it has completed execution.

In terms of what you can do for repeated computation, it seems as if you could use SwingUtilities invokeLater method. You are already experimenting with calling run() directly, meaning you're already thinking about using a Runnable rather than a raw Thread. Try using the invokeLater method on just the Runnable task and see if that fits your mental pattern a little better.

Here is the example from the documentation:

 Runnable doHelloWorld = new Runnable() {
     public void run() {
         // Put your UI update computations in here.
         // BTW - remember to restrict Swing calls to the AWT Event thread.
         System.out.println("Hello World on " + Thread.currentThread());
     }
 };

 SwingUtilities.invokeLater(doHelloWorld);
 System.out.println("This might well be displayed before the other message.");

If you replace that println call with your computation, it might just be exactly what you need.

EDIT: following up on the comment, I hadn't noticed the Android tag in the original post. The equivalent to invokeLater in the Android work is Handler.post(Runnable). From its javadoc:

/**
 * Causes the Runnable r to be added to the message queue.
 * The runnable will be run on the thread to which this handler is
 * attached.
 *
 * @param r The Runnable that will be executed.
 *
 * @return Returns true if the Runnable was successfully placed in to the
 *         message queue.  Returns false on failure, usually because the
 *         looper processing the message queue is exiting.
 */

So, in the Android world, you can use the same example as above, replacing the Swingutilities.invokeLater with the appropriate post to a Handler.

Answer 5

What you should do is create a Runnable and wrap it with a new Thread each time you want to run the Runnable. It would be really ugly to do but you can Wrap a thread with another thread to run the code for it again but only do this is you really have to.