Question

Here is a snippet from a little program I am making using threads.

JOptionPane.showMessageDialog(null, "Before: " + thread.isAlive());
if (!thread.isAlive()) {
    JOptionPane.showMessageDialog(null, "Thread is not alive.");
    thread.start();
}
JOptionPane.showMessageDialog(null, "After: " + thread.isAlive());

This code is activated using a button. When I press the button the first time, I correctly get “Before: false” and then “After: true”.
When I press the button again, I incorrectly get “Before: false” and then “After: true”, but expect Before: true, since I am not destroying the thread or overriding the variable.

I believe that this is what is causing IllegalStateException which I am getting (correct me if I am wrong on that too!)

Can anyone explain to me what I am doing wrong?

EDIT:

public class SomeClass extends Applet
{

private ClassThatExtendsThread thread;

public void init()
{
    super.init();

    //Some UI elements are created here.

    thread = new ClassThatExtendsThread (/*there are some parameters*/);
} 

Answer 1

Once a thread has completed running it is considered dead. Calling isAlive on the same thread at this point will always return a result of false. The JavaDoc for Thread does mention this:

public final boolean isAlive()

Tests if this thread is alive. A thread is alive if it has been started and has not
yet died.

If you are not re-instantiating your thread instance in-between calls to your code snippet then you will definitely get an IllegalStateException. This is because you attempt to start a thread that has already been terminated:

if (!thread.isAlive()) {
    JOptionPane.showMessageDialog(null, "Thread is not alive.");
    thread.start();
}

For future reference, note that you can query a thread state via the getState method, which should help with analyzing errors, at the least.