Question

Please explain to me why do the following code throw IllegalThreadStateException?

try
{
    if(thread1 != null)
    {
        if(thread1.isAlive()); 
        {
            thread1.interrupt();   //it is ok
            thread1.join();
        }
    }
    if(thread2 != null)
    {
        if(thread2.isAlive());
        {
            thread2.interrupt(); //throw IllegalThreadStateException
            thread2.join();
        }
    }
}
catch(IllegalThreadStateException e)
{
    System.exit(0);
}

It threw IllegalThreadStateException when run statement thread2.interrupt(). But thread1.interrupt() is ok.

Please explain to me.

Many thanks!

Answer 1

The line:

if(thread2.isAlive()); // <-- Note this semicolon well!

is not doing what you think it’s doing 🙂 The code segment:

if(thread2.isAlive());
{
    thread2.interrupt(); //throw IllegalThreadStateException
    thread2.join();
}

will check if the thread is alive and, if so, execute that empty statement before the semicolon. It will then execute the interrupt/join sequence inside the braces no matter what the state of the thread was. That’s because:

{
    doSomething();
}

is a perfectly valid Java construct, even without an if or while preceding it. Change:

if(thread2.isAlive());

to:

if(thread2.isAlive())

(and do the same for the thread1 check as well).

---

And you should also call getMessage() on the exception to find out the specific details, if any.

It may contain extra information for easier root cause analysis.