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IllegalThreadStateException comes in two situations -:

• Calling a start() on the thread when it's already executing run() method -:
  When a thread is already running and executing its functions inside the run() method and amidst its execution, a call to the start() method on the same thread, leads to an IllegalThreadStateException.




• Calling start() on thread when it has finished executing run() method -:
  Once a thread finishes its execution, it cannot be started again, i.e. it is just dead. Hence, when a thread has already finished executing the run() method, calling the start() method on the same dead thread object(because someone feels it will restart a dead thread), leads to an IllegalTheadStateException.

Calling start() on the thread when it's already executing run() method

In this code, we are trying to create IllegalThreadStateException by calling start() method on the thread object, which is already executing its run() method

class ThRun implements Runnable		    //implementing the Runnable interface
{


public void run()  //Entry point of new thread
{
	for(int i=0;i<5;i++)
	{	
		System.out.println(i);	
	}
}


public static void main(String... ar)
{
ThRun newTh= new ThRun();
Thread th= new Thread(newTh, "Thread2");    //Calling Thread's constructor & passing the object
					    //of class that  implemented  Runnable interface
					    //& the name of new thread.

//Stating the thread(Thread2).
th.start();

//Starting thread(Thread2) again, when it's already running, causing IllegalStateException.
th.start();	
			
//This won't be printed because the main thread is halted by IllegalStateException
System.out.println("Main Thread"); 
}
}

Output is - :

0Exception in thread "main"
1
2
3
4java.lang.IllegalThreadStateException

        at java.lang.Thread.start(Unknown Source)
        at ThRun.main(Thread8.java:21)

Program Analysis

• In the main() method, we have created a new thread with the name, Thread2, pointed by th.
• Method call th.start() starts the execution of the Thread2.
• Once again, a method call th.start() in main(), while Thread2 was already executing its run() method, leads to IllegalStateException, which halts the execution of main() method.
• While, Thread2 keeps on performing it's work and finishes executing its run() method and ends.

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Calling start() on thread when it has finished executing run() method

In this code, we are trying to create IllegalThreadStateException by calling start() method on the thread object, which has already finished executing its run() method and its in a dead state.

class ThRun implements Runnable
{


public void run()    //Entry point of new thread
{

	for(int i=0;i<5;i++)
	{	
		System.out.println(i);	
	}
}


public static void main(String... ar)
{
ThRun newTh= new ThRun();
Thread th= new Thread(newTh,"Thread2");   //Calling Thread's constructor & passing the object
					    //of class that  implemented  Runnable interface
					    //& the name of new thread.

//Stating the thread(Thread2).
th.start();
	
try
{
	System.out.println("Main Thread is going to sleep");
	Thread.sleep(3000); //Making main thread sleep for 3000ms / 3 seconds
	System.out.println("Main thread is awakened");
}
catch(Exception e)
{
	System.out.println(e);
}

//Calling start() method on a dead thread, causing IllegalStateException
th.start();

//This won't be printed because the main thread is halted by IllegalStateException
System.out.println("Main Thread"); 
}
}

Output is - :

Main Thread is going to sleep
0
1
2
3
4
Main thread is awakened
Exception in thread "main" java.lang.IllegalThreadStateException
        at java.lang.Thread.start(Unknown Source)
        at ThRun.main(Thread8.java:32)

Program Analysis

• In the main() method, we have created a new thread with the name, Thread2, pointed by th.
• Method call th.start() starts the execution of the Thread2.
• Method call sleep() on the main Thread makes it sleep for 3000ms(in this time Thread2 finishes its run)
• Main thread wakes up and once again a method call th.start() in main(), while Thread2 has already finished its execution, leads to IllegalStateException, which halts the execution of main() method.

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