Java - IllegalThreadStateException

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The Java IllegalThreadStateException is thrown when a thread is not in an appropriate state for the requested operation. The IllegalThreadStateException is a runtime exception that is thrown when a thread is not in an appropriate state for the requested operation.

Following are the reason when JVM throws an IllegalThreadStateException in Java:

• When a thread is not in an appropriate state for the requested operation, JVM throws an IllegalThreadStateException.
• When a thread is not in a new state and you try to start it, JVM throws an IllegalThreadStateException.

Constructors of IllegalThreadStateException

There are two constructors of IllegalThreadStateException class:

• IllegalThreadStateException(): This constructor is used to create an IllegalThreadStateException object without any message.
• IllegalThreadStateException(String message): This constructor is used to create an IllegalThreadStateException object with a message.

Methods of IllegalThreadStateException

There are some methods of IllegalThreadStateException class:

Method	Description
getMessage()	It is used to return the message of the exception.
toString()	It is used to return the detail message string of the exception.
printStackTrace()	It is used to print the stack trace of the exception.

Example of IllegalThreadStateException

In this example, we are trying to start a thread that is already started, so JVM will throw an IllegalThreadStateException.

public class IllegalThreadStateExceptionExample {

   public static void main(String[] args) {
      Thread thread = new Thread();

         thread.start();  //  First start - Works fine
         thread.start();  // Second start - Throws IllegalThreadStateException 
   }
}

Output

Following is the output of the above code:

Exception in thread "main" java.lang.IllegalThreadStateException
	at java.base/java.lang.Thread.start(Thread.java:1525)
	at IllegalThreadStateExceptionExample.main(IllegalThreadStateExceptionExample.java:7)

As you can see in the output, JVM throws an IllegalThreadStateException because we are trying to start a thread that is already started.

Handling IllegalThreadStateException

In this example, we are trying to start a thread that is already started, so JVM will throw an IllegalThreadStateException. We are handling this exception using try-catch block.

public class IllegalThreadStateExceptionExample {

   public static void main(String[] args) {
      Thread thread = new Thread();

      try {
         thread.start();  //  First start - Works fine
         thread.start();  // Second start - Throws IllegalThreadStateException
      } catch (IllegalThreadStateException e) {
         System.out.println("IllegalThreadStateException: A thread cannot be restarted once started!");
      }
   }
}

Output

Following is the output of the above code:

IllegalThreadStateException: A thread cannot be restarted once started!

How to avoid IllegalThreadStateException?

There are some ways to avoid IllegalThreadStateException in Java:

• Before starting a thread, check if it is already started or not.
• Always check the state of the thread before starting it.

By following the above ways, you can avoid the IllegalThreadStateException in Java.

Let's see an example to avoid IllegalThreadStateException:

public class IllegalThreadStateExceptionExample {

   public static void main(String[] args) {
      Thread thread = new Thread();

      if(!thread.isAlive()) {
         thread.start();
      }
   }
}

Output

Following is the output of the above code:

As you can see in the output, we are checking the state of the thread before starting it, so JVM does not throw an IllegalThreadStateException.