No, a Java thread can be started only once during its lifetime. After a thread has been started and its execution has either completed or is in progress, attempting to call start() again on the same thread object will result in an IllegalThreadStateException.
Key Points: • A thread object can transition from the New state to the Runnable state only once. • Calling start() more than once on the same thread is not allowed. • Java throws IllegalThreadStateException if a thread is restarted. • To execute the same task again, create a new Thread object. • Once a thread reaches the Terminated state, it cannot be reused.
Why Can't a Thread Be Started Twice?
A thread follows a fixed lifecycle:
NEW | start() | RUNNABLE | RUNNING | TERMINATED
After reaching the Terminated state, the thread's execution is complete. The JVM does not allow moving the thread back to the New state.
Example: Suppose a worker completes its assigned task.
Once the task is finished:
• The thread is terminated. • The same thread object cannot be restarted. • A new thread must be created for a new execution.
Code Example:
public class Demo {
public static void main(String[] args) {
Thread thread = new Thread(() -> {
System.out.println(
"Thread Running");
});
thread.start();thread.start(); // Exception
}
}Output:
Thread Running
Exception in thread "main" java.lang.IllegalThreadStateException
Correct Approach
If the task needs to run again, create a new thread object.
Code Example:
Runnable task = () -> {
System.out.println(
"Task Executed");
};
Thread thread1 =
new Thread(task);
thread1.start();
Thread thread2 =
new Thread(task);
thread2.start();Output:
Task Executed
Task Executed
Real-World Example
Consider a food delivery application.
Thread 1:
• Process Order #101
After completion:
• Thread terminates.
For Order #102:
• Create a new thread.
Do not reuse the old terminated thread.
Common Interview Mistake
Some developers confuse:
thread.start();
with
thread.run();Calling run() directly:
• Does not create a new thread. • Executes in the current thread.
Calling start():
• Creates a new execution path. • Invokes run() internally.
Interview Tip: A concise interview answer is:
"No, a thread in Java cannot be started more than once. Once start() is called, the thread moves through its lifecycle and eventually reaches the Terminated state. Calling start() again on the same thread object throws an IllegalThreadStateException. To execute the task again, a new Thread instance must be created."