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Thread Lifecycle

During its lifetime, a thread in Java undergoes various states from its creation to its termination. Understanding these states is critical for writing robust multithreaded applications.

According to the java.lang.Thread.State enum, a thread can be in one of the following states:

  1. NEW
  2. RUNNABLE
  3. BLOCKED
  4. WAITING
  5. TIMED_WAITING
  6. TERMINATED

Visualizing the Lifecycle​

The following state diagram illustrates how a thread transitions between different states:

Detailed Explanation of States​

1. NEW​

A thread is in the New state when an instance of the Thread class is created, but the start() method has not yet been invoked on it.

  • At this point, the thread is an empty object; no system resources have been allocated.

2. RUNNABLE​

When the start() method is called, the thread enters the Runnable state.

  • In this state, the thread is either currently executing (Running) or is ready to run and waiting for its turn to get CPU time from the thread scheduler.
  • Java does not distinguish between a "Ready" state and a "Running" state; both are considered RUNNABLE.

3. BLOCKED​

A thread enters the Blocked state when it is trying to enter a synchronized block or method, but the lock for that block/method is currently held by another thread.

  • The thread will remain in the blocked state until it successfully acquires the monitor lock.

4. WAITING​

A thread is in the Waiting state when it is waiting indefinitely for another thread to perform a particular action.

  • A thread enters this state by calling methods like Object.wait() (without timeout) or Thread.join() (without timeout).
  • It will leave this state and return to RUNNABLE only when another thread calls notify() or notifyAll() on the same object, or when the thread it is joined with finishes execution.

5. TIMED_WAITING​

A thread enters the Timed Waiting state when it is waiting for another thread to perform an action for up to a specified waiting time.

  • A thread enters this state by calling methods like Thread.sleep(milliseconds), Object.wait(milliseconds), or Thread.join(milliseconds).
  • It returns to RUNNABLE when the time expires or the specific condition is met (like a notification).

6. TERMINATED (Dead)​

A thread enters the Terminated state when it has finished its execution (i.e., its run() method has completed normally) or if it was terminated abruptly due to an unhandled exception.

  • Once a thread is terminated, it cannot be restarted. Calling start() on a dead thread throws an IllegalThreadStateException.