Thread Lifecycle

The thread lifecycle in Java represents the various stages a thread goes through from its creation to termination. Here’s a detailed look at each stage in the lifecycle of a thread:

1. New (or Born) State

• Definition: A thread is in this state when an instance of the Thread class or a class implementing the Runnable interface is created.
• Transition: The thread remains in this state until the start() method is called on the thread object.
• Example:

  Thread thread = new Thread();
  // The thread is now in the New state
  

2. Runnable State

• Definition: After the start() method is called, the thread enters the runnable state. A thread in this state is ready to run and is waiting for the CPU to allocate time for its execution.
• Key Points:
  • The thread might not be running immediately after calling start(); it depends on the thread scheduler.
  • The thread can transition between runnable and running states based on the availability of the CPU.
• Example:

  thread.start();
  // The thread is now in the Runnable state, waiting for CPU time
  

3. Running State

• Definition: When the thread scheduler picks the thread from the runnable pool and assigns CPU time, the thread enters the running state and executes its run() method.
• Key Points:
  • The thread stays in this state as long as it has CPU time and does not get preempted or voluntarily give up its time.
  • The actual execution of the run() method happens in this state.
• Example:

  public void run() {
      // Code executed in the Running state
  }
  

4. Blocked/Waiting State

• Definition: A thread enters the blocked or waiting state when it is waiting for some resource or condition to be met.
• Key Points:
  • Blocked: Waiting for a monitor lock to enter a synchronized block/method.
  • Waiting: Explicitly waiting for another thread to perform a particular action (e.g., calling wait() on an object, join() on another thread, or sleep()).
• Example:

  synchronized(someObject) {
      someObject.wait(); // The thread is now in the Waiting state
  }
  

5. Timed Waiting State

• Definition: A thread enters this state when it calls methods like sleep(), wait(long timeout), or join(long timeout) that cause it to wait for a specified period.
• Key Points:
  • The thread automatically transitions back to the runnable state after the specified wait time has elapsed.
• Example:

  Thread.sleep(1000); // The thread is now in the Timed Waiting state for 1 second
  

6. Terminated (or Dead) State

• Definition: A thread enters the terminated state when it has completed its execution or has been explicitly stopped.
• Key Points:
  • Once a thread reaches this state, it cannot be restarted.
  • Typically, a thread enters this state upon returning from the run() method or if the stop() method is called.
• Example:

  public void run() {
      // Thread's task
  }
  // After the run() method completes, the thread is in the Terminated state
  

Summary Diagram of Thread Lifecycle

Here’s a simple visualization of the thread lifecycle:

New (Born)
   |
   v
Runnable <-> Running <-> Blocked/Waiting
                ^
                |
                v
            Timed Waiting
                |
                v
            Terminated

Understanding the thread lifecycle is crucial for managing multithreaded applications effectively. Proper synchronization and resource management can help prevent issues like deadlocks and ensure smooth concurrent execution.