Introduction

• The Thread Life Cycle in Java represents the different states a thread goes through — from creation to termination.
  • New → Runnable → Running → Waiting/Blocked → Dead
• It shows how a thread is managed by the JVM and the Thread Scheduler, depending on operations like start(), sleep(), wait(), or when the thread finishes execution.
• Understanding the life cycle helps developers control thread execution and write efficient multi-threaded programs.

• There are total 5 states in thread life-cycle:-
  1. New (Created)
  2. Runnable (Ready)
  3. Running (Execution)
  4. Non-Runnable (Inactive)
  5. Terminated (Dead)

1. New (Created) State

• In this state, a thread object is created but not yet started.
• The thread is not alive until start() is called.

• MyThread t1 = new MyThread(); // thread created, but not started

  If you directly call run() instead of start(), it executes as a normal method, not a separate thread.

2. Runnable (Ready) State

• After calling start(), the thread moves to Runnable.
• The thread is ready but waiting for CPU allocation.

• t1.start(); // moves from New -> Runnable state

  Thread is managed by the Thread Scheduler.

3. Running (Execution) State

• In this state, the Thread Scheduler selects a thread from Runnable and gives it CPU.
• The thread executes its run() method.

• public void run()
  {
      System.out.println("Thread is running...");
  }

4. Non-Runnable (Inactive) State

• In this state, the thread is temporarily inactive but still alive.
• It can occur due to wait(), sleep(), join(), or waiting for a lock.
• Example:

  Thread.sleep(2000); // timed waiting
  obj.wait();        // waiting
  

5. Dead (Terminated) State

• In this state, run() method finishes and the thread enters Dead state.
• Once the thread has completed its task, it cannot be restarted again using start().

Summary of Thread Life Cycle