Understanding the Java Thread Life Cycle and Its States

Give a briefing on the life cycle of a thread, I hope you can expand on it from the perspective of Java source code

In Java, a thread is a lightweight process that allows concurrent execution of code. Understanding the life cycle of a thread is crucial for effective multithreading in Java. The life cycle of a thread can be divided into several states, each representing a specific stage in the thread's life. Below is a detailed briefing on the thread life cycle, along with relevant Java source code examples.

Thread Life Cycle States

1. New (Born) State:

   • When a thread is created but not yet started, it is in the New state.
   • A thread enters this state when an instance of the Thread class is created.

   Example:

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

2. Runnable State:

   • A thread enters the Runnable state when the start() method is called. In this state, the thread is ready to run and waiting for CPU time.
   • It can also be in this state if it is waiting to be scheduled by the thread scheduler.

   Example:

   thread.start(); // The thread is now in the Runnable state
   

3. Blocked State:

   • A thread enters the Blocked state when it is trying to acquire a lock that is held by another thread. It cannot proceed until the lock is released.

   Example:

   synchronized (lock) {
       // Thread may enter Blocked state if another thread holds the lock
   }
   

4. Waiting State:

   • A thread enters the Waiting state when it is waiting for another thread to perform a particular action (like notifying or interrupting).
   • This can happen when a thread calls Object.wait(), Thread.join(), or LockSupport.park().

   Example:

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

5. Timed Waiting State:

   • A thread enters the Timed Waiting state when it is waiting for another thread to perform an action for a specified waiting time.
   • This can occur when a thread calls Thread.sleep(milliseconds), Object.wait(milliseconds), or Thread.join(milliseconds).

   Example:

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

6. Terminated State:

   • A thread enters the Terminated state when it has completed its execution or has been terminated due to an exception.
   • Once a thread is in this state, it cannot be restarted.

   Example:

   // After the run() method completes, the thread enters the Terminated state
   

Thread Life Cycle Diagram

The life cycle of a thread can be visualized as follows:

New -> Runnable -> Running -> Terminated
          |         ^
          |         |
          v         |
       Blocked <----|
          |
          v
       Waiting <----|
          |
          v
      Timed Waiting

Summary of State Transitions

• New to Runnable: When start() is called.
• Runnable to Running: When the thread scheduler allocates CPU time to the thread.
• Running to Blocked: When a thread tries to acquire a lock held by another thread.
• Running to Waiting: When a thread calls wait(), join(), or LockSupport.park().
• Running to Timed Waiting: When a thread calls sleep(), wait(milliseconds), or join(milliseconds).
• Blocked/Waiting/Timed Waiting to Runnable: When the condition for waiting is met (e.g., lock is released, notified, or time expires).
• Running to Terminated: When the run() method completes or an unhandled exception occurs.

Conclusion

Understanding the life cycle of a thread in Java is essential for writing efficient multithreaded applications. By managing the states and transitions effectively, developers can optimize resource usage and improve application performance. The Java Thread class and the Runnable interface provide the necessary tools to create and manage threads, allowing for concurrent execution of tasks.