JAVA EE: Java Tutorial : Java Threads (Life cycle of a Thread in Java

Tuesday, 29 November 2016

Java Tutorial : Java Threads (Life cycle of a Thread in Java | Java thread life cycle_V1)

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Introduction to Thread Life Cycle in Java

In Java multithreading, a thread always exists in any one of several defined states. From the moment it is created until it finishes execution, the life cycle of a thread is strictly controlled and managed by the Java Virtual Machine (JVM) and the thread scheduler.

The 5 Core States of a Java Thread

1. New State

A thread is in the New state when you create an instance of the Thread class (or subclass), but before the start() method is called. At this stage, the thread is not yet alive in the scheduling queue.

2. Runnable State

Once the start() method is invoked, the thread transitions to the Runnable state. It is ready for execution and waiting for the thread scheduler to pick it up.

3. Running State

When the thread scheduler selects the thread from the runnable pool, it enters the Running state and begins executing the code inside its run() method.

4. Blocked / Non-Runnable State

A thread is still alive but temporarily inactive and not eligible to run. It enters this state due to:

Calling Thread.sleep()
Waiting for I/O operations to finish
Waiting for a lock / resource (synchronization block)
Calling wait() or suspend()

Once the sleep time expires, I/O completes, the lock becomes available, or a notification is received (notify()/notifyAll()), the thread transitions back to the Runnable state.

5. Terminated (Dead) State

A thread enters the Terminated state when its run() method finishes execution or if an unhandled exception terminates it. Once terminated, the thread cannot be restarted.

Summary of State Transitions

New → Runnable → Running → Blocked/Waiting → Runnable → Running → Terminated