Master Java Thread Lifecycle: From NEW to TERMINATED Explained

Java thread lifecycle is crucial for multithreaded development and a common interview question.

Thread Lifecycle Overview

Diagram shows all states.

States: NEW, RUNNABLE, WAITING, TIMED_WAITING, BLOCKED, TERMINATED.

NEW – initial state, thread created but not started.

RUNNABLE – thread ready to run or executing.

WAITING – waiting indefinitely for another thread.

TIMED_WAITING – waiting with a timeout.

BLOCKED – waiting to acquire a lock.

TERMINATED – thread has finished execution.

These states can be grouped into normal flow and abnormal situations.

Normal flow: NEW → RUNNABLE → TERMINATED.

Abnormal situations: thread may enter WAITING, TIMED_WAITING, or BLOCKED before returning to RUNNABLE.

NEW

A thread is in NEW state after creation and before start() is called.

Example:

Thread t = new MyThread();

RUNNABLE

After calling start(), the thread moves from NEW to RUNNABLE.

In RUNNABLE, the thread may be executing or waiting for CPU time allocated by the JVM scheduler.

Example:

Thread t = new MyThread();
t.start();

WAITING

A thread enters WAITING when it calls methods such as Object.wait(), Thread.join(), or LockSupport.park() without a timeout.

Example: threadA calls threadB.join() and waits until threadB finishes.

TIMED_WAITING

Similar to WAITING but with a time limit, entered via methods like Thread.sleep(), Object.wait(long), Thread.join(long), LockSupport.parkNanos(), or LockSupport.parkUntil().

BLOCKED

A thread is BLOCKED when it attempts to enter a synchronized block or method that is already held by another thread.

Example: threadA holds synchronized method A(), threadB trying to call A() becomes BLOCKED.

TERMINATED

The thread has completed execution, either normally or due to an exception.

Summary

The lifecycle diagram shows the normal path NEW → RUNNABLE → TERMINATED and the possible abnormal paths involving WAITING, TIMED_WAITING, and BLOCKED, after which the thread returns to RUNNABLE before termination.