Thread Lifecycle (States of Thread)

A thread in Java goes through several states during its lifecycle. Each state represents what the thread is currently doing. Java officially defines six thread states in the Thread.State enum.

NEW → RUNNABLE → RUNNING → BLOCKED → WAITING → TIMED_WAITING → TERMINATED

Explanation:

1. NEW (Not Started State)

• This is the initial state.
• A thread has been created using the Thread class, but its start() method has not been called yet.
• It exists in memory but has not started executing.

Thread t = new Thread();  // NEW

2. RUNNABLE (Ready-to-Run State)

• After calling start(), the thread enters the RUNNABLE state.
• It is eligible to run but not yet running.
• JVM scheduler will decide when to execute it.

t.start(); // RUNNABLE

3. RUNNING (Actively Executing)

• Thread is actually executing its run() method.
• This is not a separate Java state but part of RUNNABLE in the JVM model.
• Conceptually: RUNNABLE → RUNNING → RUNNABLE.

4. BLOCKED (Waiting for Monitor Lock)

• A thread becomes blocked when it tries to enter a synchronized block or method but another thread already holds the lock.
• It waits until the lock becomes available.

Example:

synchronized(obj) { ... }

5. WAITING (Waiting Indefinitely)

A thread enters the waiting state when it is waiting indefinitely until another thread performs a specific action.

Common causes:

• wait()
• notifyAll()
• join() without timeout
• park() (LockSupport)

The thread remains here until it is explicitly awakened.

Example:

obj.wait();

6. TIMED_WAITING (Wait for a Fixed Time)

This state occurs when a thread is waiting for a specific time duration.

Common causes:

• sleep(1000)
• wait(500)
• join(2000)
• parkNanos() / parkUntil()

After the time expires, the thread becomes runnable again.

Example:

Thread.sleep(1000);

7. TERMINATED (Dead State)

• This is the final state.
• The thread has completed execution of its run() method or exited due to an exception.
• It cannot be restarted.

Simple Lifecycle Diagram

        +---------+
        |  NEW    |
        +----+----+
             |
          start()
             |
        +---------+
        | RUNNABLE|
        +----+----+
             |
       scheduler picks
             |
        +---------+
        | RUNNING |
        +----+----+
       /     |     \\
 lock missing|wait() \\sleep()
           BLOCKED  WAITING  TIMED_WAITING
             \\       |         /
              +------|--------+
                     |
               task completed
                     |
              +-------------+
              | TERMINATED  |
              +-------------+

How to Check Thread State

Java provides a simple method to check the current state:

System.out.println(thread.getState());