Thread Lifecycle & State Transitions

1Concept

A Java thread transitions across 6 states defined in `java.lang.Thread.State`: 1. `NEW` (Created, not yet started); 2. `RUNNABLE` (Executing in JVM or waiting for OS CPU scheduling); 3. `BLOCKED` (Waiting to acquire an intrinsic monitor lock); 4. `WAITING` (Waiting indefinitely via `wait()` or `join()`); 5. `TIMED_WAITING` (Waiting with timeout via `sleep(ms)`); 6. `TERMINATED` (Completed).

2Architecture Diagram

NEW --(.start())--> RUNNABLE <-----------------------------------+
                       |                                           |
         +-------------+-------------+                             |
         v                           v                             |
      BLOCKED                     WAITING / TIMED_WAITING          |
 (Waiting for monitor)      (wait(), sleep(), join())              |
         |                           |                             |
         +---------------------------+-----------------------------+
                       |
                       v
                  TERMINATED

3Code Example

public class ThreadStateInspectionDemo {
    public static void main(String[] args) throws InterruptedException {
        Thread worker = new Thread(() -> {
            try {
                Thread.sleep(100);
            } catch (InterruptedException ignored) {}
        });

        System.out.println("State after instantiation: " + worker.getState());
        worker.start();
        System.out.println("State immediately after start: " + worker.getState());

        Thread.sleep(20);
        System.out.println("State while sleeping: " + worker.getState());

        worker.join();
        System.out.println("State after completion: " + worker.getState());
    }
}

4Expected Output

State after instantiation: NEW
State immediately after start: RUNNABLE
State while sleeping: TIMED_WAITING
State after completion: TERMINATED

5Key Takeaways

• ✓A terminated thread cannot be restarted; calling `.start()` on a dead thread throws `IllegalThreadStateException`.
• ✓`Thread.sleep()` does NOT release acquired monitor locks.
• ✓Thread dumps (`jstack <pid>`) report exact states of all threads for deadlock diagnostics.