Java Thread Lifecycle: The 6 Thread States & Transitions Explained

At any point during execution, every Java thread resides in exactly one of the six states defined in the java.lang.Thread.State enum.

Figure 11: Java Thread Lifecycle: Transition triggers between NEW, RUNNABLE, BLOCKED, WAITING, and TERMINATED.

The 6 Thread States Explained

• 1. NEW: The thread object has been instantiated (new Thread()), but start() has not yet been called.
• 2. RUNNABLE: The thread is executing or ready to execute in the JVM, actively managed by the OS thread scheduler.
• 3. BLOCKED: The thread is paused waiting to acquire a monitor lock to enter a synchronized block or method.
• 4. WAITING: The thread is waiting indefinitely for another thread to perform a specific action (e.g. Object.wait() or Thread.join() without timeout).
• 5. TIMED_WAITING: The thread is sleeping or waiting up to a specified maximum duration (e.g. Thread.sleep(1000), wait(5000)).
• 6. TERMINATED: The thread has finished executing its run() method or was aborted by an uncaught exception.

Synchronizing Threads with Thread.join()

public class ThreadJoinDemo {
    public static void main(String[] args) throws InterruptedException {
        Thread worker = new Thread(() -> {
            System.out.println("Worker: Downloading large database backup...");
            try { Thread.sleep(2000); } catch (Exception e) {}
            System.out.println("Worker: Download finished!");
        });

        worker.start();

        System.out.println("Main: Waiting for worker to finish...");
        worker.join(); // Main thread pauses until worker terminates!

        System.out.println("Main: Resuming. All data downloaded successfully.");
    }
}