Thread Lifecycle

In the JVM, a thread has a defined lifecycle, transitioning through various states represented by the Thread.State enum. Recognizing what causes a thread to move from one state to another is vital for debugging resource bottlenecks and locks.

The Six Thread States

The Thread.State enum defines the following six states:

• NEW: A thread object has been instantiated but start() has not yet been called.
• RUNNABLE: The thread is actively executing in the JVM or is ready and waiting for CPU allocation from the OS scheduler.
• BLOCKED: The thread is waiting to acquire an monitor/intrinsic lock to enter a synchronized method or block.
• WAITING: The thread is waiting indefinitely for another thread to perform a specific action (e.g. calling object.wait() or thread.join()).
• TIMED_WAITING: The thread is waiting for another thread for up to a specified waiting time (e.g. calling Thread.sleep(ms), object.wait(ms), or thread.join(ms)).
• TERMINATED: The thread has completed its execution (its run() method has finished).

Code Walkthrough

This program demonstrates how to query and print a thread's state at various points in its lifecycle.

public class ThreadLifecycleDemo {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(() -> {
            try {
                // TIMED_WAITING state demonstration
                Thread.sleep(500);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });
System.out.println("State after creation: " + thread.getState()); // NEW
thread.start();
        System.out.println("State after start(): " + thread.getState()); // RUNNABLE
Thread.sleep(100);
        System.out.println("State during sleep(): " + thread.getState()); // TIMED_WAITING
thread.join(); // Wait for completion
        System.out.println("State after execution completes: " + thread.getState()); // TERMINATED
    }
}

Interview-Relevant Information