Interview question

What is Thread Lifecycle in Java? Explain all thread states. Java में Thread Lifecycle क्या है? सभी states explain करें।

Answer

Thread lifecycle consists of five states: New, Runnable, Running, Blocked/Waiting, and Terminated. Understanding state transitions is crucial for multithreading.

State	Description	Transition
New	Thread created but not started	start() called
Runnable	Ready to run, waiting for CPU	Scheduler selects
Running	Currently executing	Context switch
Blocked/Waiting	Waiting for resource/event	Resource available
Terminated	Finished execution	End of run()

public class ThreadLifecycleDemo {
    public static void main(String[] args) {
        Thread t = new Thread(() -> {
            System.out.println('Thread is running');
            try {
                Thread.sleep(2000);  // Waiting state
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println('Thread finished');
        });
        
        System.out.println('State: ' + t.getState());  // NEW
        
        t.start();
        System.out.println('State: ' + t.getState());  // RUNNABLE
        
        try {
            t.join();  // Wait for thread to finish
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        
        System.out.println('State: ' + t.getState());  // TERMINATED
    }
}

// Thread state transitions:
// New -> Runnable: start() called
// Runnable -> Running: Scheduler selects
// Running -> Runnable: Context switch
// Running -> Waiting: sleep(), wait(), join()
// Waiting -> Runnable: notify(), time expires
// Running -> Terminated: run() ends

// Thread.State enum values:
// NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED

Thread States:

1. NEW: Thread create हुआ, start() नहीं हुआ
2. RUNNABLE: CPU के लिए ready
3. RUNNING: Actively executing
4. BLOCKED/WAITING: Resource के लिए wait
5. TERMINATED: Complete हो गया

State Transitions:
NEW -> start() -> RUNNABLE
RUNNABLE -> Scheduler -> RUNNING
RUNNING -> sleep()/wait() -> WAITING
WAITING -> notify()/time -> RUNNABLE
RUNNING -> end() -> TERMINATED

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