Java · Theory

Thread Life Cycle in Java

Theory

108/270

Thread Life Cycle in Java

A thread goes through states: New (created) → Runnable (start() called, waiting for CPU) → Running (CPU is executing it) → Blocked/Waiting (sleep, join, lock) → Terminated (finished).

You cannot restart a terminated thread. start() only from New. Calling start() twice throws IllegalThreadStateException.

Draw the states in viva. Don’t invent extra states.

Let's take this on the board with one tiny Main class — no extra files. Thread Life Cycle in Java — start() schedules the thread; main also prints — order may vary. Start from main, go line by line, and stop at each print. That output is the proof for Thread Life Cycle.

Diagram

New  --start()-->  Runnable
                       │ CPU
                       ▼
                    Running
                   /       \
           sleep/join      done
              │              │
              ▼              ▼
          Waiting        Terminated

Exam tip

Draw five states. Say start() only once. Cannot restart a dead thread.

Example

// Thread demo
public class Main {
  public static void main(String[] args) {
    Thread t = new Thread(() -> System.out.println("Hi from thread"));
    t.start();
    System.out.println("Hi from main");
  }
}

Thread Life Cycle in Java — start() schedules the thread; main also prints — order may vary.

Short notes

DefNew → Runnable → Running → Waiting/Blocked → Terminated.
Rulestart() from New. Cannot restart after Terminated.
Trapstart() twice → IllegalThreadStateException.
Remembersleep/join/lock can move a thread to waiting.

Java · Theory

Thread Life Cycle in Java

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Theory

108/270

Thread Life Cycle in Java

A thread goes through states: New (created) → Runnable (start() called, waiting for CPU) → Running (CPU is executing it) → Blocked/Waiting (sleep, join, lock) → Terminated (finished).

You cannot restart a terminated thread. start() only from New. Calling start() twice throws IllegalThreadStateException.

Draw the states in viva. Don’t invent extra states.

Diagram

New  --start()-->  Runnable
                       │ CPU
                       ▼
                    Running
                   /       \
           sleep/join      done
              │              │
              ▼              ▼
          Waiting        Terminated

Exam tip

Draw five states. Say start() only once. Cannot restart a dead thread.

Example

// Thread demo
public class Main {
  public static void main(String[] args) {
    Thread t = new Thread(() -> System.out.println("Hi from thread"));
    t.start();
    System.out.println("Hi from main");
  }
}

Thread Life Cycle in Java — start() schedules the thread; main also prints — order may vary.

Short notes

DefNew → Runnable → Running → Waiting/Blocked → Terminated.
Rulestart() from New. Cannot restart after Terminated.
Trapstart() twice → IllegalThreadStateException.
Remembersleep/join/lock can move a thread to waiting.

Thread Life Cycle in Java

Example

Short notes

Questions

Thread Life Cycle in Java

Example

Short notes

Questions