Lifecycle and States of a Thread in Java

A thread in Java can exist in any one of the following states at any given time. A thread lies only in one of the shown states at any instant:

1. New State
2. Runnable State
3. Blocked State
4. Waiting State
5. Timed Waiting State
6. Terminated State

The diagram below represents various states of a thread at any instant:

Life Cycle of a Thread 

There are multiple states of the thread in a lifecycle as mentioned below:

1. New Thread: When a new thread is created, it is in the new state. The thread has not yet started to run when the thread is in this state. When a thread lies in the new state, its code is yet to be run and has not started to execute.
2. Runnable State: A thread that is ready to run is moved to a runnable state. In this state, a thread might actually be running or it might be ready to run at any instant of time. It is the responsibility of the thread scheduler to give the thread, time to run. A multi-threaded program allocates a fixed amount of time to each individual thread. Each and every thread get a small amount of time to run. After running for a while, a thread pauses and gives up the CPU so that other threads can run.
3. Blocked: The thread will be in blocked state when it is trying to acquire a lock but currently the lock is acquired by the other thread. The thread will move from the blocked state to runnable state when it acquires the lock.
4. Waiting state: The thread will be in waiting state when it calls wait() method or join() method. It will move to the runnable state when other thread will notify or that thread will be terminated.
5. Timed Waiting: A thread lies in a timed waiting state when it calls a method with a time-out parameter. A thread lies in this state until the timeout is completed or until a notification is received. For example, when a thread calls sleep or a conditional wait, it is moved to a timed waiting state.
6. Terminated State: A thread terminates because of either of the following reasons: 
   
   • Because it exits normally. This happens when the code of the thread has been entirely executed by the program.
   • Because there occurred some unusual erroneous event, like a segmentation fault or an unhandled exception.

Thread States in Java

In Java, to get the current state of the thread, use Thread.getState() method to get the current state of the thread. Java provides java.lang.Thread.State enum that defines the ENUM constants for the state of a thread, as a summary of which is given below: 

1. New 

Thread state for a thread that has not yet started. 

public static final Thread.State NEW

2. Runnable 

Thread state for a runnable thread. A thread in the runnable state is executing in the Java virtual machine but it may be waiting for other resources from the operating system such as a processor. 

public static final Thread.State RUNNABLE

3. Blocked 

Thread state for a thread blocked waiting for a monitor lock. A thread in the blocked state is waiting for a monitor lock to enter a synchronized block/method or reenter a synchronized block/method after calling Object.wait(). 

public static final Thread.State BLOCKED

4. Waiting 

 Thread state for a waiting thread. A thread is in the waiting state due to calling one of the following methods: 

• Object.wait with no timeout
• Thread.join with no timeout
• LockSupport.park

public static final Thread.State WAITING

5. Timed Waiting 

Thread state for a waiting thread with a specified waiting time. A thread is in the timed waiting state due to calling one of the following methods with a specified positive waiting time: 

• Thread.sleep
• Object.wait with timeout
• Thread.join with timeout
• LockSupport.parkNanos
• LockSupport.parkUntil

public static final Thread.State TIMED_WAITING

6. Terminated 

Thread state for a terminated thread. The thread has completed execution. 

public static final Thread.State TERMINATED

Example of Demonstrating Thread States

Below is a real-world example of a ticket booking system that demonstrates different thread states:

Example:

// Java program to demonstrate thread states 
// using a ticket booking scenario
class TicketBooking implements Runnable {
    @Override
    public void run() {
        
        try {
            
            // Timed waiting
            Thread.sleep(200); 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("State of bookingThread while mainThread is waiting: " +
                TicketSystem.mainThread.getState());

        try {
            
            // Another timed waiting
            Thread.sleep(100); 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class TicketSystem implements Runnable {
    public static Thread mainThread;
    public static TicketSystem ticketSystem;

    @Override
    public void run() {
        TicketBooking booking = new TicketBooking();
        Thread bookingThread = new Thread(booking);

        System.out.println("State after creating bookingThread: " + bookingThread.getState());

        bookingThread.start();
        System.out.println("State after starting bookingThread: " + bookingThread.getState());

        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("State after sleeping bookingThread: " + bookingThread.getState());

        try {
            
            // Moves mainThread to waiting state
            bookingThread.join(); 
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("State after bookingThread finishes: " + bookingThread.getState());
    }

    public static void main(String[] args) {
        ticketSystem = new TicketSystem();
        mainThread = new Thread(ticketSystem);

        System.out.println("State after creating mainThread: " + mainThread.getState());

        mainThread.start();
        System.out.println("State after starting mainThread: " + mainThread.getState());
    }
}

Output:

Explanation:

• When a new thread is created, the thread is in the NEW state. When the start() method is called on a thread, the thread scheduler moves it to the Runnable state.
• Whenever the join() method is called on a thread instance, the main thread goes to Waiting for the booking thread to complete.
• Once the thread's run method completes, its state becomes Terminated.