Thread States

In this article we will learn about different thread states along with an example program that demonstrates thread life cycle.

Life cycle of a thread refers to all the actions or activities done by a thread from its creation to termination. A thread can be in any one of the following five states during its life cycle:

• New: A thread is created but didn’t begin its execution.
• Runnable: A thread that either is executing at present or that will execute when it gets the access to CPU.
• Terminated: A thread that has completed its execution.
• Waiting: A thread that is suspended because it is waiting for some action to occur. For example, it is waiting because of a call to non-timeout version of wait() method or join() method or sleep() method.
• Blocked: A thread that has suspended execution because it is waiting to acquire a lock or waiting for some I/O event to occur.

The life cycle of a thread is illustrated in the following diagram:

The state of a thread can be retrieved using the getState() method of the Thread class. The syntax of this method is as follows:

Thread.State getState()

The getState() method returns one of the values in State enumeration. The constants in State enumerations are:

1. NEW
2. RUNNABLE
3. TERMINATED
4. BLOCKED
5. WAITING
6. TIMED_WAITING

Below example demonstrates the use of getState() method:

class MyThread implements Runnable
{
	Thread t;
	MyThread(String name)
	{
		t = new Thread(this, name);
		System.out.println("Child thread: " + t);
		System.out.println("State: " + t.getState());
		t.start();
	}

	@Override
	public void run()
	{
		System.out.println("State: " + t.getState());
		try
		{
			for(int i = 1; i <= 3; i++)
			{
				System.out.println(t.getName() + ": " + i);
				Thread.sleep(2000);
			}
		}
		catch(InterruptedException e)
		{
			System.out.println(t.getName() + " is interrupted!");
		}
		System.out.println(t.getName() + " is terminated");
	}
}

public class Driver
{	
	public static void main(String[] args)
	{
		MyThread thread1 = new MyThread("First Thread");
		try
		{
			System.out.println("Main thread is waiting...");
			thread1.t.sleep(1000);
			System.out.println("State: " + thread1.t.getState());
			thread1.t.join();
			System.out.println("State: " + thread1.t.getState());
		}
		catch(InterruptedException e)
		{
			System.out.println("Main thread is interrupted!");
		}
		System.out.println("Main thread terminated");
	}
}

Output for the above program is:

Child thread: Thread[First Thread,5,main]
State: NEW
Main thread is waiting…
State: RUNNABLE
First Thread: 1
State: TIMED_WAITING
First Thread: 2
First Thread: 3
First Thread is terminated
State: TERMINATED
Main thread terminated

Suryateja Pericherla

Suryateja Pericherla, at present is a Research Scholar (full-time Ph.D.) in the Dept. of Computer Science & Systems Engineering at Andhra University, Visakhapatnam. Previously worked as an Associate Professor in the Dept. of CSE at Vishnu Institute of Technology, India.

He has 11+ years of teaching experience and is an individual researcher whose research interests are Cloud Computing, Internet of Things, Computer Security, Network Security and Blockchain.

He is a member of professional societies like IEEE, ACM, CSI and ISCA. He published several research papers which are indexed by SCIE, WoS, Scopus, Springer and others.