Life cycle of a Thread (Thread States)

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The life cycle of a thread in a typical operating system can be described in terms of its different states. The thread states can vary slightly depending on the specific implementation, but the following are the most common states:

  1. New: The thread is created, but not yet started. The thread has been allocated resources, but its code has not yet been executed.
  2. Runnable: The thread is ready to run, but the processor has not yet allocated CPU time to it. In this state, the thread is waiting for its turn to be executed.
  3. Running: The thread is currently being executed by the processor. While in this state, the thread is actively running its code.
  4. Blocked: The thread is temporarily suspended because it is waiting for some external event to occur, such as I/O completion or a signal from another thread. The thread is not using any CPU time while blocked.
  5. Waiting: The thread is waiting for another thread to complete before it can continue. In this state, the thread is not using any CPU time.
  6. Terminated: The thread has completed its execution and has been terminated. The thread resources are released by the operating system.

These states can be visualized in a state diagram, where the transitions between states are caused by events such as I/O completion, timer expiration, or signals from other threads. The state diagram can help to illustrate the behavior of a thread in a particular system and can be used to help debug issues with threading applications.

Explanation of Different Thread States:

In a multi-threaded environment, a thread can exist in different states, depending on its current activity and availability of resources. Here are the explanations of the different thread states:

  1. New State: A thread enters the new state when it is created, but not yet started. In this state, the thread has been allocated system resources, such as stack and memory, but its code has not yet been executed.
  2. Runnable State: A thread enters the runnable state when it is ready to run, but the CPU has not yet allocated time for it. In this state, the thread is waiting for the CPU to assign it processing time.
  3. Running State: A thread enters the running state when the CPU has allocated processing time to it. In this state, the thread is executing its instructions.
  4. Blocked State: A thread enters the blocked state when it is temporarily suspended and cannot continue execution. A thread can enter the blocked state due to reasons such as waiting for I/O completion or waiting for a lock to be released.
  5. Waiting State: A thread enters the waiting state when it waits for another thread to complete its execution. A thread can enter the waiting state due to reasons such as calling a join() method on another thread or waiting for a condition variable.
  6. Terminated State: A thread enters the terminated state when its execution is complete, either due to normal termination or an error. In this state, the thread releases its resources, such as memory and stack.

These thread states are usually represented in a state diagram to provide a graphical representation of the thread’s lifecycle. Understanding the different thread states can help in designing and debugging multi-threaded applications.

Implementation of Thread States:

The implementation of thread states varies depending on the operating system and the programming language used to create the threads. However, in general, the implementation of thread states involves setting a thread’s state variable to a specific value, depending on the thread’s activity.

For example, in Java, a thread’s state can be obtained using the getState() method, which returns a Thread.State enumeration. The possible thread states in Java are:

  1. NEW: A thread is in the NEW state if it has been created but not yet started.
  2. RUNNABLE: A thread is in the RUNNABLE state if it is ready to run but has not been allocated CPU time.
  3. BLOCKED: A thread is in the BLOCKED state if it is temporarily suspended while waiting for a lock to be released.
  4. WAITING: A thread is in the WAITING state if it is waiting indefinitely for another thread to perform a specific action.
  5. TIMED_WAITING: A thread is in the TIMED_WAITING state if it is waiting for a specified period for another thread to perform a specific action.
  6. TERMINATED: A thread is in the TERMINATED state if its execution has completed.

In C and C++, thread states are typically implemented using a state variable or an enum type, and the thread’s state is changed by the operating system or the application code depending on the thread’s activity.

Overall, the implementation of thread states involves maintaining and updating a thread’s state variable to reflect the thread’s current activity and availability of resources. This information is used by the operating system or the programming language runtime to manage and schedule the execution of threads in a multi-threaded environment.

Java Program for Demonstrating Thread States:

Sure, here’s a Java program that demonstrates the different thread states using the Thread class:

public class ThreadStateDemo implements Runnable {

    @Override
    public void run() {
        try {
            Thread.sleep(1000); // sleep for 1 second
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ThreadStateDemo demo = new ThreadStateDemo();

        Thread thread1 = new Thread(demo); // new state
        Thread thread2 = new Thread(demo); // new state
        Thread thread3 = new Thread(demo); // new state

        thread1.start(); // runnable state
        thread2.start(); // runnable state
        thread3.start(); // runnable state

        Thread.sleep(500); // wait for 0.5 seconds

        System.out.println(thread1.getState()); // prints "TIMED_WAITING"
        System.out.println(thread2.getState()); // prints "TIMED_WAITING"
        System.out.println(thread3.getState()); // prints "TIMED_WAITING"

        Thread.sleep(600); // wait for 0.6 seconds

        System.out.println(thread1.getState()); // prints "TERMINATED"
        System.out.println(thread2.getState()); // prints "TERMINATED"
        System.out.println(thread3.getState()); // prints "TERMINATED"
    }
}

In this program, the ThreadStateDemo class implements the Runnable interface, and the run() method is overridden to sleep for 1 second. The main method creates three threads and starts them, which puts them in the runnable state.

After waiting for 0.5 seconds, the program prints the state of each thread using the getState() method, which should be “TIMED_WAITING” because the threads are sleeping. Finally, after waiting for another 0.6 seconds, the program prints the state of each thread again, which should be “TERMINATED” because the threads have completed their execution.

This program demonstrates the different thread states in Java and how they can be used to manage and monitor the execution of threads in a multi-threaded application.