Thread States and Lifecycle in Java

Thread states and lifecycle are important concepts in Java concurrency, as they determine the behavior and execution of threads. Understanding these concepts is crucial for writing efficient and synchronized multi-threaded applications. In this article, we will explore the different thread states and discuss the lifecycle of a thread in Java.

Thread States

A thread in Java can be in one of the following states:

  1. New: A thread is in the new state when it is created but has not yet started. In this state, the thread has not been assigned any system resources and has not been started yet.

  2. Runnable: A thread is in the runnable state when it is ready to run, and the underlying thread scheduler can select it for execution. This state includes both running and ready-to-run threads. The thread may be temporarily suspended or waiting for system resources (e.g., CPU) before execution.

  3. Blocked: A thread is in the blocked state when it is temporarily unable to run. This can occur when a thread is waiting for a monitor lock to enter a synchronized section or when it is waiting to acquire a lock on an object that is already acquired by another thread.

  4. Waiting: A thread is in the waiting state when it is waiting indefinitely for another thread to perform a particular action. This can be triggered by calling the wait() method, waiting for a notification from another thread.

  5. Timed Waiting: A thread is in the timed waiting state when it is waiting for a specific duration for another thread to perform an action. This can be triggered by calling methods like sleep(long millis) or join(long millis).

  6. Terminated: A thread is in the terminated state when it has completed its execution or terminated abruptly due to an exception.

Thread Lifecycle

The lifecycle of a thread in Java consists of the following stages:

  1. New: At this stage, a thread is created, and its associated objects and variables are initialized. The thread is not yet eligible for execution.

  2. Runnable: Once the start() method is called on a thread, it transitions from the new state to the runnable state. At this stage, the thread is ready to run, and the underlying thread scheduler can execute it when resources are available.

  3. Running: When the thread scheduler selects a thread from the runnable state, it moves to the running state. The run() method of the thread is executed during this stage.

  4. Blocked: A running thread can transition to the blocked state if it encounters a blocking condition, such as waiting to enter a synchronized section or waiting for a lock on an object.

  5. Waiting: A running thread can move to the waiting state if it calls the wait() method, waiting for another thread to notify it.

  6. Timed Waiting: Similar to the waiting state, a running thread can enter the timed waiting state if it calls methods like sleep(long millis) or join(long millis).

  7. Terminated: A thread can enter the terminated state when it completes its execution or terminates abruptly due to an uncaught exception.

It's important to note that a thread can transition between these states as it executes its code or awaits certain conditions.

Conclusion

Understanding thread states and the lifecycle of a thread in Java is essential for developing efficient and synchronized multi-threaded applications. By knowing the various states, you can better control and manage the execution and synchronization of threads.

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