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In Java, every thread goes through a lifecycle with distinct states from its creation until its termination. Understanding these states is essential for developing robust multithreaded applications and for debugging threading issues. A thread's state reflects what it is currently doing, and the Java Virtual Machine (JVM) manages transitions between these states during execution.
Thread States
The following table summarizes the main states of a thread along with their descriptions:
| State | Description |
|---|---|
| New | The thread has been created but has not yet started execution. |
| Runnable | The thread is ready to run and is waiting for CPU time. This includes threads that are actually running. |
| Blocked | The thread is blocked waiting for a monitor lock (e.g., waiting to enter a synchronized block). |
| Waiting | The thread is waiting indefinitely for another thread to perform a specific action (e.g., via wait()). |
| Timed Waiting | The thread is waiting for another thread to perform an action for a specified period (e.g., via sleep() or join(timeout)). |
| Terminated | The thread has completed its execution or has been terminated. |
Examples
Example 1: Observing Thread States
This example demonstrates how to observe the lifecycle of a thread by printing its state at different stages using the getState() method.
Example Explanation:
- Initial State (NEW):
- Before calling
start(), the thread is created but not yet scheduled.
- Before calling
- After
start()(RUNNABLE):- The thread becomes eligible to run, entering the RUNNABLE state.
- During Sleep (TIMED_WAITING):
- Inside
run(),Thread.sleep(2000)causes the thread to enter TIMED_WAITING (this may not be visible if printed at an exact moment).
- Inside
- After Completion (TERMINATED):
- Once
run()finishes, the thread enters the TERMINATED state.
- Once
Understanding the lifecycle and states of a thread is crucial for effective multithreading in Java. Threads transition through various states such as New, Runnable, Blocked, Waiting, Timed Waiting, and Terminated during their execution. By using methods like start(), sleep(), and join(), you can manage thread execution and synchronize operations. The provided examples illustrate how to observe these state transitions and how to use join() to coordinate thread completion.
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