Introduction
Understanding the lifecycle of a thread is essential for effectively managing and utilizing multithreading in Java. A thread in Java goes through various states from its creation to termination. The Thread class provides methods to manage the lifecycle of a thread, allowing you to control its execution and ensure proper synchronization and communication between threads.
Key Points:
- Thread Lifecycle States: A thread can be in one of several states during its lifecycle.
- State Management: The
Threadclass provides methods to control and manage the state of a thread. - Synchronization: Proper synchronization is crucial to manage the transition between thread states.
Table of Contents
- Thread Lifecycle States
- Diagram of Thread Lifecycle
- Detailed Explanation of Each State
- Thread Lifecycle Methods
- Example: Demonstrating Thread Lifecycle
- Best Practices
- Conclusion
1. Thread Lifecycle States
A thread in Java can be in one of the following states:
- New: A thread that has been created but not yet started.
- Runnable: A thread that is ready to run and waiting for CPU time.
- Blocked: A thread that is waiting for a monitor lock to enter or re-enter a synchronized block/method.
- Waiting: A thread that is waiting indefinitely for another thread to perform a particular action.
- Timed Waiting: A thread that is waiting for another thread to perform a particular action for a specified waiting time.
- Terminated: A thread that has exited.
2. Diagram of Thread Lifecycle
Here is a text-based diagram representing the lifecycle of a thread in Java:
+---------------------------------+
| |
| New |
| (Thread created, but not yet |
| started) |
| |
+---------------------------------+
|
| start()
|
v
+---------------------------------+
| |
| Runnable |
| (Thread is ready to run and |
| waiting for CPU time) |
| |
+---------------------------------+
|
| Running
v
+---------------------------------+
| |
| Blocked |
| (Thread is blocked and waiting |
| for a monitor lock) |
| |
+---------------------------------+
|
| Waiting or Timed Waiting
v
+---------------------------------+
| |
| Waiting |
| (Thread is waiting indefinitely|
| for another thread to perform |
| a specific action) |
| |
+---------------------------------+
|
| Timed Waiting
v
+---------------------------------+
| |
| Timed Waiting |
| (Thread is waiting for a |
| specified period) |
| |
+---------------------------------+
|
| notify() / notifyAll()
v
+---------------------------------+
| |
| Terminated |
| (Thread has exited after |
| completing its execution) |
| |
+---------------------------------+
3. Detailed Explanation of Each State
New
- State: The thread is created but not yet started.
- Methods:
new Thread()
Runnable
- State: The thread is ready to run and waiting for CPU time.
- Methods:
start()
Blocked
- State: The thread is blocked and waiting for a monitor lock to enter or re-enter a synchronized block/method.
- Methods: Enters this state when attempting to enter a synchronized block/method but another thread holds the lock.
Waiting
- State: The thread is waiting indefinitely for another thread to perform a particular action.
- Methods:
wait(),join(),park()
Timed Waiting
- State: The thread is waiting for another thread to perform a particular action for a specified period.
- Methods:
sleep(long millis),wait(long timeout),join(long millis),parkNanos(long nanos),parkUntil(long deadline)
Terminated
- State: The thread has exited after completing its execution.
- Methods: The thread enters this state when the
run()method completes or the thread is terminated abruptly by an exception.
4. Thread Lifecycle() Methods
start()
Starts the thread, causing the run() method to be called. Transitions the thread from the New state to the Runnable state.
run()
Contains the code that defines the task of the thread. When a thread is started, its run() method is invoked.
sleep(long millis)
Causes the currently executing thread to sleep (temporarily cease execution) for the specified number of milliseconds. Transitions the thread to the Timed Waiting state.
join()
Waits for the thread to die. If called on a thread, the calling thread transitions to the Waiting state until the specified thread completes.
interrupt()
Interrupts the thread, causing it to throw an InterruptedException if it is currently sleeping, waiting, or otherwise blocked.
wait()
Causes the current thread to wait until another thread invokes the notify() or notifyAll() methods. Transitions the thread to the Waiting state.
notify()
Wakes up a single thread that is waiting on the object’s monitor. Transitions a waiting thread to the Runnable state.
notifyAll()
Wakes up all threads that are waiting on the object’s monitor. Transitions all waiting threads to the Runnable state.
Example:
public class ThreadLifecycleExample {
public static void main(String[] args) {
Thread t1 = new Thread(() -> {
try {
System.out.println("Thread is running...");
Thread.sleep(1000); // Timed Waiting
synchronized (ThreadLifecycleExample.class) {
ThreadLifecycleExample.class.wait(); // Waiting
}
} catch (InterruptedException e) {
System.out.println("Thread was interrupted.");
}
});
System.out.println("State: " + t1.getState()); // NEW
t1.start();
System.out.println("State: " + t1.getState()); // RUNNABLE
try {
Thread.sleep(500);
System.out.println("State: " + t1.getState()); // TIMED_WAITING
synchronized (ThreadLifecycleExample.class) {
ThreadLifecycleExample.class.notify();
}
t1.join();
System.out.println("State: " + t1.getState()); // TERMINATED
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
5. Example: Demonstrating Thread Lifecycle
Example: Thread Lifecycle with Detailed Transitions
public class ThreadLifecycleDemo {
public static void main(String[] args) {
Thread t1 = new Thread(() -> {
try {
System.out.println("Thread started, current state: " + Thread.currentThread().getState());
Thread.sleep(1000); // Timed Waiting
synchronized (ThreadLifecycleDemo.class) {
ThreadLifecycleDemo.class.wait(); // Waiting
}
System.out.println("Thread resumed, current state: " + Thread.currentThread().getState());
} catch (InterruptedException e) {
System.out.println("Thread was interrupted.");
}
});
System.out.println("State before start: " + t1.getState()); // NEW
t1.start();
System.out.println("State after start: " + t1.getState()); // RUNNABLE
try {
Thread.sleep(500);
System.out.println("State while sleeping: " + t1.getState()); // TIMED_WAITING
synchronized (ThreadLifecycleDemo.class) {
ThreadLifecycleDemo.class.notify();
}
Thread.sleep(100); // Allow time for the thread to resume
System.out.println("State after notify: " + t1.getState()); // RUNNABLE or WAITING
t1.join();
System.out.println("State after join: " + t1.getState()); // TERMINATED
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
Output:
State before start: NEW
State after start: RUNNABLE
Thread started, current state: RUNNABLE
State while sleeping: TIMED_WAITING
State after notify: RUNNABLE
Thread resumed, current state: RUNNABLE
State after join: TERMINATED
6. Best Practices
- Avoid Busy Waiting: Use proper synchronization techniques (
wait(),notify(),notifyAll()) instead of busy-waiting loops. - Minimize Synchronized Sections: Keep synchronized sections as short as possible to avoid performance bottlenecks.
- Handle InterruptedException: Properly handle
InterruptedExceptionto ensure threads can be interrupted gracefully. - Use Thread Pools: For managing multiple short-lived tasks, use thread pools provided by the
ExecutorServiceto efficiently manage resources. - Avoid Deadlocks: Design your synchronization strategy to avoid deadlocks by using techniques like lock ordering.
- Use High-Level Concurrency Utilities: Leverage high-level concurrency utilities from the
java.util.concurrentpackage for complex synchronization and concurrency tasks.
7. Conclusion
Understanding the lifecycle of a thread in Java is crucial for effectively managing and utilizing multithreading. The Thread class provides methods to control and manage the state of a thread, allowing you to ensure proper synchronization and communication between threads. By following best practices and leveraging high-level concurrency utilities, you can write efficient, thread-safe, and maintainable multithreaded applications.