Java Thread.join()
The join()
method in Java is used to wait for a thread to complete its execution. When one thread calls the join()
method on another thread, it pauses its execution until the thread it called join()
on has finished. This method is typically used to coordinate the execution order of multiple threads, ensuring that one thread continues only after another has finished.
Basic Usage of join()
Method
Wait for a Thread to Finish: If thread A calls the
join()
method on thread B, thread A will wait until thread B finishes its execution before resuming.Wait for a Specified Time: The
join()
method can also take a timeout parameter, allowing the calling thread to wait for a specified amount of time. If the thread being joined does not finish within the timeout period, the calling thread will resume execution.
Below is a simple example to demonstrate the usage of the join()
method:
1 | public class JoinExample { |
Explanation of the Example
Creating Threads: Two threads,
thread1
andthread2
, are created. Each thread prints a message, sleeps for a specified time to simulate work, and then prints a completion message.Starting Threads: Both threads are started using the
start()
method.Joining Threads:
- The main thread calls
thread1.join()
, causing it to wait untilthread1
finishes its execution. - After
thread1
completes, the main thread prints a message and then callsthread2.join(1500)
, causing it to wait forthread2
to finish or timeout after 1.5 seconds. - After waiting for
thread2
, the main thread prints another message indicating whetherthread2
has finished or the wait has timed out.
- The main thread calls
Main Thread Completion: Finally, the main thread prints a message indicating its completion.
Practical Applications
- Ensuring Order of Execution: When you need to ensure that one thread completes its task before another thread starts or continues.
- Dependency Management: Managing dependencies between tasks running in different threads.
- Resource Cleanup: Ensuring that resources are released or cleaned up only after certain tasks have completed.
Key Points
- Thread State: When a thread is waiting for another thread to complete, it is in a blocked state.
- InterruptedException: The
join()
method can throw anInterruptedException
if the waiting thread is interrupted. It is essential to handle this exception appropriately. - Timeout: The timeout versions of
join()
(join(long millis)
andjoin(long millis, int nanos)
) allow for more fine-grained control over how long a thread should wait.
The join()
method is a powerful tool for thread synchronization and can be very useful in scenarios where the completion of one task depends on the completion of another.