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Learn how to solve the issue of a stuck main thread in reactive programming using Project Reactor. Follow our guide for efficient asynchronous execution!
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Understanding Why the Main Thread is Stuck in Reactive Programming with Reactor

If you're diving into the world of reactive programming with Project Reactor, you might face an issue where your main thread appears to be stuck during the execution of your code. This can be puzzling, especially when you expect asynchronous behavior. In this guide, we'll explore a common scenario that can cause this issue and how to solve it effectively.

The Problem: Main Thread Gets Stuck

Consider the following code snippet, which uses Project Reactor to create a simple flux of fruit names and subscribes to it:

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What Happens Here?

When you run this test case, you may observe that the output is delayed. Instead of printing "hello main thread" immediately, the main thread seems to be stuck for 5 seconds. You might wonder why this is happening and why the subscriber does not execute on its own thread, allowing the main thread to continue.

The Root Cause: Subscription on the Main Thread

The key to understanding this behavior lies in how Project Reactor handles subscriptions by default. In the provided code, the subscription takes place on the main thread. When you call subscribe, the subscriber processes the emitted items in the same thread, which in this case is the main thread.

Important Points to Note

Synchronous Execution: By default, Reactor executes pipelines on the calling thread. This is different from frameworks like CompletableFuture<T>, which run in a different thread pool by default.

Thread Blocking: The Thread.sleep(5000) call blocks the main thread, preventing it from reaching the next line of code that prints "hello main thread".

The Solution: Use subscribeOn to Enable Asynchronous Execution

To allow the subscriber to operate in an asynchronous manner, you need to tell Reactor to run the subscription on a different thread. This can be accomplished using the subscribeOn method along with a suitable scheduler:

Updated Code Example

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Explanation of the Changes

Schedulers.parallel(): In the updated code, Schedulers.parallel() tells Reactor to run the subscription on a different thread, freeing the main thread to continue executing subsequent lines of code immediately.

Asynchronous Behavior: With this adjustment, you can expect that "hello main thread" will print instantly, while the fruit names will follow after 5 seconds, indicating that they are processed on a different thread.

Conclusion

By understanding how subscriptions work in Project Reactor and the default synchronous behavior of reactive streams, you can effectively manage threading in your applications. Remember, if you want non-blocking execution, always ensure to use subscribeOn with your chosen scheduler. This approach not only enhances the responsiveness of your application but also leverages the true power of reactive programming.

If you have any questions or encounters related to this topic, feel free to leave a comment! Happy coding!