Why subscribeOn(Schedulers.io()) Might Not Work as Expected in Kotlin's RxJava

Описание: Discover why using `subscribeOn(Schedulers.io())` in your Kotlin RxJava code might not execute as anticipated and learn how to solve the issue effectively.
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This video is based on the question https://stackoverflow.com/q/63417737/ asked by the user 'ericn' ( https://stackoverflow.com/u/541624/ ) and on the answer https://stackoverflow.com/a/63418434/ provided by the user 'Aarjav' ( https://stackoverflow.com/u/5130921/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding subscribeOn(Schedulers.io()) in Kotlin's RxJava

When working with RxJava in Kotlin, developers often face challenges with scheduling threads. One common issue many encounter is the difference in behavior between subscribeOn(Schedulers.trampoline()) and subscribeOn(Schedulers.io()), where the former works seamlessly while the latter seems to fail, producing no output. In this post, we'll explore why this happens and how you can work around it effectively.

The Problem

Let's take a look at two blocks of code that illustrate this problem. Both blocks are intended to perform similar operations, but they schedule their execution on different schedulers.

Code Example with Schedulers.trampoline()

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Output:

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In this case, everything executes as expected, outputting the thread names and the final result.

Code Example with Schedulers.io()

Now, let's consider the code that utilizes Schedulers.io().

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Output: (No output)

Despite the expectation of output, this block produces no output at all. Why is this the case?

Understanding the Issue

The key reason behind this behavior is that the program is exiting before the asynchronous code has had a chance to execute. When using Schedulers.trampoline(), the execution happens on the same thread, allowing for immediate completion of work. However, with Schedulers.io() (and other schedulers), the work is scheduled on a different thread, which might lead the main thread to complete and terminate the program before the scheduled work executes.

Clarification on Schedulers

Schedulers.io(): Utilizes an unbounded thread pool designed for I/O-bound work.

Schedulers.computation(): Utilizes a bounded thread pool meant for CPU-intensive tasks.

Schedulers.trampoline(): Executes tasks immediately on the current thread, blocking until completed.

The Solution

To ensure that your program doesn't terminate early, you can introduce a mechanism to wait for the completion of the scheduled work. Here’s how you can modify your code to use Schedulers.io() properly:

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New Output:

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Conclusion

In conclusion, if you find that your subscribeOn(Schedulers.io()) or other schedulers aren’t behaving as expected, remember that your main program might be exiting before the scheduled work has completed. By adding a sleep or some form of wait, you can provide sufficient time for the async operations to finish before the main thread terminates. This way, all scheduled operations can execute, yielding the desired results.

Final Thoughts

Always be mindful of threading behavior in reactive programming! Understanding how different schedulers handle thread management is key to mastering RxJava in Kotlin.