Скачать или смотреть Understanding Coroutine Execution: Why Does 'a' Print After 1 Second Even When Thread is Blocked?

Explore the behavior of coroutines in Kotlin. Learn why println('a') is executed during a blocked thread.
---
This video is based on the question https://stackoverflow.com/q/63027705/ asked by the user 'Marti Serra Molina' ( https://stackoverflow.com/u/13833124/ ) and on the answer https://stackoverflow.com/a/63027905/ provided by the user 'Commander Tvis' ( https://stackoverflow.com/u/10117056/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why is there code that is executed when the thread is blocked using coroutines?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Coroutine Execution: Why Does 'a' Print After 1 Second Even When Thread is Blocked?

In the world of concurrent programming, the behavior of threads and coroutines can be one of the more perplexing topics to navigate. A common point of confusion arises when working with Kotlin coroutines, particularly when dealing with thread blocking and delayed execution.

The Problem at Hand

Consider a simple piece of Kotlin code. It launches a coroutine that prints 'a' after a 1-second delay, yet it appears to execute while the main thread is blocked for 3 seconds. The question many developers have is: how is it possible for println('a') to execute when the thread is supposedly blocked by Thread.sleep()?

Here's a closer look at the code snippet in question:

[[See Video to Reveal this Text or Code Snippet]]

Understanding the Execution Flow

To break this down, let’s analyze how Kotlin coroutines work in conjunction with threading and blocking:

1. Coroutine Launching

In the code above, we are using GlobalScope.launch(Dispatchers.Default) to start a coroutine.

This coroutine is scheduled on a thread managed by the default dispatcher, which utilizes a thread pool for executing coroutines.

2. Delay vs. Sleep

The delay(1000L) function call is part of the coroutine. It tells the coroutine to pause (or delay) its work for 1 second without blocking the underlying thread.

On the other hand, Thread.sleep(3000L) blocks the main thread for 3 seconds. During this time, it cannot process any other tasks.

3. Coroutine Execution While Blocked

Because the coroutine runs on a separate thread from the one that is blocked by Thread.sleep, the delay(1000L) executes without any problems.

After 1 second, the coroutine completes its delay and the println("a") statement is executed, printing 'a' to the console.

Key Takeaways

Coroutines are non-blocking: Even when a thread is blocked, other threads may continue execution. In this case, the coroutine's delay allows it to finish even while the main thread waits.

Thread Pool Management: The Dispatchers.Default uses a pool of threads to handle coroutine execution. This enables concurrent execution of coroutines while the main thread is doing other work (or being blocked).

Understanding Blocking vs. Non-Blocking: The distinction between Thread.sleep() (blocking) and delay() (non-blocking) is crucial when working with concurrent code.

Conclusion

Kotlin coroutines provide a powerful paradigm for managing concurrency in applications. Understanding how coroutines operate independently of thread blocking is key to harnessing their full potential. In our case, println executing after a delay while the main thread is blocked illustrates the effective design of Kotlin's coroutine system, clearly demonstrating that blocking operations do not prevent other concurrent tasks from being completed.

By keeping these concepts in mind, developers can write more efficient and responsive applications that leverage Kotlin's coroutines to their full advantage.