Understanding @Async Inside synchronized Methods in Java

---

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: @Async method inside synchronized method Java

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding @Async Inside synchronized Methods in Java: A Comprehensive Guide

In the world of Java programming, working with @Async methods and synchronized blocks can be a confusing experience, especially for those relatively new to Java Spring and Spring Boot. If you've ever tried to call an asynchronous method within a synchronized method, you've likely faced peculiar results.

In this guide, we’ll examine a specific problem and provide clarity on how threading and synchronization interact in Java. By the end, you’ll have a deeper understanding of how to manage asynchronous tasks in a multithreaded environment without causing unintended behavior.

The Problem: Conflicting Thread Execution

Let's consider a scenario where you have a method annotated with @Async that is being called within a synchronized method. This leads to unexpected behavior: two threads appear to be executing the synchronized method simultaneously.

Here's the essential part of the problem:

Code Example

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

In the example above, the compute() method is synchronized, which implies that only one thread should access this method at a time. Nonetheless, the output suggests that multiple threads are, in fact, entering compute() simultaneously.

Observed Output

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

Unraveling the Behavior: How It Works

Asynchronous Execution

When you call the asynchronous method reset() within the synchronized method compute(), here's what's occurring behind the scenes:

Thread Spawning: The reset() method launches in a new thread. This thread operates independently of the calling thread, which means that it doesn’t block the execution of compute().

Immediate Continuation: Following the reset() call, the compute() method immediately invokes the return statement, completing its execution. Thus, another thread can enter compute() before the reset() method of the first thread has finished executing.

Debunking the Confusion

Despite appearances, you are not experiencing simultaneous access to the compute() method. It only appears that way due to the asynchronous nature of the reset() method. The completion of reset() and its asynchronous thread's output is registered after the main flow of compute() has finished executing.

Key Takeaways

Single Entry: The synchronized nature of compute() ensures that one thread can execute it at any time, but asynchronous operations allow for parallel execution.

Thread Management: If you see output from different threads, it is a result of how the threads are scheduled rather than an actual violation of the synchronized block.

Understanding Context Switching: The Java Virtual Machine handles context switching, allowing other threads to run while the current thread is waiting, as long as the current thread is not holding locks that might block new threads from entering synchronized methods.

Conclusion

Working with Java’s threading models can often introduce complexities, particularly with @Async and synchronized. Understanding these mechanisms empowers you to leverage asynchronous programming effectively while maintaining thread safety within your applications.

If you have encountered similar challenges or have additional questions regarding threading in Java, feel free to share your thoughts or seek clarification. Engaging with this vibrant community can help broaden your understanding and drive better coding practices.