Скачать или смотреть Resolving Java Threading Issue: Distinct Outputs for CompletableFuture Threads

A guide on troubleshooting Java CompletableFuture threading issues, focusing on solving problems where threads return unexpected outputs and handling NullPointerExceptions.
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This video is based on the question https://stackoverflow.com/q/75866186/ asked by the user 'Richard62' ( https://stackoverflow.com/u/21509206/ ) and on the answer https://stackoverflow.com/a/75866869/ provided by the user 'John Williams' ( https://stackoverflow.com/u/8041003/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Java Threading Issues with CompletableFuture

Java's CompletableFuture provides a powerful way to handle asynchronous programming and improve the performance of applications. However, sometimes developers encounter unexpected behaviors, such as multiple threads returning the same output despite processing different inputs. In this guide, we will address a common issue involving threading with CompletableFuture and provide a clear solution.

The Problem: Unexpected Outputs and NullPointerException

Imagine you're processing a mainframe file, reading records line-by-line and creating a JSONObject for each record. You want to speed up the process by utilizing multiple threads, with each thread handling its own record. However, you notice that some threads return the same output as others, leading to confusion and errors.

Moreover, you might encounter a NullPointerException that disrupts the execution of your program. Here's a simplified version of what's going wrong:

Code Example: As seen in the provided code, there are issues with how CompletableFuture is utilized, particularly in the way the collected results from these futures are handled.

Analyzing the Existing Code

Let's break down the critical parts of the provided code:

Thread Pool Setup: A fixed thread pool of size 5 is created.

Data Processing: A list of strings is defined, and for each string, a byte array is derived and processed through asynchronous completions.

Collecting Results: A CompletableFuture array is initialized but left unpopulated, leading to the NullPointerException.

The Key Issues

Duplicate Outputs: Since each thread uses a shared buf variable, concurrent access could lead to threads retrieving the same data inadvertently.

NullPointerException: Occurs because the jObjArray is not populated with completed futures before the allOf method is called.

The Solution: Correcting the Code

To resolve these issues, we need to make a few adjustments to our code. Here’s the proposed solution:

Populate the CompletableFuture Array:

Before calling CompletableFuture.allOf(), ensure that the jObjArray is populated with completed futures from jObjList.

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

Avoid Sharing Mutable State:

Modify the code to avoid sharing the buf variable. Instead, create a new byte[] for each thread, so each thread has its own reference to its data:

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

Call All Futures Together:

Now, you can safely call CompletableFuture.allOf(jObjArray), ensuring there are no null references, and obtain results without errors.

Final Code Preview

The corrective code block would resemble the following structure:

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

Conclusion

By carefully managing your thread states and ensuring that each future is populated correctly prior to processing, you can eliminate duplication of outputs in your CompletableFuture implementation. Moreover, avoiding the shared mutable state is crucial in multi-threaded applications.

Takeaway

Understanding and resolving threading issues in Java can be complex, but breaking down the problem into manageable sections and applying these targeted solutions can lead to a better, more robust application. Happy coding!