How to Properly Add Objects to a synchronized ArrayList Using MultiThreading in Java

Описание: Learn how to efficiently add Employee objects to a synchronized ArrayList in Java using multithreading techniques, ensuring your list is populated correctly.
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This video is based on the question https://stackoverflow.com/q/65613760/ asked by the user 'Patryk Chojnacki' ( https://stackoverflow.com/u/14595047/ ) and on the answer https://stackoverflow.com/a/65614068/ provided by the user 'juwil' ( https://stackoverflow.com/u/8344485/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Properly Add Objects to a synchronized ArrayList Using MultiThreading in Java

When working with multithreading in Java, especially while modifying shared data structures like a synchronized ArrayList, developers often face issues that lead to unexpected results, such as an empty list when it's supposed to be populated. In this guide, we'll explore the challenges of adding objects to a synchronized list from multiple threads and provide a clear solution to ensure your objects are stored correctly.

The Problem Explained

Imagine you're attempting to create multiple Employee objects in parallel threads. You expect each thread to add its Employee object to a synchronized list. However, upon checking the size of the list, you find that it's still empty. This problem commonly arises because threads may complete their tasks asynchronously, which can lead to situations where the main thread prints the size of the list before any of the worker threads have had a chance to add their objects.

Example of the Code Encountering the Problem

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In this code snippet, even though we've shut down the executor service, we might inadvertently print out the size of the list before all the threads have completed their execution.

The Solution

Ensuring All Threads Complete Execution

To ensure that the main thread waits for all the worker threads to finish before checking the size of the list, you need to use the awaitTermination method. This method blocks the main thread until all tasks have completed execution after a shutdown request.

Modified Code Example

Here’s how you can modify the code to ensure that all threads are finished before getting the size of the list:

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By adding the above block after the executorService.shutdown() line, you're effectively pausing the main thread until all of the tasks submitted to the executor service are completed.

Final Revised Code

Here’s how the complete, corrected code looks:

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Conclusion

By incorporating the awaitTermination method, you can ensure that your multithreaded program works smoothly without prematurely accessing shared resources like the synchronized ArrayList. This approach not only enhances the reliability of your code but also avoids common pitfalls associated with concurrent programming.

If you're embarking on multithreading in Java, remember that managing the execution flow of your threads is crucial for achieving the desired outcome. With the right practices in place, you can efficiently manage operations across multiple threads, making your application more robust and well-functioning.

Feel free to leave a comment or ask any questions if you need further clarification on multithreading in Java!