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Learn how to efficiently update a `HashMap` in Java when using `CompletableFuture`, whether in a blocking or non-blocking way.
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Modifying a HashMap After a CompletableFuture is Completed in Java

When developing applications in Java dealing with asynchronous tasks, it's common to encounter scenarios where you might need to update a HashMap after a CompletableFuture has completed its execution. This can be particularly relevant in multi-module systems where tasks, like database transactions, are performed asynchronously. This guide will delve into a solution that allows for both blocking and non-blocking updates to ensure your program's data remains consistent and efficient.

The Problem

You may be working with a method that handles database transactions asynchronously. After the transaction completes, you need to update a HashMap with new data. However, you might encounter challenges with managing the state of that HashMap if the code continues to execute without waiting for the CompletableFuture to finish.

Consider the following method that executes a transaction in a non-blocking manner:

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In the above example, once runTransaction is invoked, it runs asynchronously, which may hinder your ability to immediately modify your HashMap with the result unless properly handled.

Solutions

Let's explore two effective approaches to tackle this common problem: a blocking model and a non-blocking model.

1. Blocking Approach

You can convert the asynchronous nature of CompletableFuture into a synchronous context. This means waiting for the CompletableFuture to complete before proceeding to update the HashMap. The get() method can be used for this purpose. Here's how you could implement this:

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Blocking Nature: By using get(), the program will pause until the runTransaction method completes. This guarantees that your HashMap won't be modified until the transaction is confirmed to be complete.

Exception Handling: Properly handling exceptions is crucial, especially when interfacing with external systems like databases.

2. Non-Blocking Approach

If you prefer to keep the code non-blocking (allowing your application to continue processing while waiting for the transaction to complete), you can use the thenRun method. This method lets you define a callback that is executed once the CompletableFuture finishes. Here's an example:

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Non-Blocking Nature: Using thenRun, your application won't wait for the transaction to complete before executing other tasks, promoting better resource utilization.

Code Readability: This method maintains the readability of code and explicitly defines the action to take once the asynchronous process is finished.

Conclusion

Whether you choose to take the blocking or non-blocking approach, both strategies help ensure that your HashMap is updated appropriately once your CompletableFuture completes its execution. Depending on your application requirements, you can select the method that best suits your needs.

In summary, managing asynchronous tasks and data updates is critical for building responsive Java applications. By effectively handling CompletableFuture, you will improve your application’s performance and reliability.

If you have any further questions or need additional clarification on handling CompletableFuture in Java, feel free to ask in the comments!