Скачать или смотреть How to Wait for a Mono Value in Mono.flatMap?

Learn how to properly handle `Mono` values in spring reactive programming with `flatMap`, especially in a scenario where you need to wait for other `Mono` operations to complete.
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This video is based on the question https://stackoverflow.com/q/75516602/ asked by the user 'EdgarHan' ( https://stackoverflow.com/u/9030675/ ) and on the answer https://stackoverflow.com/a/75516952/ provided by the user 'Nikolas Charalambidis' ( https://stackoverflow.com/u/3764965/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Wait for a Mono Value in Mono.flatMap?

In the world of Spring WebFlux and reactive programming, managing asynchronous data flows can often lead to confusion. One common issue developers encounter is how to wait for a value from a Mono operation while working within the flatMap context. If you've found yourself struggling with issues like getting a null value when expecting a response, you're not alone!

The Problem

In a recent example, a developer had two Mono calls that retrieved a Student and a School. These were combined using Mono.zip(). The intended flow was to create a Person object that combines these values and then fetch an additional piece of information called passInfo. However, despite following the reactive programming principles, the developer noticed that the passInfo was always null.

Here’s a simplified overview of the problematic code snippet:

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

The root cause of the problem lies in the way the asynchronous operations were handled. The attempt to use a subscription within the flatMap did not effectively wait for the result before proceeding to return the person.

The Solution

To resolve this issue, we can take advantage of method chaining and properly utilize the doOnSuccess() method that helps us execute additional actions when the Mono completes successfully. Here, we will outline the steps to refactor the code effectively.

Simplified Code Example

Here’s a simplified version of how the code can be refactored:

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

Breakdown of the Solution

Create the Person Object: Start with the Mono.just() method, where we immediately create the Person from the Student and School Mono calls. This ensures that we have the person ready to work with.

Fetch PassInfo: Call the getPassOrfail() method, which retrieves the relevant information based on the student.id and school.number. This is done in a reactive way, and we do not block the main thread.

Update the Person Object: Use doOnSuccess() to update the person's information once passInfo is successfully retrieved. This approach ensures that the setPassInfo() method is called once passInfo is available.

Return the Updated Person: Finally, with thenReturn(person), we return the updated person object, which now includes the passInfo.

Conclusion

By reorganizing the structure of your reactive pipeline using these principles, you can effectively manage asynchronous calls and ensure that you're waiting for the necessary data to appear. This not only eliminates the issue of receiving null values but also enhances the overall clarity and robustness of your code.

Next time you run into similar issues, remember to utilize the full power of chaining in Mono operations, and reap the benefits of reactive programming in your Spring applications.