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Discover how to seamlessly convert a `Flux` to a `Mono` when testing with `webTestClient` in Spring WebFlux. Learn about the key operators to achieve this effortlessly.
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How to Return a Mono Using webTestClient in Spring WebFlux Testing

In the world of reactive programming with Spring WebFlux, you often encounter situations where you need to handle asynchronous data streams efficiently. One common task is working with endpoints that return a Mono type—a single asynchronous value. However, when testing these services with webTestClient, you might run into issues if your setup expects a Flux instead of the expected Mono. This post addresses that very issue and outlines how to navigate it effectively.

The Problem

Assume you have a REST service set up in Spring WebFlux, which returns a Mono<Person> from an endpoint, like /person/{id}. While writing your JUnit tests using webTestClient, you might notice that your code is trying to return a Flux<Person> instead of the Mono that you anticipated.

Here's a basic snippet that illustrates the issue:

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In this snippet, the returnResult method returns a FluxExchangeResult<T>, which, in turn, provides a Flux type. So, how do you work around this limitation to achieve your desired outcome—a Mono?

The Solution

You can obtain a Mono from the Flux returned by the webTestClient. There are two primary operators that you can use for this conversion: single() and next(). The choice between them will depend on your specific use case.

Options for Conversion

Using single():

This operator retrieves the first element from the Flux. However, it has a strict constraint—it will throw an error if no elements are emitted or if more than one element is emitted.

Use this when you are sure that there should only be one item in the Flux.

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Using next():

This operator is more flexible. It allows for zero or more items to be emitted. It simply takes the first emitted item and ignores any subsequent ones.

This is a suitable choice when your use case allows for the possibility of no return or multiple results, as it won’t throw an error for those scenarios.

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Summary

In conclusion, if your testing setup with webTestClient is returning a Flux when you need a Mono, you can easily convert the Flux to a Mono by utilizing the single() or next() operators. Remember to choose the operator that best aligns with your expected data conditions.

With these simple adjustments, you'll be able to make your tests more robust and aligned with the reactive programming principles that Spring WebFlux promotes.

Now, you're well-equipped to handle Mono returns in your WebFlux tests like a pro!