How to Block a Reactor Http Thread When Calling Third Party APIs with Spring Boot WebClient

Learn how to handle blocking calls in a Spring Boot WebFlux application by managing Reactor's non-blocking nature effectively when calling third-party APIs.
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How to Handle Blocking Calls in a Spring Boot WebClient

When working with Spring Boot and WebClient in a reactive programming model, you might encounter situations where you need to perform blocking calls to third-party APIs. However, the reactive stack inherently discourages blocking operations, which can lead to unexpected errors. In this post, we'll explore a common issue related to blocking calls and provide a practical solution.

The Problem: Blocking the Http Thread

You may have structured your code as follows:

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When you try to execute this method, you might face the following error:

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Understanding the Error

This error is raised because the block() method is a blocking call, which is not supported on the non-blocking reactor-http threads managed by Spring WebFlux. By blocking the thread, you disrupt the asynchronous nature of WebClient and may encounter performance issues or deadlocks.

The Solution: Implementing a Separate Business Process Thread Pool

To handle blocking calls safely, you can implement a separate thread pool specifically for business process tasks. This method allows you to maintain the reactive programming paradigm while enabling your need for blocking calls.

Step-by-step Implementation

Create the Api Client Class

Here's how you can modify your api client class:

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Initialize a Thread Pool as a Spring Bean

To ensure your thread pool is managed by the Spring context, you should define it as a bean:

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Autowire the Thread Pool

Lastly, make sure to autowire the ThreadPoolTaskExecutor in your service class:

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Conclusion

By following these steps, you can effectively handle the need for blocking calls in your Spring Boot WebClient implementation without compromising the reactive programming model. This approach allows you to submit non-blocking API requests while minimizing the risks associated with blocking the reactor HTTP threads.

Remember, while implementing blocking operations in a reactive application may be necessary in some cases, it's always best to evaluate whether there are better, non-blocking alternatives before proceeding.