Resolving Blocking Calls When Sending Multipart Form Data via WebClient in Spring Boot

Learn how to overcome the issues with `blocking calls` when sending multipart/form data through the WebClient in Spring Boot with a step-by-step guide.
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Overcoming Blocking Calls in Multipart Form Data with WebClient in Spring Boot

When developing applications with Spring Boot, particularly when using the reactive WebClient, you might encounter certain challenges. One of the more frustrating issues is facing blocking calls while sending multipart/form data. This can result in exceptions that not only disrupt your flow but also hinder the performance of your application. In this guide, we will tackle this problem and walk you through a solution that allows you to successfully send multipart/form requests without running into blocking issues.

The Problem: Blocking Calls in WebClient

Suppose you are trying to send a multipart/form request that includes files and string fields using the WebClient in Spring Boot. During the process, you might run into a BlockingOperationError, which indicates that the call is being blocked. This generally happens due to the usage of certain I/O operations, such as FileInputStream, that are not designed for reactive programming.

Here’s a snippet of what the issue might look like:

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

This error tells us that an I/O operation is blocking your thread, which goes against the principles of asynchronous and non-blocking I/O that reactive programming promotes.

The Solution: Using BodyInserters for Non-Blocking Multipart Requests

To resolve this issue, we will create the body of our POST request in a non-blocking way. Instead of directly using the multipart builder in a blocking manner, we can utilize Spring's BodyInserters to insert the multipart data reactively. Here’s how to implement this solution step-by-step.

Step 1: Build Your Multipart Data

First, we need to prepare the multipart data using MultipartBodyBuilder. This component helps in creating multipart body requests seamlessly. An example might look like this:

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When your builder is ready, you can then proceed to send it with the WebClient.

Step 2: Employ BodyInserters for Reactive Multiple Form Data

Instead of the typical call to bodyValue, use BodyInserters to create your body. This way, the parts will be sent in a non-blocking manner:

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

Key Points to Remember

Reactive Programming: Always make sure to avoid blocking I/O operations in reactive frameworks to maintain efficiency and prevent performance bottlenecks.

Using BodyInserters: This method allows you to insert multipart data reactively and thus helps avoid blocking operations.

Error Handling: Properly handle responses according to the HTTP status codes to ensure that your application behaves as expected during failures.

Conclusion

Sending multipart/form data without causing blocking calls can be tricky in Spring Boot when using WebClient. By employing BodyInserters along with reactive programming principles, you can effectively overcome these issues.

Following the structured approach highlighted above will not only fix the immediate problem but also enhance your application's overall performance and reliability when handling multipart data requests.

Feel free to implement this in your project and ease your WebClient interactions! If you have any more questions or need further guidance, don't hesitate to reach out.