Why isn't my Spring WebFilter being applied to my webclient API requests?

Learn why your `Spring WebFilter` is not triggering when using `WebClient` in your Spring application, and discover how to properly implement request filtering using `ExchangeFilterFunction`.
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Understanding the Problem: Why Your Spring WebFilter Isn't Triggering

If you're working on a Spring application and have implemented a WebFilter, you might encounter an issue where your filter does not seem to be invoked during API requests made via a WebClient. This can be frustrating, especially if you've set up everything correctly and added debug statements to help trace the execution flow. In this guide, we will uncover the reason behind this issue and guide you on how to properly implement request filtering for your client requests.

The Scenario

You have created a WebFilter that is intended to add an authorization token to the headers of outgoing API requests made by your application. However, upon testing, you find that the println statement inside the filter is never executed. Below is a simplified version of what you might be seeing:

Your Current WebFilter Implementation

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The API Call Implementation

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The Core Issue: WebFilter vs. Client Requests

The fundamental issue here is a misunderstanding of how WebFilter works. In Spring, the WebFilter is designed primarily for handling server-side web requests, not client-side requests. This means that any attempts to use WebFilter for WebClient requests will not yield results, as the filter won't be executed during client-side calls.

Solution: Use ExchangeFilterFunction

To effectively intercept and manipulate client requests, you should utilize ExchangeFilterFunction. This allows you to define custom filters for your WebClient instance. Below is an example of how to implement this:

Define the Filter

Firstly, you need to create an ExchangeFilterFunction that will enable you to modify the client requests:

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Attach the Filter to WebClient

After defining your filter function, you will need to configure your WebClient to include this filter:

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Conclusion

In summary, if you're trying to apply a WebFilter to your WebClient requests and not seeing the expected behavior, remember that WebFilter is not intended for client-side processing. By adopting ExchangeFilterFunction, you will be able to effectively intercept client requests and manipulate them as needed.

Make sure to structure your code accordingly, and your filtering logic will be applied as intended. Happy coding!