Adding Interceptors to WebClient in Spring Boot

In Spring Boot applications, WebClient is a powerful tool for making HTTP requests to external services. However, sometimes you need to intercept and modify these requests or responses for various reasons such as logging, authentication, or adding custom headers. This is where interceptors come into play. Interceptors allow you to intercept and manipulate the HTTP requests and responses before they are sent or received. In this article, we’ll explore how to add interceptors to WebClient in Spring Boot with examples.

What is WebClient?

WebClient is a non-blocking, reactive web client provided by Spring Framework. It offers a modern alternative to the RestTemplate for making HTTP requests in Spring-based applications. WebClient is built on top of Project Reactor, which provides support for reactive programming.

Adding Interceptors to WebClient

To add interceptors to WebClient in Spring Boot, you need to implement the ExchangeFilterFunction interface, which allows you to intercept the requests and responses. Then, you can register these interceptors with WebClient using the filter() method.

Let’s look at an example of how to create a simple interceptor to log the request and response details:

import org.springframework.web.reactive.function.client.ExchangeFilterFunction;
import org.springframework.web.reactive.function.client.WebClient;

public class LoggingInterceptor implements ExchangeFilterFunction {

    @Override
    public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction next) {
        System.out.println("Request: " + request.method() + " " + request.url());
        request.headers().forEach((name, values) -> values.forEach(value -> System.out.println(name + ": " + value)));

        return next.exchange(request)
            .doOnNext(response -> {
                System.out.println("Response Status: " + response.statusCode());
                response.headers().asHttpHeaders().forEach((name, values) -> values.forEach(value -> System.out.println(name + ": " + value)));
            });
    }
}

In this example, we’ve created a LoggingInterceptor class that implements the ExchangeFilterFunction interface. The filter() method logs the details of the request and response and then delegates the request to the next filter in the chain using the exchange() method.

Now, let’s see how to register this interceptor with WebClient:

import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;

public class Main {

    public static void main(String[] args) {
        WebClient webClient = WebClient.builder()
            .baseUrl("https://api.example.com")
            .filter(new LoggingInterceptor())
            .build();

        Mono<String> responseMono = webClient.get()
            .uri("/resource")
            .retrieve()
            .bodyToMono(String.class);

        responseMono.subscribe(System.out::println);
    }
}

In this example, we’ve created a WebClient instance and registered the LoggingInterceptor using the filter() method. Now, every request made by this WebClient will be intercepted by the LoggingInterceptor, and the request and response details will be logged.

Conclusion

Interceptors are a powerful feature of WebClient in Spring Boot that allow you to intercept and manipulate HTTP requests and responses. In this article, we’ve covered how to add interceptors to WebClient with examples. You can use interceptors for various purposes such as logging, authentication, adding custom headers, etc. Experiment with different interceptors to suit your application’s needs and enhance the functionality of your WebClient-based HTTP requests.