API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) · Lesson

Custom Request/Response Filters

Develop your own custom filters to inject specific logic for modifying incoming requests or outgoing responses.

Custom Request/Response Filters is a free API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why Custom Filters?

Spring Cloud Gateway offers many built-in filters, but sometimes you need very specific logic. Custom filters let you inject your own code into the request/response flow.

This allows for highly tailored processing, such as custom authentication checks, unique logging formats, or specific header manipulations.

The GatewayFilter Interface

To create a custom filter, you'll implement the GatewayFilter interface. This interface has one main method: filter().

ServerWebExchange exchange: Provides access to the HTTP request and response.
GatewayFilterChain chain: Allows you to pass the request to the next filter or the target service.

You'll also often implement Ordered to define your filter's execution priority.

Basic Request Logger Filter

Let's create a simple filter that logs the incoming request path and method. This filter will run before the request is forwarded to the backend service.

Notice how chain.filter(exchange) is called to continue the processing.

import org.springframework.cloud.gateway.filter.GatewayFilter;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.core.Ordered;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

public class MyLoggingFilter implements GatewayFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        System.out.println("Incoming Request:");
        System.out.println("  Path: " + exchange.getRequest().getPath());
        System.out.println("  Method: " + exchange.getRequest().getMethod());
        return chain.filter(exchange); // Pass to next filter/service
    }

    @Override
    public int getOrder() {
        return -1; // Execute early
    }

    public static void main(String[] args) {
        MyLoggingFilter filter = new MyLoggingFilter();
        System.out.println("This filter logs request details.");
        System.out.println("In a Spring Gateway app, this code runs for each matched request.");
        System.out.println("The 'filter' method is the core logic, 'getOrder' sets priority.");
    }
}

Registering Your Custom Filter

Once you've created your filter class, you need to tell Spring Cloud Gateway to use it. You can register it either in your application.yml or programmatically.

For simple filters, registering directly on a route is common. For more complex, reusable filters, a filter factory is often used (covered later).

spring:
  cloud:
    gateway:
      routes:
        - id: my_custom_route
          uri: http://localhost:8081
          predicates:
            - Path=/api/**
          filters:
            - MyLoggingFilter # Name of your @Component filter

Accessing Request & Response

The ServerWebExchange object is your gateway to everything about the current HTTP interaction. It contains the ServerHttpRequest and ServerHttpResponse.

exchange.getRequest(): Get details about the incoming request (headers, body, path).
exchange.getResponse(): Modify the outgoing response (headers, status code).

Remember, these are reactive types, so operations often return Mono or Flux.

Modifying Request Headers

You might need to add or modify headers before forwarding a request to a backend service. For example, injecting an authentication token or a tracing ID.

Since ServerHttpRequest is immutable, you must build a new request using mutate() and then update the exchange.

import org.springframework.cloud.gateway.filter.GatewayFilter;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.core.Ordered;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

public class AddAuthHeaderFilter implements GatewayFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        String authToken = "my-secret-token-123"; // Get from config/service
        ServerWebExchange mutatedExchange = exchange.mutate()
            .request(builder -> builder.header("X-Auth-Token", authToken))
            .build();
        System.out.println("Added X-Auth-Token header to request.");
        return chain.filter(mutatedExchange); // Use the mutated exchange
    }

    @Override
    public int getOrder() {
        return 0; // After logging, before forwarding
    }

    public static void main(String[] args) {
        AddAuthHeaderFilter filter = new AddAuthHeaderFilter();
        System.out.println("This filter adds an 'X-Auth-Token' header.");
        System.out.println("It uses 'exchange.mutate().request(...).build()' to modify the request.");
    }
}

Modifying Response Headers

Custom filters can also modify the response *after* the backend service has sent its reply. This is useful for adding security headers, caching directives, or custom metadata.

The trick is to use exchange.getResponse().beforeCommit() to hook into the response lifecycle, or reactive chaining.

import org.springframework.cloud.gateway.filter.GatewayFilter;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.core.Ordered;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

public class AddResponseHeaderFilter implements GatewayFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        return chain.filter(exchange).then(Mono.fromRunnable(() -> {
            // This code runs AFTER the response from backend is received
            exchange.getResponse().getHeaders().add("X-Gateway-Processed", "true");
            System.out.println("Added X-Gateway-Processed header to response.");
        }));
    }

    @Override
    public int getOrder() {
        return 1; // After request processing
    }

    public static void main(String[] args) {
        AddResponseHeaderFilter filter = new AddResponseHeaderFilter();
        System.out.println("This filter adds 'X-Gateway-Processed' to the response.");
        System.out.println("It uses 'chain.filter(exchange).then(Mono.fromRunnable(...))' for post-processing.");
    }
}

Controlling Filter Order

When you have multiple custom filters, their execution order matters. The Ordered interface (and its getOrder() method) lets you define this.

Lower values (e.g., -1) mean higher priority (execute earlier).
Higher values (e.g., 1) mean lower priority (execute later).
Filters with the same order might run in an undefined sequence.

Use Ordered.HIGHEST_PRECEDENCE or LOWEST_PRECEDENCE for extreme cases.

Custom GatewayFilter Factories

For more advanced scenarios, especially when your filter needs configuration (e.g., a specific header name, a threshold value), you can create a GatewayFilterFactory.

This allows you to define a filter with parameters that can be configured directly in your application.yml, making your filters highly reusable and flexible.

You'd extend AbstractGatewayFilterFactory and define a configuration class for its properties.

Filter Logic Check

Consider a custom GatewayFilter that needs to both add a request header and then, after the backend responds, add a response header. Which parts of the filter() method would you modify?

Recap: Custom Filter Power

You've learned how to create custom GatewayFilters in Spring Cloud Gateway. These filters are powerful tools for injecting specific logic into your API Gateway's request and response flow.

Implement GatewayFilter and Ordered.
Use ServerWebExchange for request/response access.
Mutate requests and use reactive chaining for response modification.
Control execution order with getOrder().

Custom filters give you fine-grained control to tailor your gateway's behavior precisely.

Frequently asked questions

Is the “Custom Request/Response Filters” lesson free?

Yes — the full text of “Custom Request/Response Filters” is free to read here on the web, and the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) course, upgrade to CoddyKit PRO.

What will I learn in “Custom Request/Response Filters”?

Develop your own custom filters to inject specific logic for modifying incoming requests or outgoing responses. You practise API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway)?

No prior experience is required. API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Custom Request/Response Filters” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this API Gateway & Reverse Proxy (Nginx + Spring Cloud Gateway) lesson?

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