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Why Custom Filters?

The GatewayFilter Interface

Basic Request Logger Filter

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

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

Modifying Request Headers

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

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

Custom GatewayFilter Factories

Filter Logic Check

Recap: Custom Filter Power