Filter Factory in Spring Cloud Gateway

A Filter Factory in Spring Cloud Gateway is used to create reusable filters that intercept and modify HTTP requests and responses during routing. It helps implement cross-cutting concerns without changing the backend services.

• Applies common functionalities such as authentication, logging, rate limiting, and header manipulation.
• Supports both built-in and custom filter factories for flexible request and response processing.
• Can be configured globally or for specific routes using the gateway configuration.

Types of Filters in Spring Cloud Gateway

Spring Cloud Gateway provides two types of filters:

1. GatewayFilter Factory

A GatewayFilter is a route-specific filter in Spring Cloud Gateway that is applied only to a particular route. It is used when you want to perform request or response processing for a specific microservice.

• Applies only to selected routes.
• Used for route-specific tasks such as header manipulation, authentication, and path rewriting.

Example: This filter only applies to the employee route.

routes:
- id: employee-service
uri: http://localhost:8081
predicates:
- Path=/employee/**
filters:
- AddRequestHeader=source,gateway

2. GlobalFilter

A GlobalFilter is a filter that executes for every request and response passing through the API Gateway. It is commonly used for cross-cutting concerns such as logging, authentication, monitoring, and tracing.

• Applies to all routes in the gateway.
• Used for common functionalities across all microservices.

Example: The LoggingFilter logs every request that enters the Spring Cloud Gateway.

@Component
public class LoggingFilter implements GlobalFilter {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange,
                             GatewayFilterChain chain) {

        System.out.println(
            "Request URI: "
            + exchange.getRequest().getURI());

        return chain.filter(exchange);
    }
}

Some Common Built-in Gateway Filter Factories

Sometimes built-in filters are not enough. Spring Cloud Gateway allows us to create our own Filter Factory.

Custom Gateway Filter Factory

A Custom Gateway Filter Factory allows developers to create their own filters when the built-in filters provided by Spring Cloud Gateway do not meet specific business requirements. It provides a reusable way to implement custom request and response processing logic.

• Used to implement custom business logic such as request validation, custom authentication, and logging.
• Can be configured and reused across multiple routes.

Steps to Create a Custom Gateway Filter Factory

Step 1: Create a Custom Filter Factory Class

Extend the AbstractGatewayFilterFactory class and implement the apply() method.

@Component
public class LoggingGatewayFilterFactory
        extends AbstractGatewayFilterFactory<
        LoggingGatewayFilterFactory.Config> {

    public LoggingGatewayFilterFactory() {
        super(Config.class);
    }

    @Override
    public GatewayFilter apply(Config config) {

        return (exchange, chain) -> {

            System.out.println(
                    "Request Received: "
                    + exchange.getRequest().getURI());

            return chain.filter(exchange)
                    .then(Mono.fromRunnable(() ->
                            System.out.println(
                                    "Response Sent")));
        };
    }

    public static class Config {

    }
}

Step 2: Register the Filter as a Spring Bean

Annotate the class with @Component.

@Component
public class LoggingGatewayFilterFactory
extends AbstractGatewayFilterFactory<Config> {
}

Step 3: Configure the Filter in application.yml

Add the custom filter to the desired route.

spring:
cloud:
gateway:
routes:
- id: employee-service
uri: http://localhost:8081
predicates:
- Path=/employee/
filters:
- Logging

Step 4: Run the Application

Start the Spring Cloud Gateway application using maven commands.

mvn spring-boot:run

Step 5: Test the Endpoint

Send a request through the gateway.

GET http://localhost:8080/employee/1

Console Output:

Request Received: /employee/1

Response Sent

GlobalFilter vs GatewayFilter