In a microservices architecture, client requests are typically routed through an API Gateway. This central component provides a unified entry point to all backend services, handling responsibilities like routing, security, rate limiting, logging, and load balancing.

Spring Cloud Gateway, built on top of Spring Boot and Project Reactor, offers a powerful, non-blocking API gateway framework that’s easy to integrate into existing Java systems.

In this guide, you’ll learn how to implement an API Gateway using Spring Boot and Spring Cloud Gateway, enabling efficient communication and control across your microservices.

What Is an API Gateway?

An API Gateway serves as a reverse proxy that routes client requests to the appropriate service. It abstracts service endpoints and enforces cross-cutting concerns such as:

  • Routing and path rewriting
  • Authentication and authorization
  • CORS and headers
  • Request throttling
  • Circuit breaking and fallback

Adding Spring Cloud Gateway to Your Project

Include the dependency in your pom.xml:

<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-gateway</artifactId>
</dependency>

Add the Spring Cloud BOM to manage versions:

<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-dependencies</artifactId>
<version>2023.0.0</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>

Defining Routes in application.yml

Configure routes to different services:

spring:
cloud:
gateway:
routes:
- id: user-service
uri: http://localhost:8081
predicates:
- Path=/api/users/**
filters:
- StripPrefix=2

        - id: order-service
          uri: http://localhost:8082
          predicates:
            - Path=/api/orders/**
          filters:
            - StripPrefix=2

This maps /api/users/** to the user service and removes the /api prefix.

Adding Custom Filters

You can implement global or route-specific filters for logging, authorization, etc.

Example: A custom filter to log request paths

@Component
public class LoggingFilter implements GlobalFilter {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        System.out.println("Request Path: " + exchange.getRequest().getPath());
        return chain.filter(exchange);
    }
}

Rate Limiting and Throttling

Add Redis and enable rate limiting:

spring:
cloud:
gateway:
routes:
- id: product-service
uri: http://localhost:8083
predicates:
- Path=/api/products/**
filters:
- name: RequestRateLimiter
args:
redis-rate-limiter.replenishRate: 10
redis-rate-limiter.burstCapacity: 20

You’ll need Redis running and include the spring-cloud-starter-redis dependency.

Securing the Gateway

Integrate with Spring Security and JWT to authenticate requests.

Add the dependency:

<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
</dependency>

Configure JWT validation:

spring:
security:
oauth2:
resourceserver:
jwt:
issuer-uri: https://auth.myapp.com

Secure routes with SecurityWebFilterChain:

@Bean
public SecurityWebFilterChain securityFilterChain(ServerHttpSecurity http) {
return http
.authorizeExchange(ex -> ex
.pathMatchers("/api/public/**").permitAll()
.anyExchange().authenticated()
)
.oauth2ResourceServer().jwt()
.and().build();
}

Load Balancing with Service Discovery

Use Spring Cloud Discovery (e.g., with Eureka or Consul) to resolve service URIs dynamically.

<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>

Enable load-balanced routing:

spring:
cloud:
gateway:
discovery:
locator:
enabled: true
lowerCaseServiceId: true

Then reference services by name:

routes:
- id: inventory
  uri: lb://inventory-service
  predicates:
  - Path=/api/inventory/**
    ```

---

#### Adding Circuit Breakers with Resilience4j

Protect services with timeouts and fallback handlers:

```yml
filters:
- name: CircuitBreaker
  args:
  name: inventoryCB
  fallbackUri: forward:/fallback/inventory

Create a fallback controller:

@RestController
public class FallbackController {
@GetMapping("/fallback/inventory")
public Mono<String> fallback() {
return Mono.just("Inventory service is temporarily unavailable");
}
}

CORS Configuration

Handle cross-origin requests in application.yml:

spring:
cloud:
gateway:
globalcors:
corsConfigurations:
'[/**]':
allowedOrigins: "*"
allowedMethods: "*"

Or create a custom CorsConfigurationSource.

Monitoring Gateway Traffic

Use Micrometer with Prometheus and Grafana to track:

  • Request rates
  • Response status codes
  • Latency per route

Add:

<dependency>
<groupId>io.micrometer</groupId>
<artifactId>micrometer-registry-prometheus</artifactId>
</dependency>

Expose actuator metrics:

management:
endpoints:
web:
exposure:
include: prometheus,health,info

Conclusion

A well-designed API Gateway is essential in any modern microservices architecture. With Spring Cloud Gateway, you can build a powerful and reactive API management layer that handles routing, security, load balancing, and resilience — all within the Spring Boot ecosystem.

By applying these patterns and best practices, your architecture becomes more maintainable, secure, and ready for scale.