Spring Boot 4 Microservices & REST APIs · Lesson

API Gateway with Spring Cloud Gateway

Set up a Spring Cloud Gateway to route requests, apply filters, and secure your microservices.

API Gateway with Spring Cloud Gateway is a free Spring Boot 4 Microservices & REST APIs lesson on CoddyKit — lesson 3 of 3. 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 Spring Boot 4 Microservices & REST APIs learning path, one of 3 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why an API Gateway?

Microservices are great for building scalable applications, but they can introduce complexity. Imagine having many services, each with its own address and port.

An API Gateway acts as a single, unified entry point for all client requests. It funnels incoming traffic to the correct microservice, simplifying how clients interact with your backend.

What is Spring Cloud Gateway?

Spring Cloud Gateway (SCG) is a powerful, reactive API Gateway built on Spring Framework 5, Project Reactor, and Spring Boot 2. It's designed for high performance and scalability.

It provides flexible routing based on requests.
Enables dynamic filtering of requests and responses.
Integrates seamlessly with other Spring Cloud projects.

Setting Up Your Gateway Project

To create a Spring Cloud Gateway application, start with a standard Spring Boot project. The key is to add the spring-cloud-starter-gateway dependency.

This dependency pulls in everything needed to transform your Spring Boot app into an intelligent gateway. Remember to include the spring-cloud-dependencies in your dependencyManagement section.

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.2.5</version>
    <relativePath/>
  </parent>
  <groupId>com.coddykit</groupId>
  <artifactId>gateway</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <name>gateway</name>
  <description>Demo project for Spring Cloud Gateway</description>
  <properties>
    <java.version>17</java.version>
    <spring-cloud.version>2023.0.1</spring-cloud.version>
  </properties>
  <dependencies>
    <dependency>
      <groupId>org.springframework.cloud</groupId>
      <artifactId>spring-cloud-starter-gateway</artifactId>
    </dependency>
  </dependencies>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-dependencies</artifactId>
        <version>${spring-cloud.version}</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
</project>

Defining Basic Routes

Routes are the core of an API Gateway. They tell the gateway how to forward incoming requests to the correct backend service. Routes are defined using predicates and a target URI.

Here, the Path=/hello/** predicate matches any request path starting with /hello/ and routes it to http://localhost:8081.

server:
  port: 8080

spring:
  application:
    name: api-gateway
  cloud:
    gateway:
      routes:
        - id: hello_route
          uri: http://localhost:8081
          predicates:
            - Path=/hello/**

Testing Your Gateway Route

To test the route, you'd typically have a backend microservice running. Let's imagine a simple 'hello-service' running on http://localhost:8081 that responds to any /hello/** path.

Start your Gateway application (e.g., on port 8080).
Make a request to http://localhost:8080/hello/world.
The Gateway intercepts it and forwards it to http://localhost:8081/hello/world.
The response from 'hello-service' is then sent back to the client via the Gateway.

Introducing Gateway Filters

Gateway Filters are functions that allow you to modify requests and responses as they pass through the gateway. They are incredibly powerful for implementing cross-cutting concerns.

Request Filters: Modify the request before it reaches the target service.
Response Filters: Modify the response before it's sent back to the client.

Filters can be applied globally to all routes or specifically to individual routes.

Applying a Simple Filter: AddRequestHeader

Let's enhance our hello_route with a filter. The AddRequestHeader filter is a built-in filter that adds a specified header to the request before forwarding it.

This is useful for injecting correlation IDs, security tokens, or origin information into requests sent to downstream services.

server:
  port: 8080

spring:
  application:
    name: api-gateway
  cloud:
    gateway:
      routes:
        - id: hello_route_with_header
          uri: http://localhost:8081
          predicates:
            - Path=/hello/**
          filters:
            - AddRequestHeader=X-Request-Source, Gateway

Centralizing Security with Gateway

An API Gateway is an ideal place to centralize security mechanisms for your microservices. Instead of implementing authentication and authorization in every service, the gateway can handle it once.

Authentication: Validate user credentials or tokens (e.g., JWT).
Authorization: Check if the authenticated user has permission for the requested resource.
If checks pass, the request proceeds; otherwise, it's rejected at the gateway level.

Beyond Built-in Filters: Custom Filters

While Spring Cloud Gateway provides many useful built-in filters, you might need custom logic. You can create your own filters by implementing the GlobalFilter and Ordered interfaces.

Custom filters are perfect for unique logging requirements, advanced metrics collection, or bespoke security checks that apply across your entire API landscape.

Gateway Concepts Check

Test your understanding of API Gateway fundamentals!

Recap: API Gateway Power

Great job! In this lesson, we explored the crucial role of an API Gateway in a microservices environment, specifically using Spring Cloud Gateway.

We understood its purpose as a single entry point.
Learned to configure basic routing with predicates.
Discovered how filters can modify requests and responses.
Touched upon its importance in centralizing security.

API Gateways are essential for building robust, secure, and manageable microservice systems.

Frequently asked questions

Is the “API Gateway with Spring Cloud Gateway” lesson free?

Yes — the full text of “API Gateway with Spring Cloud Gateway” is free to read here on the web, and the Spring Boot 4 Microservices & REST APIs course includes 3 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Boot 4 Microservices & REST APIs course, upgrade to CoddyKit PRO.

What will I learn in “API Gateway with Spring Cloud Gateway”?

Set up a Spring Cloud Gateway to route requests, apply filters, and secure your microservices. You practise Spring Boot 4 Microservices & REST APIs 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 Spring Boot 4 Microservices & REST APIs?

No prior experience is required. Spring Boot 4 Microservices & REST APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 3, so you can start here or from the beginning and move at your own pace.

How long does the “API Gateway with Spring Cloud Gateway” 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 Spring Boot 4 Microservices & REST APIs lesson?

Yes. Every Spring Boot 4 Microservices & REST APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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