How do you implement a custom gateway filter in Spring?

Table of Contents

• Introduction
• Types of Filters in Spring Cloud Gateway
  • Key Filter Interfaces
  • Steps to Implement a Custom Gateway Filter
  • Example 1: Implementing a Custom GatewayFilter for a Specific Route
    • Step 1: Create the Custom Gateway Filter
    • Step 2: Register the Custom Filter with a Route
      • Option 1: Configure Filter in application.yml
      • Option 2: Register the Filter Programmatically
  • Example 2: Implementing a Global Filter
    • Step 1: Create the Global Filter
    • Step 2: No Need to Register the Global Filter
  • Example 3: Custom Post-Filter for Modifying the Response
• Conclusion

Introduction

In Spring Cloud Gateway, filters are an essential mechanism for adding cross-cutting functionality to requests and responses. Filters are used for a wide range of purposes, including request logging, security checks, modifying request headers, rate limiting, authentication, and more. While Spring Cloud Gateway provides a set of built-in filters, you can easily extend its functionality by creating custom filters to suit your needs.

This guide explains how to implement a custom gateway filter in Spring Cloud Gateway and covers both pre-filters (executed before the request is routed) and post-filters (executed after the response is generated).

Types of Filters in Spring Cloud Gateway

Spring Cloud Gateway offers two primary types of filters:

1. Pre-filters: These filters are executed before the request is routed to the destination service. They are typically used for tasks like request validation, modifying headers, authentication, and logging.
2. Post-filters: These filters are executed after the response is generated by the downstream service but before it is sent back to the client. They are often used for tasks like response logging, modifying response headers, and custom error handling.

Key Filter Interfaces

• GatewayFilter: This is the main interface for implementing custom filters. It allows you to manipulate both the request and response during the request/response lifecycle.
• GlobalFilter: This is a special type of filter that applies globally across all routes in your application. You can use it for operations like logging or security that should be applied universally.

Steps to Implement a Custom Gateway Filter

There are two ways to implement a custom filter:

1. Custom GatewayFilter: A filter specific to a route.
2. GlobalFilter: A filter that is applied to all routes globally.

Example 1: Implementing a Custom GatewayFilter for a Specific Route

Step 1: Create the Custom Gateway Filter

To create a custom filter, implement the GatewayFilter interface and override the filter method. Here’s an example of a custom filter that adds a header to the incoming request.

Step 2: Register the Custom Filter with a Route

Now, you need to register the filter for a specific route. You can do this in the application.yml configuration file or programmatically using RouteLocator.

Option 1: Configure Filter in application.yml

Here’s how to configure the filter for a specific route in the application.yml:

Option 2: Register the Filter Programmatically

You can also register the filter programmatically using RouteLocatorBuilder in a Java configuration class:

Example 2: Implementing a Global Filter

If you want the custom filter to apply globally across all routes, implement the GlobalFilter interface. Here’s an example of a global filter that logs the details of every request.

Step 1: Create the Global Filter

Step 2: No Need to Register the Global Filter

Once you define a GlobalFilter as a Spring Bean (annotated with @Component), it will be applied automatically to all routes in the Spring Cloud Gateway application without any additional configuration.

Example 3: Custom Post-Filter for Modifying the Response

You can also create a post-filter that modifies the response before it is sent back to the client. Here's an example of a post-filter that logs the response status code.

This filter is useful for logging purposes, analytics, or monitoring.

Conclusion

Implementing custom gateway filters in Spring Cloud Gateway allows you to customize the way requests and responses are handled. Filters are versatile and can be used for a wide range of purposes, such as:

• Modifying headers (e.g., adding authentication tokens).
• Logging requests and responses for debugging or monitoring.
• Rate limiting or throttling to control the traffic to backend services.
• Authentication and authorization to enforce security policies across routes.

Spring Cloud Gateway provides two types of filters:

1. **GatewayFilter** for route-specific filters.
2. **GlobalFilter** for filters applied globally to all routes.

By creating custom filters, you can extend the functionality of Spring Cloud Gateway and ensure that it meets the specific needs of your microservices architecture. Whether you’re working with pre-filters or post-filters, the flexibility to 

modify the request and response pipeline is a powerful feature in Spring Cloud Gateway.