Enhancing Request Auditing in Spring Boot Using OncePerRequestFilter

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In modern web applications, auditing every request hitting the service is crucial for various purposes such as logging, security, and monitoring. Spring Boot offers robust tools to help implement such features efficiently. One of the essential tools in this arsenal is the OncePerRequestFilter. This article explores the concept of filters in Spring Boot, the differences between filters and interceptors, and provides a step-by-step guide to implementing a request auditing feature using OncePerRequestFilter.

Understanding Filters in Spring Boot

Filters in Spring Boot are components that can inspect and manipulate incoming HTTP requests and outgoing HTTP responses. They provide a way to perform common tasks, such as logging, authentication, and data compression before the request reaches the controller and after the response leaves the controller.

Filters vs. Interceptors

Both filters and interceptors are used to process requests and responses, but they have different scopes and use cases:

Implementing Request Auditing Using OncePerRequestFilter

OncePerRequestFilter is a special type of filter in Spring Boot that ensures the filter is executed only once per request. This is particularly useful for tasks like request auditing, where duplicate processing could lead to inaccurate data.

Here’s a step-by-step explanation for OncePerRequestFilter is invoked during the request processing lifecycle in a Spring Boot application:

1. Request Incoming: When an HTTP request is received by the Spring Boot application, the servlet container (e.g., Tomcat, Jetty) begins processing it.
2. Servlet Container Filter Chain: Before the application’s controllers and endpoints are invoked, the request passes through a chain of servlet filters. This is where our custom OncePerRequestFilter is placed in the filter chain.
3. Filter Invocation: When the request reaches our OncePerRequestFilter, the doFilterInternal method of our filter is invoked where it simply passes the request to the controller.
4. Controller Execution: The request is further processed by the Spring MVC framework, which maps the request to the appropriate controller method.
5. Controller Logic: The controller method executes its logic and generates a response.
6. Response Outgoing: The response is then passed back through the filter chain in the reverse order (starting with the filter that was invoked last on the way in) before being sent to the client. Our OncePerRequestFilter is invoked again during the response phase and now we are storing the request and response in read-only mode.
7. Filter Chain Completion: Once the response has passed through the filter chain, the servlet container sends it to the client, completing the request-response cycle.

processRequestResponse() is used in finally — Because if any exceptions happen in filterChain.doFilter then the actual purpose of auditing will not happen. Be it any exception or any failure that needs to be tracked. So it is placed in finally.
The above filter needs to be registered as shown below

addUrlPatterns() is the place that helps us to configure which endpoints need to be audited. In the above case, all the endpoints in that service are audited. This can be customizable.

Using OncePerRequestFilter in Spring Boot is an effective way to implement request auditing. It ensures that each request is processed only once, avoiding duplicate logs or processing. By understanding the differences between filters and interceptors, developers can choose the right tool for their specific use case. The provided example demonstrates how easy it is to set up a custom filter to log requests and responses, enhancing the monitoring and debugging capabilities of your Spring Boot application.