What is the significance of the OncePerRequestFilter class?

Introduction
Conclusion

Introduction

In Spring Boot, filtering HTTP requests is an essential part of handling cross-cutting concerns such as authentication, logging, monitoring, and request modification. Spring provides a special filter class called OncePerRequestFilter, which ensures that a filter is executed once for each request.

This class plays a crucial role in simplifying filter management, ensuring that filters are executed in a consistent manner. In this guide, we'll explore the significance of OncePerRequestFilter, how it works, and where it is typically used in a Spring Boot application.

What is `OncePerRequestFilter`?

The OncePerRequestFilter class is part of the org.springframework.web.filter package and extends GenericFilterBean, which is a basic filter implementation in Spring. Its key feature is that it ensures that the filter's logic is applied exactly once during the request lifecycle.

This class is particularly useful when you want to perform operations that should only occur once per request, like authentication, logging, or exception handling. It helps in scenarios where multiple filters might be involved, ensuring that you don't accidentally apply logic multiple times for the same request.

How `OncePerRequestFilter` Works

OncePerRequestFilter works by overriding the doFilterInternal method. This method contains the logic that you want to execute for every request. The class ensures that your custom filter logic runs only once for each HTTP request, no matter how many other filters are involved in the request processing chain.

The doFilterInternal method is called for each incoming request, and it is guaranteed that the filter chain does not run multiple times per request.

Key Features of `OncePerRequestFilter`

Ensures Single Execution per Request:
The primary advantage of OncePerRequestFilter is that it guarantees that your custom filter logic is executed only once for each request. This is especially important when the request may pass through multiple filters, and you want to avoid redundant execution.
Avoids Redundant Logic:
In complex applications with multiple filters (such as those used for authentication, logging, caching, etc.), using OncePerRequestFilter can prevent issues related to executing the same logic more than once. For example, authentication logic might need to be applied once per request, and without this class, there could be a risk of executing authentication multiple times, leading to inefficiencies or errors.
Extends GenericFilterBean:
OncePerRequestFilter extends GenericFilterBean, which is a Spring filter class that provides basic functionality for creating custom filters. This means that OncePerRequestFilter inherits many of the capabilities of the generic filter class, such as easy integration with Spring’s filter chain.
Can Be Used for Cross-Cutting Concerns:
You can use OncePerRequestFilter for tasks such as:
- Authentication: Ensuring the JWT token or session cookie is validated only once during the request.
- Logging: For logging request details once before processing.
- Exception Handling: Ensuring exceptions are caught and handled appropriately without being duplicated.
- Metrics: Tracking performance metrics by ensuring that the tracking logic runs once per request.

Implementing a Custom Filter with `OncePerRequestFilter`

Here’s an example of how to create a custom filter using OncePerRequestFilter for authentication:

Why Use `OncePerRequestFilter`?

Reduces Complexity:
If your application uses multiple filters (for example, security, logging, or validation filters), it can become challenging to control when each filter is applied. By using OncePerRequestFilter, you simplify the logic and ensure that each filter runs only once for each request.
Prevents Duplicate Execution:
Without using OncePerRequestFilter, you might risk executing certain logic multiple times, especially in complex filter chains. This redundancy can lead to issues like repeated logging or unnecessary database calls. OncePerRequestFilter helps avoid such duplication by ensuring each filter executes just once per request.
Optimized for Request Processing:
In web applications, performance is important. By using OncePerRequestFilter, Spring ensures that you don’t incur the overhead of processing the same request multiple times in the filter chain.

Practical Use Cases for `OncePerRequestFilter`

Authentication Filters: In an application that requires token-based authentication (such as JWT), you can create a custom authentication filter that checks the token only once per request and sets the user’s authentication context.
Logging: You may want to log every incoming request but ensure that the logging is done only once. Using OncePerRequestFilter ensures that your logging logic is executed exactly once, regardless of how many filters are chained.
Metrics Collection: Collecting metrics like request duration or the number of hits per endpoint should also be done once per request to avoid redundant calculations.
Exception Handling: Catching and handling exceptions in a centralized filter is another valid use case for OncePerRequestFilter.

Conclusion

The OncePerRequestFilter class in Spring Boot provides a robust way to ensure that custom filter logic is applied only once during the request lifecycle. This is particularly useful when handling cross-cutting concerns like authentication, logging, or metrics collection. By using OncePerRequestFilter, developers can ensure that their filters are applied in a predictable and efficient manner, avoiding duplicate operations and reducing unnecessary complexity in the request processing chain.