Understanding Filters in Spring Boot

Introduction

Filters in Spring Boot play a crucial role in processing HTTP requests and responses. They allow developers to modify request headers, log requests, authenticate users, and optimize performance before the request reaches the controller. In this article, we will dive deep into the concept of filters in Spring Boot, their implementation, and best practices.

What Are Filters in Spring Boot?

Filters in Spring Boot are components that sit between the client and the request-handling logic. They process incoming requests and outgoing responses before they reach their final destination. Filters can be used for authentication, logging, compression, and more.

Spring Boot filters extend the javax.servlet.Filter interface and can be configured using Java configuration or annotations.

Types of Filters in Spring Boot

Spring Boot provides different types of filters to cater to various needs:

1. Authentication Filters: Used to authenticate requests before they reach the controller.
2. Logging Filters: Used to log request and response details.
3. Compression Filters: Used to compress responses before sending them to clients.
4. Security Filters: Used for security enhancements like CORS, CSRF protection, etc.

Creating a Custom Filter in Spring Boot

Let’s create a simple logging filter in Spring Boot that logs request details.

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import java.io.IOException;
import java.util.logging.Logger;

public class LoggingFilter implements Filter {
    private static final Logger logger = Logger.getLogger(LoggingFilter.class.getName());

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        // Initialization logic if needed
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        logger.info("Incoming request: " + request.getRemoteAddr());
        chain.doFilter(request, response);
        logger.info("Response sent");
    }

    @Override
    public void destroy() {
        // Cleanup logic if needed
    }
}

Registering a Filter in Spring Boot

You can register a filter in Spring Boot using a @Bean configuration.

import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class FilterConfig {
    @Bean
    public FilterRegistrationBean<LoggingFilter> loggingFilter() {
        FilterRegistrationBean<LoggingFilter> registrationBean = new FilterRegistrationBean<>();
        registrationBean.setFilter(new LoggingFilter());
        registrationBean.addUrlPatterns("/*");
        return registrationBean;
    }
}

Using Spring Boot’s OncePerRequestFilter

Spring Boot provides OncePerRequestFilter, which ensures a filter is executed only once per request.

import org.springframework.web.filter.OncePerRequestFilter;

import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.logging.Logger;

public class CustomOncePerRequestFilter extends OncePerRequestFilter {
    private static final Logger logger = Logger.getLogger(CustomOncePerRequestFilter.class.getName());

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {
        logger.info("Processing request in OncePerRequestFilter: " + request.getRequestURI());
        filterChain.doFilter(request, response);
    }
}

Common Use Cases for Filters

1. Authentication and Authorization – Implementing JWT token-based authentication.
2. Request Logging – Capturing and analyzing HTTP request details.
3. Rate Limiting – Preventing excessive requests from a single user.
4. CORS Handling – Managing cross-origin resource sharing.
5. Response Modification – Adding headers or transforming response content.

Best Practices for Using Filters in Spring Boot

• Use OncePerRequestFilter when the filter should run only once per request.
• Order filters properly using @Order annotation to avoid conflicts.
• Use FilterRegistrationBean to register filters dynamically.
• Avoid performing business logic inside filters.

External References

• Spring Boot Filters Documentation
• Java Servlet Filters
• Spring Security Filters

FAQs

1. What is the purpose of filters in Spring Boot?
   Filters help preprocess requests and responses to perform actions like authentication, logging, or modification before they reach controllers.
2. How do I create a custom filter in Spring Boot?
   Implement the javax.servlet.Filter interface and register it using FilterRegistrationBean or annotations.
3. What is OncePerRequestFilter in Spring Boot?
   It ensures that a filter runs only once per request, even in case of forwarded requests.
4. Can I have multiple filters in Spring Boot?
   Yes, multiple filters can be registered, and their execution order can be managed using the @Order annotation.
5. How do I set the order of filters in Spring Boot?
   Use the @Order annotation or set an order value in FilterRegistrationBean.
6. What is the difference between an interceptor and a filter?
   Filters work at the servlet level, while interceptors operate within the Spring MVC framework and have access to Spring context.
7. Can filters modify the request body in Spring Boot?
   Yes, but modifying request bodies requires wrapping the HttpServletRequestWrapper.
8. How do I enable CORS using a filter in Spring Boot?
   Create a custom filter that sets CORS headers on responses.
9. Is it necessary to manually register filters in Spring Boot?
   Not always. Spring Boot can auto-detect @Component annotated filters.
10. How can I disable a filter in Spring Boot?
    Use FilterRegistrationBean#setEnabled(false) or exclude it from component scanning.

By understanding filters in Spring Boot, Java professionals can enhance application security, improve performance, and ensure smoother request processing. Filters are an essential component in designing robust and scalable web applications in Spring Boot.