Creating a Filter Class in Spring Boot: A Step-by-Step Guide

Spring Boot is a powerful framework for building Java-based applications, and it provides a wide range of features to make development easier and more efficient. One of these features is the ability to add custom filters to your application, which can be used to perform a variety of tasks such as authentication, logging, and more. In this blog post, we’ll show you how to add a filter class in Spring Boot and provide an example of how to use it.



Step 1: Create a new class for your filter

To create a filter class in Spring Boot, you’ll first need to create a new class that implements the javax.servlet.Filter interface. This class should have the following methods:

  • void init(FilterConfig filterConfig): This method is called when the filter is first initialized and can be used to perform any setup tasks that are required.
  • void doFilter(ServletRequest request, ServletResponse response, FilterChain chain): This method is called for each request and is where you’ll perform the main logic of your filter.
  • void destroy(): This method is called when the filter is destroyed and can be used to perform any cleanup tasks that are required.

Here’s an example of a simple filter class that logs the request and response:

import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class LoggingFilter implements Filter {

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        // No setup required
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse res = (HttpServletResponse) response;

        System.out.println("Request: " + req.getMethod() + " " + req.getRequestURI());
        chain.doFilter(req, res);
        System.out.println("Response: " + res.getStatus());
    }

    @Override
    public void destroy() {
        // No cleanup required
    }
}


Step 2: Register your filter

Once you have created your filter class, you’ll need to register it with Spring Boot. You can do this by using the @WebFilter annotation and specifying the URL patterns that you want the filter to apply to.

Here’s an example of how you might register the above logging filter:

@WebFilter(filterName = "loggingFilter", urlPatterns = "/*")
public class LoggingFilter implements Filter {
    ...
}


Step 3: Test your filter

Now that you’ve created and registered your filter, you can test it by running your Spring Boot application and making a request to one of the URL patterns that you specified in the @WebFilter annotation. In the above example, the filter will be applied to all requests, so you can test it by making a request to any URL in your application.

You should see the request and response logged to the console, and you’ll know that your filter is working correctly.



Step 4: FilterRegistrationBean

by adding them to the FilterRegistrationBean and registering them in the WebConfig. Here’s an example of how you might do this:

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

In this example, we’re creating a FilterRegistrationBean for our LoggingFilter class and adding it to the Spring context. We’re also specifying that the filter should be applied to all URL patterns by using the addUrlPatterns method.

It’s also worth mentioning that you can specify filter order by setOrder(int) method, this can be useful when you have multiple filters and you want to set the order of execution.



Conclusion

Adding a filter class in Spring Boot is a powerful feature that allows you to add custom functionality to your application in a clean and organized way. By following the steps outlined in this blog post, you’ll be able to create your own filter class and register it with Spring Boot, so you can start using it to perform tasks such as authentication, logging, and more. With a little bit of practice, you’ll be able to create your own filters and take advantage of the many benefits that they offer.



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