How to Define a Spring Boot Filter?

 1. Overview

In this quick tutorial, we’ll explore how to define custom filters and specify their invocation order with the help of Spring Boot.

2. Defining Filters and the Invocation Order

Let’s start by creating two filters:

1. TransactionFilter – to start and commit transactions
2. RequestResponseLoggingFilter – to log requests and responses

In order to create a filter, we need to simply implement the Filter interface:

@Component
@Order(1)
public class TransactionFilter implements Filter {

    @Override
    public void doFilter
      ServletRequest request, 
      ServletResponse response, 
      FilterChain chain) throws IOException, ServletException {
 
        HttpServletRequest req = (HttpServletRequest) request;
        LOG.info(
          "Starting a transaction for req : {}", 
          req.getRequestURI());
 
        chain.doFilter(request, response);
        LOG.info(
          "Committing a transaction for req : {}", 
          req.getRequestURI());
    }

    // other methods 
}

@Component
@Order(2)
public class RequestResponseLoggingFilter implements Filter {

    @Override
    public void doFilter(
      ServletRequest request, 
      ServletResponse response, 
      FilterChain chain) throws IOException, ServletException {
 
        HttpServletRequest req = (HttpServletRequest) request;
        HttpServletResponse res = (HttpServletResponse) response;
        LOG.info(
          "Logging Request  {} : {}", req.getMethod(), 
          req.getRequestURI());
        chain.doFilter(request, response);
        LOG.info(
          "Logging Response :{}", 
          res.getContentType());
    }

    // other methods
}

In order for Spring to be able to recognize a filter, we needed to define it as a bean with the @Component annotation.

And, to have the filters fire in the right order – we needed to use the @Order annotation.

4. A Quick Example

Let’s now create a simple endpoint and send an HTTP request to it:

@RestController
@RequestMapping("/users")
public class UserController {
    
    @GetMapping()
    public List<User> getAllUsers() {
        // ...
    }
}

The application logs on hitting this API are :

23:54:38 INFO  com.spring.demo.TransactionFilter - Starting Transaction for req :/users
23:54:38 INFO  c.s.d.RequestResponseLoggingFilter - Logging Request  GET : /users
...
23:54:38 INFO  c.s.d.RequestResponseLoggingFilter - Logging Response :application/json;charset=UTF-8
23:54:38 INFO  com.spring.demo.TransactionFilter - Committing Transaction for req :/users

This confirms that filters are invoked in the desired order.

5. Conclusion

In this article, we’ve summarized how to define custom filters in a Spring Boot webapp.

As always, code snippets can be found over on GitHub.