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.

Further reading:

Find the Registered Spring Security Filters

Learn how to find all the registered Spring Security filters in an application.

Read more →

Configure a Spring Boot Web Application

Some of the more useful configs for a Spring Boot application.

Read more →

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.

2.1. Filter with URL Pattern

In the example above, our filters are registered by default for all the URL’s in our application. However, we may sometimes want a filter to only apply to certain URL patterns.

In this case, we have to remove the @Component annotation from the filter class definition and register the filter using a *FilterRegistrationBean*:

@Bean
public FilterRegistrationBean<RequestResponseLoggingFilter> loggingFilter(){
    FilterRegistrationBean<RequestResponseLoggingFilter> registrationBean
      = new FilterRegistrationBean<>();

    registrationBean.setFilter(new RequestResponseLoggingFilter());
    registrationBean.addUrlPatterns("/users/*");

    return registrationBean;
}

Now the filter will only apply for paths that match the /users/* pattern.

To set URL patterns for the filter, we can use the addUrlPatterns() or setUrlPatterns() methods.

3. 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.

4. 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.

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