Make simple servlet filter work with @ControllerAdvice

Question

I've a simple filter just to check if a request contains a special header with static key - no user auth - just to protect endpoints. The idea is to throw an AccessForbiddenException if the key does not match which then will be mapped to response with a class annotated with @ControllerAdvice. However I can't make it work. My @ExceptionHandler isn't called.

ClientKeyFilter

import org.springframework.beans.factory.annotation.Value
import org.springframework.stereotype.Controller

import javax.servlet.*
import javax.servlet.http.HttpServletRequest

@Controller //I know that @Component might be here
public class ClientKeyFilter implements Filter {

  @Value('${CLIENT_KEY}')
  String clientKey

  public void init(FilterConfig filterConfig) {}

  public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) {
    req = (HttpServletRequest) req
    def reqClientKey = req.getHeader('Client-Key')
    if (!clientKey.equals(reqClientKey)) {
      throw new AccessForbiddenException('Invalid API key')
    }
    chain.doFilter(req, res)
  }

  public void destroy() {}
}

AccessForbiddenException

public class AccessForbiddenException extends RuntimeException {
  AccessForbiddenException(String message) {
    super(message)
  }
}

ExceptionController

@ControllerAdvice
class ExceptionController {
  static final Logger logger = LoggerFactory.getLogger(ExceptionController)

  @ExceptionHandler(AccessForbiddenException)
  public ResponseEntity handleException(HttpServletRequest request, AccessForbiddenException e) {
    logger.error('Caught exception.', e)
    return new ResponseEntity<>(e.getMessage(), I_AM_A_TEAPOT)
  }
}

Where I'm wrong? Can simple servlet filter work with spring-boot's exception mapping?

Answer 1

As specified by the java servlet specification Filters execute always before a Servlet is invoked. Now a @ControllerAdvice is only useful for controller which are executed inside the DispatcherServlet. So using a Filter and expecting a @ControllerAdvice or in this case the @ExceptionHandler, to be invoked isn't going to happen.

You need to either put the same logic in the filter (for writing a JSON response) or instead of a filter use a HandlerInterceptor which does this check. The easiest way is to extend the HandlerInterceptorAdapter and just override and implement the preHandle method and put the logic from the filter into that method.

public class ClientKeyInterceptor extends HandlerInterceptorAdapter {

    @Value('${CLIENT_KEY}')
    String clientKey

    @Override
    public boolean preHandle(ServletRequest req, ServletResponse res, Object handler) {
        String reqClientKey = req.getHeader('Client-Key')
        if (!clientKey.equals(reqClientKey)) {
          throw new AccessForbiddenException('Invalid API key')
        }
        return true;
    }

}