NoHandlerFoundException in Spring Boot

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When developing web applications with Spring Boot, you may occasionally encounter exceptions related to the request-response lifecycle. One such exception is NoHandlerFoundException. In this post, we'll explore what causes this exception, how to handle it gracefully, and some best practices to prevent it.

What is NoHandlerFoundException? 

The NoHandlerFoundException is thrown when Spring's DispatcherServlet can't find a handler (like a Controller) for a particular request URL. Simply put, this means that your application does not have a matching endpoint for the URL the client has requested. 

Causes

Endpoint Doesn't Exist: The most straightforward reason - the client might be requesting an endpoint that does not exist in your application. 

Mistyped URL: A common cause is a simple typo in the requested URL. 

Incomplete Configuration: If certain configurations related to view resolvers or static resources are misconfigured, they might lead to this exception. 

How to Handle

Enable Custom Error Page

One of the best ways to handle this exception is by presenting a user-friendly 404 error page.

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(NoHandlerFoundException.class)
    public ModelAndView handleError404(HttpServletRequest request, Exception e) {
        ModelAndView mav = new ModelAndView("404error");
        mav.addObject("exception", e);
        return mav;
    }
}

In the above example, whenever NoHandlerFoundException is thrown, the user will be directed to a 404 error view.

Enable spring.mvc.throw-exception-if-no-handler-found

To ensure the exception is thrown and can be captured by your exception handler, add this to your application.properties:

spring.mvc.throw-exception-if-no-handler-found=true

Alongside this, you also need to set:

spring.resources.add-mappings=false

Logging

Always log the exception so that you can monitor and potentially identify malicious or erroneous requests.

@ExceptionHandler(NoHandlerFoundException.class)
public ResponseEntity<String> handleNoHandlerFoundException(NoHandlerFoundException ex) {
    log.error("404 Error", ex);
    return new ResponseEntity<>("Error 404 - Page Not Found", HttpStatus.NOT_FOUND);
}

Prevention

Consistent URL Design: Maintain consistency in your endpoint URLs. This helps in reducing client-side errors. 

Provide API Documentation: If you are developing a RESTful service, use tools like Swagger to provide clear API documentation. 

Monitoring and Alerts: Set up monitoring and alerting for 404 errors. This way, if a particular non-existing endpoint is being hit frequently, you can either create an appropriate handler or set up a redirect. 

Conclusion

NoHandlerFoundException in Spring Boot is a clear indication of a missed handler for a request URL. Proper handling and user-friendly error messages can enhance the user experience. Regularly monitoring logs and setting up alerts for such exceptions will further help in maintaining the health and reliability of your application.

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