Spring 3.2 @ControllerAdvice to handle Controller Exceptions

My Java/Spring web application has controllers that either return ModelAndView to a jsp page or return json for ajax calls.  Exceptions can occur at any time, and I noticed that on ajax success the data returned was json, but on ajax error the data returned was regular text; all ajax calls should return the same kind of data, namely json.  There had to be a way to solve this problem generically!

Spring 3.2 to the rescue - it introduced a new annotation called @ControllerAdvice (http://docs.spring.io/spring/docs/3.2.x/spring-framework-reference/html/new-in-3.2.html#new-in-3.2-webmvc-controller-advice)  that defines methods that applies to all @RequestMapping rest url methods, and in particular to help with exceptions with @ExceptionHandler.  We can use this to have a generic exception handling solution to have all ajax calls return json, and all ModelAndView return html.

• @ControllerAdvice allows you to define ONE controller class to handle ALL exceptions that could occur (no specific exceptions defined needed)
• You can control the HTTP status code returned
• You can control the json message returned by grabbing the exception message from the specific exception that occurred

Here's how you do it:

1. Your controllers won't change, the code will still continue to throw any number of specific exceptions:

@Controller
public class ExceptionController {
    
    @RequestMapping(value = "/randomException", method = RequestMethod.GET)
    public String randomException(Authentication auth, HttpServletRequest request) throws Exception {    
       
        throw new NumberFormatException(" " +
                "Test ControllerAdvice randomException[" + "NumberFormatException" + "]");

        [...]
    }

    @RequestMapping(value = "/mavException", method = RequestMethod.GET)   
    public ModelAndView modelAndViewException(Authentication auth, HttpServletRequest request) throws Exception {  
     
        throw new UnexpectedRollbackException("Test ControllerAdvice Exception for mav");
        [...]
    }
}

2. Write ONE new Controller, this will act as your ControllerAdvice controller. Notice:
• the class is annotated with @ControllerAdvice
• @ExceptionHandler is annotated above your generic "handleException" function
• handleException function takes generic Exception e as a parameter
• it checks the accept header to see how the controller function was invoked (via ajax or via form submit; randomException or mavException, respectively).  *Update*  We have to check for null first in case the request doesn't specify a response type expected.  If the request doesn't specify it, then simply return text/html.
• It returns json or html

@ControllerAdvice
public class CherryShoeControllerAdvice {

    /*
     * Handles JSON and HTML
     */
    @ExceptionHandler
    @ResponseBody
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public String handleException(HttpServletRequest request, HttpServletResponse response, Exception e) throws IOException {    
        String acceptHeader = request.getHeader("Accept");
       
        // If Accept header exists, check if it expects a response of type json, otherwise just return text/html
        // Use apache commons lang3 to escape json values
        if(acceptHeader.contains("application/json")) {
            // return as JSON
            String jsonString = 
                    "{\"success\": false, \"message\": \"" + StringEscapeUtils.escapeJson(e.getMessage()) + "\" }";
        
            System.out.println("In handleGeneric" + e.getMessage());
            return jsonString;
        } else {
            //return as HTML
            response.setContentType("text/html");
            return response.toString();
        }
    }

}

BTW - if you wanted to specify each type of exception going to a separate @ExceptionHandler you could.

BTW - The main cons for using pre-Spring 3.2 @ExceptionHandler (http://docs.spring.io/spring/docs/3.1.x/spring-framework-reference/html/mvc.html#mvc-exceptionhandlers) by itself are you have to:

• Define an ExceptionHandler for EACH controller (or have each controller inherit from one common base)
• Define EACH exception type to be handled – Or have each controller throw the specific type of exception expected