Using Spring MVC, accepting POST requests with bad

I am working on a REST api. Receiving a POST message with bad JSON (e.g. { sdfasdfasdf } ) causes Spring to return the default server page for a 400 Bad Request Error. I do not want to return a page, I want to return a custom JSON Error object.

I can do this when there is an exception thrown by using an @ExceptionHandler. So if it is a blank request or a blank JSON object (e.g. { } ), it will throw a NullPointerException and I can catch it with my ExceptionHandler and do whatever I please.

The problem then, is that Spring doesn't actually throw an exception when it is just invalid syntax... at least not that I can see. It simply returns the default error page from the server, whether it is Tomcat, Glassfish, etc.

So my question is how can I "intercept" Spring and cause it to use my exception handler or otherwise prevent the error page from displaying and instead return a JSON error object?

Here is my code:

@RequestMapping(value = "/trackingNumbers", method = RequestMethod.POST, consumes = "application/json")
@ResponseBody
public ResponseEntity<String> setTrackingNumber(@RequestBody TrackingNumber trackingNumber) {

    HttpStatus status = null;
    ResponseStatus responseStatus = null;
    String result = null;
    ObjectMapper mapper = new ObjectMapper();

    trackingNumbersService.setTrackingNumber(trackingNumber);
    status = HttpStatus.CREATED;
    result = trackingNumber.getCompany();


    ResponseEntity<String> response = new ResponseEntity<String>(result, status);

    return response;    
}

@ExceptionHandler({NullPointerException.class, EOFException.class})
@ResponseBody
public ResponseEntity<String> resolveException()
{
    HttpStatus status = null;
    ResponseStatus responseStatus = null;
    String result = null;
    ObjectMapper mapper = new ObjectMapper();

    responseStatus = new ResponseStatus("400", "That is not a valid form for a TrackingNumber object " + 
            "({\"company\":\"EXAMPLE\",\"pro_bill_id\":\"EXAMPLE123\",\"tracking_num\":\"EXAMPLE123\"})");
    status = HttpStatus.BAD_REQUEST;

    try {
        result = mapper.writeValueAsString(responseStatus);
    } catch (IOException e1) {
        e1.printStackTrace();
    }

    ResponseEntity<String> response = new ResponseEntity<String>(result, status);

    return response;
}

This was raised as an issue with Spring SPR-7439 - JSON (jackson) @RequestBody marshalling throws awkward exception - which was fixed in Spring 3.1M2 by having Spring throw a org.springframework.http.converter.HttpMessageNotReadableException in the case of a missing or invalid message body.

In your code you cannot create a ResponseStatus since it is abstract but I tested catching this exception with a simpler method locally with Spring 3.2.0.RELEASE running on Jetty 9.0.3.v20130506.

@ExceptionHandler({org.springframework.http.converter.HttpMessageNotReadableException.class})
@ResponseStatus(HttpStatus.BAD_REQUEST)
@ResponseBody
public String resolveException() {
    return "error";
}

and I received a 400 status "error" String response.

The defect was discussed on this Spring forum post.

Note: I started testing with Jetty 9.0.0.M4 but that had some other internal issues stopping the @ExceptionHandler completing, so depending on your container (Jetty, Tomcat, other) version you might need to get a newer version that plays nicely with whatever version of Spring you are using.