Automatic conversion of JSON form parameter in Spring MVC 4.0

Question

I am trying to build a Spring MVC controller which will receive a POSTed form with a parameter in JSON format, and have Spring automatically convert it to a Java object.

Request content type is application/x-www-form-urlencoded
The name of the parameter that contains a JSON string is data.json

This is the controller:

@Controller
public class MyController {
    @RequestMapping(value = "/formHandler", method = RequestMethod.POST)
    public @ResponseBody String handleSubscription(
        @RequestParam("data.json") MyMessage msg) {
        logger.debug("id: " + msg.getId());
        return "OK";
    }
}

And this is what the MyMessage object looks like:

public class MyMessage {
    private String id;
    // Getter/setter omitted for brevity
}

Perhaps not surprisingly, posting a form with parameter data.json={"id":"Hello"} results in HTTP error 500 with this exception:

org.springframework.beans.ConversionNotSupportedException:
    Failed to convert value of type 'java.lang.String' to required type 'MyMessage' 
nested exception is java.lang.IllegalStateException:
    Cannot convert value of type [java.lang.String] to required type [MyMessage]: no matching editors or conversion strategy found

If I read the MappingJackson2HttpMessageConverter docs correctly, Jackson JSON conversion is triggered by Content-Type application/json, which I obviously cannot use since this is a form POST (and I don't control the POSTing part).

Is it possible to get Spring to convert the JSON string into an instance of MyMessage, or should I just give up, read it as a String and perform the conversion myself?

Automatic conversion of JSON form parameter in Spring MVC 4.0

2 Answers