SpringMVC Http Message Converters

Quick note about spring mvc http message converter. Especially thanks to this post by Eugen Paraschiv.

Http Message Converters

An example:

Client sends a GET request to /foos with two headers Accept: application/json and Content-type: application/xml. To tell server that the request body is in xml and we want the server to return json.
Spring controller is hit. With @RequestBody annotation and Content-type of client, the request body will be converted from xml to specified java entity as argument of method by xml converter.
When spring controller returns a response. With @ResponseBody annotation and Accept of client, the return value/object will be converted from specified java entity to json by json converter.

Converters Type

By default, the following HttpMessageConverters instances are pre-enabled:

StringHttpMessageConverter – converts Strings
MappingJackson2HttpMessageConverter – converts to/from JSON
Jaxb2RootElementHttpMessageConverter – converts to/from XML
For the other converters - Message Converters

Set Up

We must declare explicitly support for annotation-driven MVC controllers

<mvc:annotation-driven/>

We could also use @EnableWebMvc annotation to do the same thing.

@RequestBody

@RequestBody indicates that the body of the HTTP Request is deserialized to that particular Java entity as method argument.

@RequestMapping(path = "/user)
public void test(@RequestBody User user) {
    ...
}

Content-Type header specified by the Client will be used to determine the appropriate converter for it. So Content-Type: application/xml here to choose a xml converter which will convert xml content in request body to User object in method argument.

@ResponseBody

@ResponseBody indicates that the return value of the method is serialized directly to the body of the HTTP Response.

@RequestMapping(value = "/getJson")
public @ResponseBody User test(){

	User user = new User();
	user.setPassword("1234");
	user.setUserName("DONG");

	return user;
}

Accept header specified by the Client will be used to choose the appropriate Http Converter to marshall the entity. So Accept: application/json here to choose a json converter which will convert user object to json fromat in the http response body.

{
    "password": 1234,
    "username": "DONG",
}