1.接收参数，无非前端发过来就两种方式
①application/x-www-form-urlencoded
②application/json

前端contentType：application/x-www-form-urlencoded
1.public ResponseVo register(@RequestParam(value="username") String username)
//或者直接用对象接收
2.public ResponseVo register(User user)

前端contentType：application/json
public ResponseVo register(@RequestBody User user)

返回Json格式
直接在ResponseVo对象上用注解 
@JsonInclude(value=这个自己选 基本选 NOT_NULL)