No converter for [class com.duan.domain.User] with preset Content-Type ‘null‘

 @RequestMapping(value="/quick10",produces = {"text/html;charset=utf-8"})
 @ResponseBody
 //期望SpringMVC自动将User转换成json格式的字符串
 public User save10(){
    User user = new User();
    user.setName("绝区零");
    user.setAge(18);
    return user;
 }

@RequestMapping(value="/quick10",produces = {"application/json;charset=utf-8"})
@ResponseBody
//期望SpringMVC自动将User转换成json格式的字符串
public User save10(){
    User user = new User();
    user.setName("绝区零");
    user.setAge(18);
    return user;
}

错误原因：由于方法是引用@ResponseBody注解，表示返回的是字符串，而此时要把@RequestMapping中的produces的值改为application/json才不会报错，如果是text/html则表示跳转到某个页面，前端无法解析为JSON字符串。