var user = {
    "username" : "test",
    "password" : "test",
    "ID" : 1
};
$.ajax({
    url: "/test1",
    type: "POST",
    data: JSON.stringify(user),
    contentType:"application/json; charset=utf-8",
    success: function() {
        alert("success")
    }
})

@RequestMapping(value = "/test1")
public String test2(@RequestBody User user) {

    //Can't receive the Param.
    //   console result : null
    System.out.println(user.getAge());

    //Nothing wrong
    //console result:
    //                  username:  test
    //                  password:  test
    System.out.println(user.getUsername());
    System.out.println(user.getPassword());
    return "test";
}