500 Internal Server Error on json post

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>mvc_1</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>

  </welcome-file-list>
  <servlet>
        <servlet-name>Jersey</servlet-name>
        <servlet-class>com.sun.jersey.spi.container.servlet.ServletContainer</servlet-class>

        <init-param>
            <param-name>com.sun.jersey.config.property.packages</param-name>
            <param-value>org.user</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>


        <servlet-name>Jersey</servlet-name>
        <url-pattern>/rest/*</url-pattern>
    </servlet-mapping>

</web-app>

var sUsername = $("#tbUsername").val();
	var sPassword = $("#tbPassword").val();
	var jsonObj = {"username":sUsername,"password":sPassword};

$.ajax
	({
		url:"http://localhost:8080/mvc_1/rest/controller/doPosting",
		type:'POST',
		dataType:'json',
		data:JSON.stringify(jsonObj),
        contentType: "application/json; charset=utf-8",
		success: function(data)
		{
			alert("success!!")
		},
		error:function(XHR,status,er)
		{
			alert(XHR.status +" : "+er+" : "+status);
		}
	}); 

@Path("/controller")
public class UserController {
	
	@POST
	@Path("/doPosting")
	@Consumes({ MediaType.APPLICATION_JSON })
	public String doPosting(User user) {

		String output = user.getUsername();
		System.out.println(output);

		String json = new Gson().toJson(user);

		System.out.println("Hi");

		return json;

	}
}