请求servlet时出现错误，我尝试了很多解决方法，但没有找到适当的解决方案。

Registration.java（Servlet类文件）

package login;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.hibernate.SessionFactory;

@WebServlet("/Registration")
public class Registration extends HttpServlet {
   private static final long serialVersionUID = 1L;


public Registration() {
    super();
 }


protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

     String username =  request.getParameter("username");
     String password =  request.getParameter("password");
     String name =  request.getParameter("name");

     Users u = new Users();
     u.setUsername(username);
     u.setPassword(password);
     u.setName(name);

     String uname = u.getUsername();
     System.out.println(uname);
     SessionFactory sessionFactory = new                 Configuration().configure().buildSessionFactory();
     Session session = sessionFactory.openSession();
        session.beginTransaction();
        session.save(u);
        session.getTransaction().commit();
        session.close();
        sessionFactory.close();

}

}

Users.java 此类用于将用户信息存储在“用户”表中

package login;
import javax.persistence.Entity;
import javax.persistence.Table;
@Entity
@Table(name="Users")
public class Users {
private String username;
private String password;
private String name;
public String getUsername() {
    return username;
}
public void setUsername(String username) {
    this.username = username;
}
public String getPassword() {
    return password;
}
public void setPassword(String password) {
    this.password = password;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}   
}

hibernate.cfg.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration SYSTEM "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
 <session-factory>
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="connection.url">jdbc:mysql://localhost:3306/login</property>
    <property name="connection.username">root</property>
    <property name="connection.password"></property>

    <property name="connection.pool_size">1</property>
    <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

    <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
    <property name="show_sql">true</property>
    <property name="hbm2ddl.auto">create</property>


      <mapping class ="login.Users" />

</session-factory>

资料夹结构

控制台日志

EVERE: Servlet.service() for servlet [Registration] in context with path [/loginapp] threw exception
org.hibernate.AnnotationException: No identifier specified for entity: login.Users
at    org.hibernate.cfg.InheritanceState.determineDefaultAccessType(InheritanceState.java:265)
at org.hibernate.cfg.InheritanceState.getElementsToProcess(InheritanceState.java:211)
at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:710)
at org.hibernate.boot.model.source.internal.annotations.AnnotationMetadataSourceProcessorImpl.processEntityHierarchies(AnnotationMetadataSourceProcessorImpl.java:245)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess$1.processEntityHierarchies(MetadataBuildingProcess.java:222)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:265)
at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.build(MetadataBuildingProcess.java:83)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:418)
at org.hibernate.boot.internal.MetadataBuilderImpl.build(MetadataBuilderImpl.java:87)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:692)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:724)
at login.Registration.doPost(Registration.java:47)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Unknown Source)