javax.servlet.ServletException:实例化servlet类org.server.db.test

我刚开始学习Hibernate和Servlet。我有一个表格（姓名、电子邮件），我填写这些字段，请求就会发送到服务器。然后我从服务器获取数据并打印它们。这是正确的！

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String name = request.getParameter("userName");
    String email = request.getParameter("email");
    String ip = request.getRemoteAddr();
    response.getWriter().println("<html>");
    response.getWriter().println("<head>");
    response.getWriter().println("<title>This is a response</title>");
    response.getWriter().println("<head>");
    response.getWriter().println("<body>");
    response.getWriter().println("<br>Your name is: " + name);
    response.getWriter().println("<br>Your email is: " + email);
    response.getWriter().println("<br>Your IP address is: " + ip);
    response.getWriter().println("</body>");
    response.getWriter().println("</html>");
}

此外，我还有一个DB（数据库和.xml文件配置正确，ping成功，映射完成）和Hibernate代码：

public static void main(String[] args) {
    AnnotationConfiguration config = new AnnotationConfiguration();
    config.configure("hibernate.cfg.xml");
    SessionFactory factory = config.buildSessionFactory();
    Session session = factory.openSession();
    session.beginTransaction();
    Data data = new Data();
    data.setUserName(name);
    data.setUserEmail(email);
    session.save(data);
    session.getTransaction().commit();
    session.close();
}

我还有一个数据类：

@Entity
public class Data {
    @Id
    @GeneratedValue
    private int Id;
    private String userName;
    private String userEmail;
//getteres, setters...
}

但是，如果我将这些代码放入Servlet，我会得到一个HTTP错误500。我可以确定的错误消息是：

javax.servlet.ServletException: Error instantiating servlet class org.server.db.test.ServletExample org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293) org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859) org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11P‌rotocol.java:602) org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489) java.lang.Thread.run(Thread.java:722)

如何将我的信息从服务器放入数据库？

我的web.xml文件：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>Example02</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
    <description></description>
    <display-name>ServletExample</display-name>
    <servlet-name>ServletExample</servlet-name>
    <servlet-class>org.server.db.test.ServletExample</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ServletExample</servlet-name>
    <url-pattern>/servlet1</url-pattern>
  </servlet-mapping>
</web-app>

hibernate cfg文件：

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- Database connection settings -->
        <property name="connection.driver_class">org.postgresql.Driver</property>
        <property name="connection.url">jdbc:postgresql://localhost:5432/db</property>
        <property name="connection.username">postgres</property>
        <property name="connection.password">***</property>
        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">2</property>
        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.PostgreSQLDialect</property>
        <!-- Enable Hibernate's current session context -->
        <property name="current_session_context_class">thread</property>
        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>
        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">create</property>
         <mapping class="org.server.db.test.Data" />
         <mapping class="org.server.db.test.ServletExample" />
    </session-factory>
</hibernate-configuration>

javax.servlet.ServletException：实例化servlet类org.server.db.test.ServletExample 时出错

到目前为止，在服务器的掩护下，以下构造失败了，出现了一个异常：

ServletExample servletExample = new ServletExample();

确切的原因在后面的堆栈中进一步提到。这是最下面的一个。阅读服务器日志找到它。这个问题至少与"将数据从UI保存到数据库"无关。

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