我正在为 Java Web Developers 和 Tomcat v7.0 使用 Eclipse Java EE IDE。我配置了服务器并将动态 Web 项目与 tomcat 相关联。我在本地工作（本地主机）。
我有我的 Connection 课程，这是：
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class Conexion {
private static Connection conexion = null;
private Conexion() {
}
public static Connection getInstancia() {
if (conexion == null) {
try {
Class.forName("com.mysql.jdbc.Driver");
conexion = DriverManager.getConnection(
"jdbc:mysql://localhost/Esquema_VentaLibros","gustavo", "123581321");
} catch (SQLException sqlex) {
sqlex.printStackTrace();
} catch(ClassNotFoundException cnfex) {
cnfex.printStackTrace();
}
return conexion;
}
else {
return conexion;
}
}
}
在我的 index.jsp 文件中，我有一个用户可以登录的表单。根据登录是否正确，我想通过名为的控制器重定向到一个视图（Welcome.jsp）或另一个视图（Denegate.jsp）ControladorLogin
所以，在我的ListaDeQueries课堂上，我有登录方法，就是这个
public class ListaDeQueries {
private static PreparedStatement pst;
private static String query = "";
public static boolean loguear(String usr, String pass) {
query = "select * from Usuario where username = ? and password = ?";
try {
pst = Conexion.getInstancia().prepareStatement(query);
pst.setString(1, usr);
pst.setString(2, pass);
pst.executeQuery();
return true;
} catch (SQLException e) {
e.printStackTrace();
return false;
}
}
}
然后在我的 Servlet 中ControladorLogin调用这样的方法，如下所示：
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
if(ListaDeQueries.loguear(request.getParameter("usuario"), request.getParameter("clave"))) {
request.getRequestDispatcher("Welcome.jsp").forward(request, response);
}
else {
request.getRequestDispatcher("Denegate.jsp").forward(request, response);
}
}
这是我的小表格index.jsp：
<form method="post" action="ControladorLogin">
Usuario: <input type="text" name="usuario"/> <br/>
Clave: <input type="password" name="clave"/> <br/>
<input type="submit" value="Entrar"/>
</form>
问题是我得到了 MySQL 驱动程序的 ClassNotFoundException :java.lang.ClassNotFoundException: com.mysql.jdbc.Driver，但我正确地添加了库。在项目的属性 > Java 构建路径 > 库 > 添加外部 JAR，然后添加此文件中的 jar： http ://www.mysql.com/downloads/connector/j/
这是完整的 StackTrace：
java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1676)
at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1521)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at edu.itla.diaz.modelo.Conexion.getInstancia(Conexion.java:18)
at edu.itla.diaz.modelo.ListaDeQueries.loguear(ListaDeQueries.java:15)
at edu.itla.diaz.controladores.ControladorLogin.doPost(ControladorLogin.java:30)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
04/12/2012 01:04:45 PM org.apache.catalina.core.StandardWrapperValve invoke
GRAVE: Servlet.service() para servlet edu.itla.diaz.controladores.ControladorLogin lanzó excepción
java.lang.NullPointerException
at edu.itla.diaz.modelo.ListaDeQueries.loguear(ListaDeQueries.java:15)
at edu.itla.diaz.controladores.ControladorLogin.doPost(ControladorLogin.java:30)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:641)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)