所以我有一个 MySQL JDBC 驱动程序的 .jar 文件，它在我的库源文件夹下，我有以下代码:

public static Connection getConnection() throws SQLException {
        Connection conn = null;
        try{
            Class.forName("com.mysql.jdbc.Driver").newInstance();
            String url = "jdbc:mysql://50.56.81.42:3306/GUEST_BOOK";
            String user = "user";
            String password = "pass";

            conn = (Connection) DriverManager.getConnection(url, user, password);
        }catch(ClassNotFoundException e){
            e.printStackTrace();
        }catch(InstantiationException e){
            e.printStackTrace();
        }catch(IllegalAccessException e){
            e.printStackTrace();
        }
            return conn;

    }

但是，它总是给我这个错误:

INFO: Server startup in 645 ms
java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1678)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1523)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Class.java:169)
    at gbook.DbHelper.getConnection(DbHelper.java:14)
    at gbook.DbHelper.getGuestBook(DbHelper.java:51)
    at org.apache.jsp.index_jsp._jspService(index_jsp.java:83)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:433)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:389)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:333)
    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:224)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:169)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:168)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:929)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:405)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:964)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:515)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:680)
Dec 1, 2011 1:41:05 PM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [jsp] in context with path [/JDBC-MySQL] threw exception [javax.servlet.ServletException: java.lang.NoClassDefFoundError: com/mysql/jdbc/Connection] with root cause
java.lang.ClassNotFoundException: com.mysql.jdbc.Connection
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1678)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1523)
    at gbook.DbHelper.getGuestBook(DbHelper.java:52)
    at org.apache.jsp.index_jsp._jspService(index_jsp.java:83)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

这是我添加 jar 文件的方式。我创建了一个 lib 文件夹并将 jar 放入其中，然后在 eclipse 中配置构建路径并将 lib 文件夹中的 jar 添加到项目中。