无法为连接 URL 'null' 创建类 ' 的 JDBC 驱动程序：我不明白此异常

为什么当我提供数据库URL时，它说空URL并在异常中给出一个空的' '类？

我在使用Tomcat时试图通过servlet连接到derby数据库。当 servlet 运行时，我得到以下异常：

org.apache.tomcat.dbcp.dbcp.SQLNestedException: Cannot create JDBC driver of class '' for connect URL 'null'

at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createConnectionFactory(BasicDataSource.java:1452)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createDataSource(BasicDataSource.java:1371)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.getConnection(BasicDataSource.java:1044)

at servlets.servlet_1.doGet(servlet_1.java:23) // ---> Marked the statement in servlet

at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
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.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)
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$Worker.runTask(ThreadPoolExecutor.java:885)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.lang.NullPointerException
at sun.jdbc.odbc.JdbcOdbcDriver.getProtocol(JdbcOdbcDriver.java:507)
at sun.jdbc.odbc.JdbcOdbcDriver.knownURL(JdbcOdbcDriver.java:476)
at sun.jdbc.odbc.JdbcOdbcDriver.acceptsURL(JdbcOdbcDriver.java:307)
at java.sql.DriverManager.getDriver(DriverManager.java:253)
at org.apache.tomcat.dbcp.dbcp.BasicDataSource.createConnectionFactory(BasicDataSource.java:1437)
... 24 more

服务 ：

package servlets;

import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.servlet.http.*;
import javax.servlet.*;
import javax.sql.DataSource;

public class servlet_1 extends HttpServlet {

    @Override 
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        try {
           // String queryString = request.getQueryString();
            System.out.println("!!!!!!!!!!!!!!!!!!!");
            Context initContext = new InitialContext();
            Context envContext = (Context)initContext.lookup("java:comp/env");
            DataSource ds = (DataSource)envContext.lookup("jdbc/PollDatasource");
            Connection connection = ds.getConnection(); // -->LINE 23
            String sqlQuery = "select * from PollResult";
            PreparedStatement statement = connection.prepareStatement(sqlQuery);
            ResultSet set = statement.executeQuery();
            System.out.println("after the final statement");
        } catch (Exception exc) {
            exc.printStackTrace();
        }
    }
}

这是什么例外？为什么我会收到此异常？

我在Tomcat中添加了以下标签：context.xml

<Resource name="jdbc/PollDatasource" auth="Container" type="javax.sql.DataSource"
driverClassName="org.apache.derby.jdbc.EmbeddedDriver"
url="jdbc:derby://localhost:1527/poll_database;create=true"
username="suhail" password="suhail"
maxActive="20" maxIdle="10" maxWait="-1" />

和这个在：web.xml

<resource-ref>
  <description>my connection</description>
  <res-ref-name>jdbc/PollDatasource</res-ref-name>
  <res-type>javax.sql.DataSource</res-type>
  <res-auth>Container</res-auth>
</resource-ref>

我在哪里犯了错误？

显示数据库 URL 的图像。

注意：在@Bryan Pendleton回答之后，我将驱动程序更改为org.apache.derby.jdbc.ClientDriver，但我得到了相同的异常。