Idea中写JavaWeb项目遇到“java.lang.ClassNotFoundException: com.mysql.jdbc.Driver”异常的解决方法

Idea中写JavaWeb项目遇到“java.lang.ClassNotFoundException: com.mysql.jdbc.Driver”异常的解决方法

今天用Idea写JavaWeb时，出现了java.lang.ClassNotFoundException: com.mysql.jdbc.Driver at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1365) at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1188) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:264) at lee.GetConnListener.contextInitialized(GetConnListener.java:17) at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4685) at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5146) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:183) at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:717) at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:690) at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:705) at org.apache.catalina.startup.HostConfig.manageApp(HostConfig.java:1728) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:289) at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819) at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801) at org.apache.catalina.mbeans.MBeanFactory.createStandardContext(MBeanFactory.java:456) at org.apache.catalina.mbeans.MBeanFactory.createStandardContext(MBeanFactory.java:405) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.apache.tomcat.util.modeler.BaseModelMBean.invoke(BaseModelMBean.java:289) at com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:819) at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:801) at javax.management.remote.rmi.RMIConnectionImpl.doOperation(RMIConnectionImpl.java:1468) at javax.management.remote.rmi.RMIConnectionImpl.access$300(RMIConnectionImpl.java:76) at javax.management.remote.rmi.RMIConnectionImpl$PrivilegedOperation.run(RMIConnectionImpl.java:1309) at javax.management.remote.rmi.RMIConnectionImpl.doPrivilegedOperation(RMIConnectionImpl.java:1401) at javax.management.remote.rmi.RMIConnectionImpl.invoke(RMIConnectionImpl.java:829) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:357) at sun.rmi.transport.Transport$1.run(Transport.java:200) at sun.rmi.transport.Transport$1.run(Transport.java:197) at java.security.AccessController.doPrivileged(Native Method) at sun.rmi.transport.Transport.serviceCall(Transport.java:196) at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:573) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:834) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:688) at java.security.AccessController.doPrivileged(Native Method) at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:687) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)
Idea没找到数据库驱动
我先想到的是没有配置好mysql-connector-java-5.0.8-bin.jar,所以我先配置了Project Structure->Moudles->Dependencies->JARs or directories找到mysql-connector-java-5.0.8-bin.jar 点击ok，然后勾上mysql-connector-java-5.0.8-bin.jar就可以了

这样配置后我发现写个普通的java测试类测试一下

可以连接上数据库`package lee;

import java.sql.Connection;
import java.sql.DriverManager;

public class Test {
public void test() {
try {
Class.forName(“com.mysql.jdbc.Driver”);
Connection conn = DriverManager.getConnection(“jdbc:mysql://localhost:3306/test”, “root”, “root”);
System.out.println(“连接数据库成功！”);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void main(String[]args){
new Test().test();
}

}
`

但是写一个ServletContextListener的实现类时就出现开始时的错误。
代码如下

import javax.servlet.*;
import javax.servlet.annotation.WebListener;
import java.sql.*;

@WebListener
public class GetConnListener implements ServletContextListener {
    public void contextInitialized(ServletContextEvent sce) {
        ServletContext application = sce.getServletContext();
        String driver = application.getInitParameter("driver");
        String url = application.getInitParameter("url");
        String user = application.getInitParameter("user");
        String pass = application.getInitParameter("pass");
        try {
            Class.forName(driver);
            Connection conn = DriverManager.getConnection(url,user,pass);
            application.setAttribute("conn",conn);
        } catch (Exception e) {
            System.out.println("获取数据库连接时异常"+e.getMessage());
            e.printStackTrace();
        }
    }
    public void contextDestroyed(ServletContextEvent sce) {
        ServletContext application = sce.getServletContext();
        Connection conn = (Connection) application.getAttribute("conn");
        if (conn!=null){
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

我又怀疑是web.xml中的com.mysql.jdbc.Driver没写对检查后发现没错，
web.xml代码：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">

    <context-param>
        <param-name>driver</param-name>
        <param-value>com.mysql.jdbc.Driver</param-value>
    </context-param>
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/test</param-value>
    </context-param>
    <context-param>
        <param-name>user</param-name>
        <param-value>root</param-value>
    </context-param>
    <context-param>
        <param-name>pass</param-name>
        <param-value>root</param-value>
    </context-param>
    <listener>
        <listener-class>lee.GetConnListener</listener-class>
    </listener>
</web-app>

然后我又debug ServletContextListener实例类

发现到第17行之前都没有问题，然后Step Over(F8)后直接跳到了第21行，发现什么问题都没有，这时就怀疑人生了。。。然后，在网上找到只要把mysql-connector-java-5.1.7-bin.jar复制到tomcat的lib目录下面！就可以了 然后启动tomecat运行就没问题了


补充：
页面效果由jsp呈现。
代码：

<%@ page import="java.sql.Connection" %>
<%@ page import="java.sql.ResultSet" %>
<%@ page import="java.sql.Statement" %><%--
  Created by IntelliJ IDEA.
  User: Admin
  Date: 2020/2/6
  Time: 16:23
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
  <h1>GetConn</h1>
  <%
    Connection conn = (Connection) application.getAttribute("conn");
    if(conn!=null){
      Statement stmt = conn.createStatement();
      stmt.executeQuery("select * from student");
      ResultSet resultSet = stmt.getResultSet();

   %>
  <table bgcolor="blue" border="1" width="300">
    <% while (resultSet.next()){
    %>
    <tr>
      <td><%=resultSet.getString(2)%></td>
    </tr>
    <%
    }
      }
    %>
    <tr>
      <td>2</td>
    </tr>
  </table>
  </body>
</html>