Question

下面是从main（即eclipse RunAs java应用程序）执行时完美运行的代码。但是，如果我在同一个包中从外部调用Statement getStatement（意思是通过put调用它的servlet），我会得到java.lang.ClassNotFoundException：com.mysql.jdbc.Driver。指向原因的任何指示？

public class DBConnectivity {

    private static Statement getStatement(String username, String password) throws SQLException, ClassNotFoundException {
        Properties prop = new Properties();
        prop.put("user", "root");
        prop.put("password", "password");

        Class.forName("com.mysql.jdbc.Driver");

        Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/keeptrack", prop);
        Statement stmt = conn.createStatement();

        return stmt;
    }

    public void put(String username, String password, String table,
            String name, String exercise, String wsj, String food)
            throws SQLException, ClassNotFoundException {
        Statement stmt = getStatement(username, password);
        // checkIfTableExists(stmt, table);     
        stmt.executeUpdate(
                "insert into " + table +
                " " +
                "values(\'" + name + "\', \'" +  exercise + "\', \'" + wsj + "\', \'" + food + "\')");
    }   


    public List<String> get(String username, String password, String table)
            throws SQLException, ClassNotFoundException {
        Statement stmt = getStatement(username, password);
        //checkIfTableExists(stmt, table);      
        ResultSet rs = stmt.executeQuery("select FOOD from " + table);

        List<String> foodData = new ArrayList<String>();

        while(rs.next()) {
            String output = rs.getString("FOOD");
            foodData.add(output);
        //  System.out.println("wohoo: " + output);
        }

        return foodData;
    }   

    public static void main(String[] args) throws SQLException, ClassNotFoundException {
        DBConnectivity conn = new DBConnectivity();
        conn.put("root", "password", "trackTable", "D", "B", "C", "D");
        conn.get("root", "password", "trackTable");
    }
}

这是调用数据库的servlet：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String username = request.getParameter("firstname");
        String password = request.getParameter("lastname");
        String firstname  = username;
        String exercise = request.getParameter("exerise");
        String wsj = request.getParameter("wsj");
        String food = request.getParameter("food");


        try {
            new DBConnectivity().put(username, password, "trackTable", firstname, exercise, wsj, food);
        } catch (ClassNotFoundException | SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

new DBConnectivity().put电话会产生ClassNotFoundException。

堆栈跟踪是：

java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1295)
at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1147)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:190)
at org.ameya.dynasty.DBConnectivity.getStatement(DBConnectivity.java:27)
at org.ameya.dynasty.DBConnectivity.get(DBConnectivity.java:63)
at org.ameya.dynasty.ServletIndex.doGet(ServletIndex.java:39)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:618)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:537)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1085)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:658)
at org.apache.coyote.http11.Http11NioProtocol$Http11ConnectionHandler.process(Http11NioProtocol.java:222)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1556)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1513)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:744)

Answer 1

您的错误：

java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

如果它在Eclipse中运行，则表示您在项目的类路径中有MySql jdbc dirver jar，但它不适用于您的Web应用程序。

项目类路径（构建路径）与部署时Web应用程序的类路径不同。并非所有项目类路径上的jar都会自动导出。

通常WEB-INF/lib中的jars可用于Web应用程序，但不包含其中的jar。

您应该将MySql jdbc驱动程序jar放在WEB-INF/lib中，或者将其添加到Servlet容器的lib文件夹中（在您的情况下通过异常堆栈跟踪判断Tomcat）。

Answer 2

您必须将mysql-connector-java-5.1.24-bin.jar库文件包含到项目中。然后它不会给出找不到的错误类。此文件位于C:\Program Files\MySQL\Connector J 5.1.20.0文件夹中。

Answer 3

在项目库中添加mysql-connector jar。您可以从以下链接下载

Download Connector Jar file

Answer 4

确保你用来创建war文件的打包器（它也可能是eclipse），将mysql驱动程序jar文件放在war的web-inf lib中。

通常，在eclipse中，您可以通过转到项目属性 - >部署程序集并确保包含驱动程序jar来确保

Answer 5

如果在项目内的其他类中执行相同的方法（main），则必须完美执行。

类加载器是相同的，但您无法访问类getStatement之外的方法DBConnectivity，因为该方法是私有的。

您可以通过反射执行此方法。

当我从外部类调用它时，为什么会出现ClassNotFoundException？

5 个答案: