java - 尝试从 servlet 调用 JDBC 时抛出 NullPointerException

Question

我正在运行我的 servlet RegistrationServlet ，我尝试创建一个数据库对象

这是 servlet:

@WebServlet("/register")
public class RegistrationServlet extends HttpServlet 
{
    private static final long serialVersionUID = 1L;

    @Override

    public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException 
    {
        // create a new connection to mysql database  , with this we put the new client in the database 

        Database myDabatase = null;

        try {
            myDabatase = new Database();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }  // create a new database 

        myDabatase.createDatabaseAndTables();  // create the tables of the database


        HttpSession session = request.getSession();
        synchronized(session) 
        {


              boolean returnValue = myDabatase.addNewClient("david", "cole", "jamie", "123456789", "johnny", "blabla");     

          if (returnValue == true)  // client was added 
          {
              String addressPath = "/WEB-INF/results/show-name.jsp";
              RequestDispatcher dispatcher = request.getRequestDispatcher(addressPath);
              dispatcher.forward(request, response);
          }

          else if (returnValue == false)  // client was not added becuase he's already registered 
          {

          }


        }
    }

}

这是完整的类(class):

但是当我在我的 servlet 中执行该行时:

boolean returnValue = myDabatase.addNewClient("david", "cole", "jamie", "123456789", "johnny", "blabla");

我得到一个 NullPointerException 说:

java.lang.ClassNotFoundException: com.mysql.jdbc.Driver
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1711)
    at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1556)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Unknown Source)
    at db.Database.<init>(Database.java:18)
    at servlets.RegistrationServlet.doGet(RegistrationServlet.java:28)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

这让我想到了这条线:

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

在 Database 类的构造函数中，正如您在跟踪跟踪中所见:

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

但是，当我执行与主程序相同的程序时:

public class Main {

    public static void main(String [ ] args) throws Exception
    {

        Database myConnection = new Database();
        myConnection.createDatabaseAndTables();
        boolean returnValue = myConnection.addNewClient("david", "cole", "jamie", "123456789", "johnny", "blabla");

    }

}

一切正常，我没有收到 NullPointerException。

那么如果我从一个 servelt 做同样的事情有什么问题呢？为什么会出现 NullPointerException？

问候

Answer 1

当您运行 servlet 时，类路径中缺少 MySQL 驱动程序。可能需要将包含驱动程序的 JAR 文件添加到 Web 应用程序的 WEB-INF/lib 目录中。