如何将JEE应用程序连接到MySQL数据库？

Question

I'm trying to connect a JEE app to a MySQL database. There is a lot of documentations and similar questions on this topic, but none of the solutions i've found works so far (Nonetheless, I may have miss one in particular so feel free to propose).

The server name is db and the database name is joinmyparty

Here is my java code to connect :

public class MySqlConnection {
    private static Connection conn;

    public static Connection getConn() {
        try {
           Context initContext =  new InitialContext() ;
           Context envContext  = (Context)initContext.lookup("java:/comp/env") ;
           DataSource ds = (DataSource) envContext.lookup("jdbc/joinmyparty") ;
           conn = ds.getConnection();

        }  catch (NamingException e) {
            System.out.println("Cannot get connection: " + e);
        }  catch (SQLException e) {
            System.out.println("Cannot get connection: " + e);
        }
        return conn;
    }

Every time I call a DAO, I use this method getConn() to open a connection and i close it in the finally block.

I wrote this into my /tomcat/conf/context.xml file :

<?xml version="1.0" encoding="UTF-8"?>

<WatchedResource>WEB-INF/web.xml</WatchedResource>
<WatchedResource>WEB-INF/tomcat-web.xml</WatchedResource>
<WatchedResource>${catalina.base}/conf/web.xml</WatchedResource>

  <Resource url="jdbc:mysql://db:3306/joinmyparty"
driverClassName="com.mysql.jdbc.Driver" password="mypassword"
username="myusername" maxWait="10000" maxIdle="30"
maxActive="100" type="javax.sql.DataSource" auth="Container"
name="jdbc/joinmyparty" validationQuery="select 1" testOnBorrow="true"/>

</Context>

I've tried to put the mysql-connector .jar into the /tomcat/lib or into the /WEB-INF/lib and into the build-path. I've also tried multiple versions of connectors (but only one at a time), especialy to get the one with same level as MySQL database.

When I call a servlet which requires to connect the database, I've a blank page with a POST() method and an error 5OO with a GET() method. Here is the log error (I tried to translate it my best since i'm not english native) :

description : this server has encountered an internal error which prevents it from fulfilling your request

exception

java.lang.NullPointerException
    com.picco.user.dao.UserDao.findByEmail(UserDao.java:40)
    com.picco.user.servlet.Index.doGet(Index.java:56)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

And here the part of the code concerned (but as I said, all codes using DAO have the same problem).

try {
            UserDao udao = new UserDao();
            User u = udao.findByEmail(myCookieVal);
            SongDao sdao = new SongDao();
            ArrayList<Song> list = sdao.getAllSongs(u.getId());
            Random rd = new Random();
            int count = udao.count();
            request.setAttribute("currentSong", list.get(rd.nextInt(list.size())));
            request.setAttribute("songList", list);
            request.setAttribute("partyCount", count);
            this.getServletContext().getRequestDispatcher("/index.jsp").forward(request, response);
        } catch(SQLException e) {
            e.printStackTrace();
        } finally {
            HsqlConnection.closeInstance();
        }

Dont hesitate to ask me for details if I did not describe enought the problem.

Answer 1

也许问题是您在实例化设置为null的连接之前尝试获取资源User。

Answer 2

The MySQL JDBC JAR belongs in the Tomcat /lib folder. You want the Tomcat class loader to find it first.

Shouldn't your DAO get a connection? I'd add it to the constructor.