javax.sql.datasource getconnection returns null

How can I fix the code below so that it does not throw an error?

The following line of code is giving me a null pointer exception:

return dataSource.getConnection();

Note that dataSource is an instance of javax.sql.DataSource which is specified in web.xml, and which works fine when called by other code.

Here is the actual method in DataAccessObject.java where the null pointer is occurring:

protected static Connection getConnection(){  
  try {
     return dataSource.getConnection();  //
  } catch (SQLException e) {
     throw new RuntimeException(e);
  }
}

The preceding method is being called by this line of code:

connection = getConnection();

Which is located in the following method in a class called CourseSummaryDAO as follows:

public List<CourseSummary> findAll(Long sid) {
    LinkedList<CourseSummary> coursesummaries = new LinkedList<CourseSummary>();
    ResultSet rs = null;
    PreparedStatement statement = null;
    Connection connection = null;
    try {
        connection = getConnection(); //
        String sql = "select * from coursetotals where spid=?";
        statement = connection.prepareStatement(sql);
        statement.setLong(1, sid);
        rs = statement.executeQuery();
        //for every row, call read method to extract column 
        //values and place them in a coursesummary instance
        while (rs.next()) {
            CourseSummary coursesummary = read("findAll", rs);
            coursesummaries.add(coursesummary);
        }
        return coursesummaries;
     }catch (SQLException e) {
        throw new RuntimeException(e);
     } 
     finally {
        close(rs, statement, connection);
    }
 }

To recreate this simply, I created the following TestCourseSummaries class:

public class TestCourseSummaries {
    public static void main(String[] args) {
    Long id = new Long(1002);
    CourseSummaryDAO myCSDAO = new CourseSummaryDAO();
    List<CourseSummary> coursesummaries = myCSDAO.findAll(id);
    for(int i = 0;i<coursesummaries.size();i++){
    System.out.println("type, numunits are: "+coursesummaries.get(i).getCourseType()+","+coursesummaries.get(i).getNumUnits());
    }
}
}

Edit

To address JustDanyul's question, I am enclosing the code that calls in my application, and the underlying DataAccessObject code which is extended by the two DAO objects in the calling code:

Here is the code in my application which triggers the error. See there are two classes that each extended DataAccessObject. Perhaps they are conflicting with each other, causing the second one not to get the database connection?

protected void doGet(HttpServletRequest req, HttpServletResponse resp) 
throws ServletException, IOException {  
   String idString = req.getParameter("id");
   Long id = new Long(idString);
   ThisObj thisobj = new ThisDAO().find(id);
   req.setAttribute("thisobj", thisobj);
   ThoseObjDAO myThoseDAO = new ThoseObjDAO();
   List<ThoseObj> thoseobjects = myThoseObjDAO.findAll(id);
   req.setAttribute("thoseobjects", thoseobjects);
   jsp.forward(req, resp);
}

And here is the code for the DataAccessObject class which is extended by the two DAO classes in the calling code:

public class DataAccessObject {
private static DataSource dataSource;
private static Object idLock = new Object();

public static void setDataSource(DataSource dataSource) {
   DataAccessObject.dataSource = dataSource;
}

protected static Connection getConnection() {
   try {return dataSource.getConnection();}
   catch (SQLException e) {throw new RuntimeException(e);}
}

protected static void close(Statement statement, Connection connection) {
   close(null, statement, connection);
}

protected static void close(ResultSet rs, Statement statement, Connection connection) {
   try {
      if (rs != null) rs.close();
      if (statement != null) statement.close();
      if (connection != null) connection.close();
   } catch (SQLException e) {throw new RuntimeException(e);}
}

protected static Long getUniqueId() {
   ResultSet rs = null;
   PreparedStatement statement = null;
   Connection connection = null;
   try {
      connection = getConnection();
      synchronized (idLock) {
         statement = connection.prepareStatement("select next_value from sequence");
         rs = statement.executeQuery();
         rs.first();
         long id = rs.getLong(1);
         statement.close();
         statement = connection.prepareStatement("update sequence set next_value = ?");
         statement.setLong(1, id + 1);
         statement.executeUpdate();
         statement.close();
         return new Long(id);
      }
   }
   catch (SQLException e) {throw new RuntimeException(e);}
   finally{close(rs, statement, connection);}
}
}

The data source is created in web.xml, as follows:

<resource-ref>
    <description>dataSource</description>
    <res-ref-name>datasource</res-ref-name>
    <res-type>javax.sql.DataSource</res-type>
    <res-auth>Container</res-auth>
</resource-ref>

Solution

I suspect that the code where it "runs fine" in, is code actually running in an application server. The example you are posting, which just runs a static void main() method, wont get any resources which has been defined in web.xml.

I am guessing that you use JDNI to setup the initial datasource. And then using something like

@Resource(name="jdbc/mydb")
private DataSource dataSource;

to set up your connection. Right?

EDIT:

After seeing your code, it seems like your data source is newer initialised at all. Just putting a element into your web.xml will not do it alone. You will also need to actually configure the dataSource, you know, specify the driver, username, password, uri etc etc etc.

I'm guessing the find() method of the DAO that works, isn't actually using the dataSource. What you have shown so far, doesn't insigunate that your have a initialised dataSource at all.

Just to give you an idea, I liked a tutorial on how you would do this with Tomcat and JDNI. (Or even better, use spring-jdbc).

http://www.mkyong.com/tomcat/how-to-configure-mysql-datasource-in-tomcat-6/