What Happens If Client JDBC DataSource JAR Is Missing?

JDBC Tutorials - Herong's Tutorial Examples

∟Derby (Java DB) JDBC DataSource Objects

∟What Happens If Client JDBC DataSource JAR Is Missing?

This section describes what will happen if you try to retrieve a client DataSource object, but the client JAR file is missing.

When you retrieve the object back from the JNDI directory service, you need to cast it back to its original class type, DataSource object, like:

  DataSource ds = (DataSource) ctx.lookup("DerbyTestDB");

Of course, this statement requires the javax.sql.DataSource class. It does not explicitly require the client DataSource class provided by the JDBC driver. But the client DataSource class is required, because the object stored in the directory service was created by the client DataSource class.

You can test this by executing the following program without the Derby JDBC JAR file in the classpath:

/* DerbyJndiLookup.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.*;
import java.sql.*;
import javax.sql.*;
import javax.naming.*;
public class DerbyJndiLookup {
  public static void main(String [] args) {
    Connection con = null;
    try {

// Starting the Directory service
      Hashtable env = new Hashtable();
      env.put(Context.INITIAL_CONTEXT_FACTORY,
        "com.sun.jndi.fscontext.RefFSContextFactory");
      env.put(Context.PROVIDER_URL, "file:/local/fscontext");
      Context ctx = new InitialContext(env);

// Looking up the DataSource object
      DataSource ds = (DataSource) ctx.lookup("DerbyTestDB");

// Getting a connection object
      con = ds.getConnection();

// Running a query
      Statement sta = con.createStatement();
      ResultSet res = sta.executeQuery(
        "SELECT * FROM HY_Address");
      System.out.println("List of Addresses: ");
      while (res.next()) {
         System.out.println(
           "  "+res.getInt("ID")
           + ", "+res.getString("StreetName")
           + ", "+res.getString("City"));
      }
      res.close();
      sta.close();

      con.close();

    } catch (Exception e) {
      System.err.println("Exception: "+e.getMessage());
      e.printStackTrace();
    }
  }
}

Here is the output of compilation and execution of the program:

herong> java -cp .;fscontext.jar;providerutil.jar \
   DerbyJndiLookup

Exception: javax.naming.Reference cannot be cast to
javax.sql.DataSource
java.lang.ClassCastException: javax.naming.Reference cannot be
cast to javax.sql.DataSource
        at DerbyJndiLookup.main(DerbyJndiLookup.java:22)

There was no compilation error. But the execution without Derby JDBC JAR resulted in an exception. It is interesting that the exception message is telling you directly what is wrong. The message just says that the object was failed to be cast to javax.sql.DataSource. The real reason was that the casting requires the object's original class, org.apache.derby.jdbc.ClientDataSource class, which is missing in the classpath, without the derbyclient.jar file.