Sometimes it is necessary to amend the class load path at run time. For example, dynamically adding jar files containing user-configurable JDBC data sources. The way this is done is truly monstrous -- the URL Path format was only revealed when a colleague looked into the Java library sources.

   1 
  2  import java.net.URL;
   3  import java.io.IOException;
   4  import java.net.URLClassLoader;
   5  import java.net.MalformedURLException;
   6 
   7  public class JarFileLoader extends URLClassLoader
   8  {
   9      public JarFileLoader (URL[] urls)
  10      {
  11          super (urls);
  12      }
  13 
  14      public void addFile (String path) throws MalformedURLException
  15      {
  16          String urlPath = "jar:file://" + path + "!/";
  17          addURL (new URL (urlPath));
  18      }
  19 
  20      public static void main (String args [])
  21      {
  22          try
  23          {
  24              System.out.println ("First attempt...");
  25              Class.forName ("org.gjt.mm.mysql.Driver");
  26          }
  27          catch (Exception ex)
  28          {
  29              System.out.println ("Failed.");
  30          }
  31 
  32          try
  33          {
  34              URL urls [] = {};
  35 
  36              JarFileLoader cl = new JarFileLoader (urls);
  37              cl.addFile ("/opt/mysql-connector-java-5.0.4/mysql-connector-java-5.0.4-bin.jar");
  38              System.out.println ("Second attempt...");
  39              cl.loadClass ("org.gjt.mm.mysql.Driver");
  40              System.out.println ("Success!");
  41          }
  42          catch (Exception ex)
  43          {
  44              System.out.println ("Failed.");
  45              ex.printStackTrace ();
  46          }
  47      }
  48  }