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Add a jar file to Java load path at run time (See related posts)

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.

import java.net.URL;
import java.io.IOException;
import java.net.URLClassLoader;
import java.net.MalformedURLException;

public class JarFileLoader extends URLClassLoader
{
    public JarFileLoader (URL[] urls)
    {
        super (urls);
    }

    public void addFile (String path) throws MalformedURLException
    {
        String urlPath = "jar:file://" + path + "!/";
        addURL (new URL (urlPath));
    }

    public static void main (String args [])
    {
        try
        {
            System.out.println ("First attempt...");
            Class.forName ("org.gjt.mm.mysql.Driver");
        }
        catch (Exception ex)
        {
            System.out.println ("Failed.");
        }

        try
        {
            URL urls [] = {};

            JarFileLoader cl = new JarFileLoader (urls);
            cl.addFile ("/opt/mysql-connector-java-5.0.4/mysql-connector-java-5.0.4-bin.jar");
            System.out.println ("Second attempt...");
            cl.loadClass ("org.gjt.mm.mysql.Driver");
            System.out.println ("Success!");
        }
        catch (Exception ex)
        {
            System.out.println ("Failed.");
            ex.printStackTrace ();
        }
    }
}

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