My code accepts a zip file as input. 
I unpack several images from the zip file into a temp directory (literally, created by File.createTempFile()) 

I have another program that will load these images as resources and put them into the database *if* I can get the temp directory onto the class path. If I can do this I won't have to write new code. Once my program exits, the temp directory should be removed from the classpath. 

Is there a standard way of going about this? Or is it just a horrific idea? 

This works pretty well: 

You can only add folders or jar files to a class loader. So if you have a single class file, you need to put it into the appropriate folder structure first.

Here is a rather ugly hack that adds to the SystemClassLoader at runtime:

import java.io.IOException; 
import java.io.File; 
import java.net.URLClassLoader; 
import java.net.URL; 
import java.lang.reflect.Method; 

public class ClassPathHacker { 

private static final Class[] parameters = new Class[]{URL.class}; 

public static void addFile(String s) throws IOException { 
   File f = new File(s); 
   addFile(f); 
}//end method 

public static void addFile(File f) throws IOException { 
   addURL(f.toURL()); 
}//end method 
 

public static void addURL(URL u) throws IOException { 

  URLClassLoader sysloader = (URLClassLoader) ClassLoader.getSystemClassLoader(); 
  Class sysclass = URLClassLoader.class; 

  try { 
     Method method = sysclass.getDeclaredMethod("addURL", parameters); 
     method.setAccessible(true); 
     method.invoke(sysloader, new Object[]{u}); 
  } catch (Throwable t) { 
     t.printStackTrace(); 
     throw new IOException("Error, could not add URL to system classloader"); 
  }//end try catch 

   }//end method 

}//end class 

PS: The reflection is necessary to access the protected method addURL. This could fail if there is a SecurityManager.

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