Can a directory be added to the class path at runtime?

Question

In order to better understand how things works in Java, I'd like to know if I can dynamically add, at runtime, a directory to the class path.

For example, if I launch a .jar using "java -jar mycp.jar" and output the java.class.path property, I may get:

java.class.path: '.:/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java'

Now can I modify this class path at runtime to add another directory? (for example before making the first call to a class using a .jar located in that directory I want to add).

Answer 1

You can use the following method:

URLClassLoader.addURL(URL url)

But you'll need to do this with reflection since the method is protected:

public static void addPath(String s) throws Exception {
    File f = new File(s);
    URL u = f.toURL();
    URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
    Class urlClass = URLClassLoader.class;
    Method method = urlClass.getDeclaredMethod("addURL", new Class[]{URL.class});
    method.setAccessible(true);
    method.invoke(urlClassLoader, new Object[]{u});
}

See the Java Trail on Reflection. Especially the section Drawbacks of Reflection

Answer 2

Update 2014: this is the code from the accepted answer, by Jonathan Spooner from 2011, slightly rewritten to have Eclipse's validators no longer create warnings (deprecation, rawtypes)

//need to do add path to Classpath with reflection since the URLClassLoader.addURL(URL url) method is protected:
public static void addPath(String s) throws Exception {
    File f = new File(s);
    URI u = f.toURI();
    URLClassLoader urlClassLoader = (URLClassLoader) ClassLoader.getSystemClassLoader();
    Class<URLClassLoader> urlClass = URLClassLoader.class;
    Method method = urlClass.getDeclaredMethod("addURL", new Class[]{URL.class});
    method.setAccessible(true);
    method.invoke(urlClassLoader, new Object[]{u.toURL()});
}

Answer 3

Yes, you can use URLClassLoader.. see example here. Doesn't use reflection.

-- edit --

Copying example from the link as suggested.

import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Hashtable;
import javax.naming.*;

public class ChangeLoader {
  public static void main(String[] args) throws MalformedURLException {
    if (args.length != 1) {
      System.err.println("usage: java ChangeLoader extra_classpath_url");
      System.exit(-1);
    }

    String url = args[0];
    // Save class loader so that we can restore later
    ClassLoader prevCl = Thread.currentThread().getContextClassLoader();

    // Use prevCl as parent, so we extend instead of replace
    ClassLoader urlCl = 
          URLClassLoader.newInstance(new URL[] {new URL(url)}, prevCl);

    try {
      Thread.currentThread().setContextClassLoader(urlCl);
      Context ctx = new InitialContext();
      System.out.println(ctx.lookup("tutorial/report.txt"));
      ctx.close();
    } catch (NamingException e) {
      e.printStackTrace();
    } finally {
      // Restore old loader
      Thread.currentThread().setContextClassLoader(prevCl);
    }
  } // main()
}