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