Java – How to use URLClassLoader to load a *.class file

javareflection

I'm playing around with Reflection and I thought I'd make something which loads a class and prints the names of all fields in the class.
I've made a small hello world type of class to have something to inspect:

kent@rat:~/eclipsews/SmallExample/bin$ ls
IndependentClass.class
kent@rat:~/eclipsews/SmallExample/bin$ java IndependentClass 
Hello! Goodbye!
kent@rat:~/eclipsews/SmallExample/bin$ pwd
/home/kent/eclipsews/SmallExample/bin
kent@rat:~/eclipsews/SmallExample/bin$ 

Based on the above I draw two conclusions:

  • It exists at /home/kent/eclipsews/SmallExample/bin/IndependentClass.class
  • It works! (So it must be a proper .class-file which can be loaded by a class loader)

Then the code which is to use Reflection: (Line which causes an exception is marked)

import java.lang.reflect.Field;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

public class InspectClass {
    @SuppressWarnings("unchecked")
    public static void main(String[] args) throws ClassNotFoundException, MalformedURLException {
        URL classUrl;
        classUrl = new URL("file:///home/kent/eclipsews/SmallExample/bin/IndependentClass.class");
        URL[] classUrls = { classUrl };
        URLClassLoader ucl = new URLClassLoader(classUrls);
        Class c = ucl.loadClass("IndependentClass"); // LINE 14
        for(Field f: c.getDeclaredFields()) {
            System.out.println("Field name" + f.getName());
        }
    }
}

But when I run it I get:

Exception in thread "main" java.lang.ClassNotFoundException: IndependentClass
    at java.net.URLClassLoader$1.run(URLClassLoader.java:200)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
    at InspectClass.main(InspectClass.java:14)

My questions:

  1. What am I doing wrong above? How do I fix it?
  2. Is there a way to load several class files and iterate over them?

Best Solution

From the Javadocs for the URLClassLoader(URL[]) constructor:

Constructs a new URLClassLoader for the specified URLs using the default delegation parent ClassLoader. The URLs will be searched in the order specified for classes and resources after first searching in the parent class loader. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be downloaded and opened as needed.

So you have two options:

  1. Refer to the directory that the .class file is in
  2. Put the .class file into a JAR and refer to that

(1) is easier in this case, but (2) can be handy if you're using networked resources.