Dynamic Class Loading in Java (Enumeration)

Question

I have a problem, I wish to use reflection to generate instances of one of a set of classes at runtime. However, I have hit a snag. I want to get all of the classes involved to register themselves so the appropriate class can be chosen from a GUI. I can do this using a static code block in each file which provides a tidy OO solution. However, the java class loader specifically loads classes when they are required, hence if a needed class has not yet been used, it is not registered.

Short of directly providing a static list of names, or running through the underlying class/java files (which would break when packaged into a jar anyway), is there any way to force classes of certain packages to be loaded?

Basically, I want the ability to add new classes, from a specified superclass, without having to change/add any other code.

Answer 1

To clarify, your problem isn't about "dynamic class loading in Java", it's about dynamic class enumeration -- you know how to load classes, you just don't know what classes you want.

A quick Google came up with this page: http://forums.sun.com/thread.jspa?threadID=341935&start=0&tstart=0

Taken from that page, here's some sample code that ought to work:

public static Class[] getClasses(String pckgname)
        throws ClassNotFoundException {
    ArrayList<Class> classes = new ArrayList<Class>();
    // Get a File object for the package
    File directory = null;
    try {
        ClassLoader cld = Thread.currentThread().getContextClassLoader();
        if (cld == null) {
            throw new ClassNotFoundException("Can't get class loader.");
        }
        String path = '/' + pckgname.replace('.', '/');
        URL resource = cld.getResource(path);
        if (resource == null) {
            throw new ClassNotFoundException("No resource for " + path);
        }
        directory = new File(resource.getFile());
    } catch (NullPointerException x) {
        throw new ClassNotFoundException(pckgname + " (" + directory
                + ") does not appear to be a valid package");
    }
    if (directory.exists()) {
        // Get the list of the files contained in the package
        String[] files = directory.list();
        for (int i = 0; i < files.length; i++) {
            // we are only interested in .class files
            if (files[i].endsWith(".class")) {
                // removes the .class extension
                classes.add(Class.forName(pckgname + '.'
                        + files[i].substring(0, files[i].length() - 6)));
            }
        }
    } else {
        throw new ClassNotFoundException(pckgname
                + " does not appear to be a valid package");
    }
    Class[] classesA = new Class[classes.size()];
    classes.toArray(classesA);
    return classesA;
}

Answer 2

Spring Framework does component scanning based on annotations.

Take a look at ClassPathScanningCandidateComponentProvider class, for example. You can do the same thing based on interface / base class and it should work for all LOCAL classes. There is no way to do this for ALL classes in java.