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.

Answer 3

Class.forName("class name here");

You'll have a String variable with the class name. Assuming you have a List of Strings containing all class names you want to be loaded (and automatically registered using static code blocks:

for(String className : classesToBeLoaded) Class.forName(className);

Answer 4

Go through this example:

public class MainClass {

  public static void main(String[] args){

    ClassLoader classLoader = MainClass.class.getClassLoader();

    try {
        Class aClass = classLoader.loadClass("com.jenkov.MyClass");
        System.out.println("aClass.getName() = " + aClass.getName());
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    }

}

Answer 5

There is a chance if your classes are in jars in the filesystem ( this may sound ridiculous but I've seen classes loaded from ldaps )

So, again if your classes are in the filesystem inside jars you can do the following:

Pseudo-java below:

  String classPath = System.getClassPath(); 
  String [] classPathParts = classpath.split(";"); // or :

  String [] allTheClasses = []

  for each ( file  in classPathParts ) {
      if( file is directory ) {
           allTheClasses.addAll( file.contents );
      } else if ( file is ".jar" ) {
           allTheClasses.addAll( getZippedNamesFrom( file ) );

      }
 }

 // At this point you would have all the classes in the array ( list ) .

 String [] packageNames = {"a.b.c", "d.e.f" };
 for each ( String  clazzName in allTheClasses ) {
       if ( packageNames.contains( clazzName.getPackageName() ) ) {
           Class.forName( clazzName ); // load it and have the static block run
           // Or run it your self
       }
 }

 // End of the story.

I did this for a similar thing a long while ago.

I use to load about 70k+ classes names using this approach in less than a second. In my case I was dynamically looking for about 10 classes so the whole process took me about 1.1 second.

Answer 6

You can use ClassFinder,

http://jmeter.apache.org/api/org/apache/jorphan/reflect/ClassFinder.html

Answer 7

I would suggest putting all the "dynamic" classes into one package. Scan that directory to pull out each filename and then use that in Class.forName() to load and register each class.

I just asked a similar question yesterday, and got some good feedback.

Create new class from a Variable in Java