Java Package Introspection

Question

How do i get all classes within a package?

Answer 1

You can't. Classes can come in via many different class loaders, including remote ones.

Answer 2

There is no global way to do that. That being said, if you know where your classes are coming from, you can walk the directory of a jar file or the file system.

Answer 3

There's a snippet from here that does exactly what you want, assuming the classes can be found locally:

private static Class[] getClasses(String packageName)
throws ClassNotFoundException, IOException {
 ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
 assert classLoader != null;
 String path = packageName.replace('.', '/');
 Enumeration<URL> resources = classLoader.getResources(path);
 List<File> dirs = new ArrayList<File>();
 while (resources.hasMoreElements()) {
  URL resource = resources.nextElement();
  dirs.add(new File(resource.getFile()));
 }
 ArrayList<Class> classes = new ArrayList<Class>();
 for (File directory : dirs) {
  classes.addAll(findClasses(directory, packageName));
 }
 return classes.toArray(new Class[classes.size()]);
}

private static List<Class> findClasses(File directory, String packageName) throws ClassNotFoundException {
 List<Class> classes = new ArrayList<Class>();
 if (!directory.exists()) {
  return classes;
 }
 File[] files = directory.listFiles();
 for (File file : files) {
  if (file.isDirectory()) {
   assert !file.getName().contains(".");
   classes.addAll(findClasses(file, packageName + "." + file.getName()));
  } else if (file.getName().endsWith(".class")) {
   classes.add(Class.forName(packageName + '.' + file.getName().substring(0, file.getName().length() - 6)));
  }
 }
 return classes;
}

Answer 4

Java doesn't have discovery.

Most products that have the ability to add (discover) new classes either have a text file describing "Program Extensions" or a specific directory where you can place either classes or jars that uses a trick like @JG described. (This is what eclipse does and is recommended for any solution where the user may add the new module by hand)

Answer 5

Here's a more complete way to solve this for jars, based on the idea posted by JG.

/**
 * Scans all classloaders for the current thread for loaded jars, and then scans
 * each jar for the package name in question, listing all classes directly under
 * the package name in question. Assumes directory structure in jar file and class
 * package naming follow java conventions (i.e. com.example.test.MyTest would be in
 * /com/example/test/MyTest.class)
 */
public Collection<Class> getClassesForPackage(String packageName) throws Exception {
  String packagePath = packageName.replace(".", "/");
  ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
  Set<URL> jarUrls = new HashSet<URL>();

  while (classLoader != null) {
    if (classLoader instanceof URLClassLoader)
      for (URL url : ((URLClassLoader) classLoader).getURLs())
        if (url.getFile().endsWith(".jar")  // may want better way to detect jar files
          jarUrls.add(url);

    classLoader = classLoader.getParent();
  }

  Set<Class> classes = new HashSet<Class>();

  for (URL url : jarUrls) {
    JarInputStream stream = new JarInputStream(url.openStream()); // may want better way to open url connections
    JarEntry entry = stream.getNextJarEntry();

    while (entry != null) {
      String name = entry.getName();
      int i = name.lastIndexOf("/");

      if (i > 0 && name.endsWith(".class") && name.substring(0, i).equals(packagePath)) 
        classes.add(Class.forName(name.substring(0, name.length() - 6).replace("/", ".")));

      entry = stream.getNextJarEntry();
    }

    stream.close();
  }

  return classes;
}

Answer 6

The above answers outline the required functionality. However, more details on the subject can be found here and here.