Came here with similar requirements:
Have a developing number of service classes in some package, which implement a common interface, and want to detect them at run time.
Partial problem is finding classes inside a specific package, where the application may be loaded from a jar file or from unpacked classes in a package/folder structure.
So I put the solutions from amicngh and an anonymous together.
// Retrieve classes of a package and it's nested package from file based class repository package esc; import java.io.File; import java.io.FileInputStream; import java.io.IOException; import java.net.MalformedURLException; import java.net.URL; import java.util.ArrayList; import java.util.Enumeration; import java.util.List; import java.util.jar.JarEntry; import java.util.jar.JarInputStream; public class GetClasses { private static boolean debug = false; /** * test function with assumed package esc.util */ public static void main(String... args) { try { final Class<?>[] list = getClasses("esc.util"); for (final Class<?> c : list) { System.out.println(c.getName()); } } catch (final IOException e) { e.printStackTrace(); } } /** * Scans all classes accessible from the context class loader which belong to the given package and subpackages. * * @precondition Thread Class loader attracts class and jar files, exclusively * @precondition Classes with static code sections are executed, when loaded and thus must not throw exceptions * * @param packageName * [in] The base package path, dot-separated * * @return The classes of package /packageName/ and nested packages * * @throws IOException, * ClassNotFoundException not applicable * * @author Sam Ginrich, http://www.java2s.com/example/java/reflection/recursive-method-used-to-find-all-classes-in-a-given-directory-and-sub.html * */ public static Class<?>[] getClasses(String packageName) throws IOException { final ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); assert classLoader != null; if (debug) { System.out.println("Class Loader class is " + classLoader.getClass().getName()); } final String packagePath = packageName.replace('.', '/'); final Enumeration<URL> resources = classLoader.getResources(packagePath); final List<Class<?>> classes = new ArrayList<Class<?>>(); while (resources.hasMoreElements()) { final URL resource = resources.nextElement(); final String proto = resource.getProtocol(); if ("file".equals(proto)) { classes.addAll(findFileClasses(new File(resource.getFile()), packageName)); } else if ("jar".equals(proto)) { classes.addAll(findJarClasses(resource)); } else { System.err.println("Protocol " + proto + " not supported"); continue; } } return classes.toArray(new Class[classes.size()]); } /** * Linear search for classes of a package from a jar file * * @param packageResource * [in] Jar URL of the base package, i.e. file URL bested in jar URL * * @return The classes of package /packageResource/ and nested packages * * @throws - * * @author amicngh, Sam [email protected] */ private static List<Class<?>> findJarClasses(URL packageResource) { final List<Class<?>> classes = new ArrayList<Class<?>>(); try { System.out.println("Jar URL Path is " + packageResource.getPath()); final URL fileUrl = new URL(packageResource.getPath()); final String proto = fileUrl.getProtocol(); if ("file".equals(proto)) { final String filePath = fileUrl.getPath().substring(1); // skip leading / final int jarTagPos = filePath.indexOf(".jar!/"); if (jarTagPos < 0) { System.err.println("Non-conformant jar file reference " + filePath + " !"); } else { final String packagePath = filePath.substring(jarTagPos + 6); final String jarFilename = filePath.substring(0, jarTagPos + 4); if (debug) { System.out.println("Package " + packagePath); System.out.println("Jar file " + jarFilename); } final String packagePrefix = packagePath + '/'; try { final JarInputStream jarFile = new JarInputStream( new FileInputStream(jarFilename)); JarEntry jarEntry; while (true) { jarEntry = jarFile.getNextJarEntry(); if (jarEntry == null) { break; } final String classPath = jarEntry.getName(); if (classPath.startsWith(packagePrefix) && classPath.endsWith(".class")) { final String className = classPath .substring(0, classPath.length() - 6).replace('/', '.'); if (debug) { System.out.println("Found entry " + jarEntry.getName()); } try { classes.add(Class.forName(className)); } catch (final ClassNotFoundException x) { System.err.println("Cannot load class " + className); } } } jarFile.close(); } catch (final Exception e) { e.printStackTrace(); } } } else { System.err.println("Nested protocol " + proto + " not supprted!"); } } catch (final MalformedURLException e) { e.printStackTrace(); } return classes; } /** * Recursive method used to find all classes in a given directory and subdirs. * * @param directory * The base directory * @param packageName * The package name for classes found inside the base directory * @return The classes * @author http://www.java2s.com/example/java/reflection/recursive-method-used-to-find-all-classes-in-a-given-directory-and-sub.html * @throws - * */ private static List<Class<?>> findFileClasses(File directory, String packageName) { final List<Class<?>> classes = new ArrayList<Class<?>>(); if (!directory.exists()) { System.err.println("Directory " + directory.getAbsolutePath() + " does not exist."); return classes; } final File[] files = directory.listFiles(); if (debug) { System.out.println("Directory " + directory.getAbsolutePath() + " has " + files.length + " elements."); } for (final File file : files) { if (file.isDirectory()) { assert !file.getName().contains("."); classes.addAll(findFileClasses(file, packageName + "." + file.getName())); } else if (file.getName().endsWith(".class")) { final String className = packageName + '.' + file.getName().substring(0, file.getName().length() - 6); try { classes.add(Class.forName(className)); } catch (final ClassNotFoundException cnf) { System.err.println("Cannot load class " + className); } } } return classes; } }
