How can I enumerate all classes in a package and add them to a List?

Question

I need to enumerate all classes in a package and add them to a List. The non-dynamic version for a single class goes like this:

List allClasses = new ArrayList(); allClasses.add(String.class); 

How can I do this dynamically to add all classes in a package and all its subpackages?

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Update: Having read the early answers, it's absolutely true that I'm trying to solve another secondary problem, so let me state it. And I know this is possible since other tools do it. See new question here.

Update: Reading this again, I can see how it's being misread. I'm looking to enumerate all of MY PROJECT'S classes from the file system after compilation.

Answer 1

****UPDATE 1 (2012)****

OK, I've finally gotten around to cleaning up the code snippet below. I stuck it into it's own github project and even added tests.

https://github.com/ddopson/java-class-enumerator

****UPDATE 2 (2016)****

For an even more robust and feature-rich classpath scanner, see https://github.com/classgraph/classgraph . I'd recommend first reading my code snippet to gain a high level understanding, then using lukehutch's tool for production purposes.

****Original Post (2010)****

Strictly speaking, it isn't possible to list the classes in a package. This is because a package is really nothing more than a namespace (eg com.epicapplications.foo.bar), and any jar-file in the classpath could potentially add classes into a package. Even worse, the classloader will load classes on demand, and part of the classpath might be on the other side of a network connection.

It is possible to solve a more restrictive problem. eg, all classes in a JAR file, or all classes that a JAR file defines within a particular package. This is the more common scenario anyways.

Unfortunately, there isn't any framework code to make this task easy. You have to scan the filesystem in a manner similar to how the ClassLoader would look for class definitions.

There are a lot of samples on the web for class files in plain-old-directories. Most of us these days work with JAR files.

To get things working with JAR files, try this...

private static ArrayList<Class<?>> getClassesForPackage(Package pkg) {     String pkgname = pkg.getName();     ArrayList<Class<?>> classes = new ArrayList<Class<?>>();     // Get a File object for the package     File directory = null;     String fullPath;     String relPath = pkgname.replace('.', '/');     System.out.println("ClassDiscovery: Package: " + pkgname + " becomes Path:" + relPath);     URL resource = ClassLoader.getSystemClassLoader().getResource(relPath);     System.out.println("ClassDiscovery: Resource = " + resource);     if (resource == null) {         throw new RuntimeException("No resource for " + relPath);     }     fullPath = resource.getFile();     System.out.println("ClassDiscovery: FullPath = " + resource);      try {         directory = new File(resource.toURI());     } catch (URISyntaxException e) {         throw new RuntimeException(pkgname + " (" + resource + ") does not appear to be a valid URL / URI.  Strange, since we got it from the system...", e);     } catch (IllegalArgumentException e) {         directory = null;     }     System.out.println("ClassDiscovery: Directory = " + directory);      if (directory != null && 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                 String className = pkgname + '.' + files[i].substring(0, files[i].length() - 6);                 System.out.println("ClassDiscovery: className = " + className);                 try {                     classes.add(Class.forName(className));                 }                  catch (ClassNotFoundException e) {                     throw new RuntimeException("ClassNotFoundException loading " + className);                 }             }         }     }     else {         try {             String jarPath = fullPath.replaceFirst("[.]jar[!].*", ".jar").replaceFirst("file:", "");             JarFile jarFile = new JarFile(jarPath);                      Enumeration<JarEntry> entries = jarFile.entries();             while(entries.hasMoreElements()) {                 JarEntry entry = entries.nextElement();                 String entryName = entry.getName();                 if(entryName.startsWith(relPath) && entryName.length() > (relPath.length() + "/".length())) {                     System.out.println("ClassDiscovery: JarEntry: " + entryName);                     String className = entryName.replace('/', '.').replace('\\', '.').replace(".class", "");                     System.out.println("ClassDiscovery: className = " + className);                     try {                         classes.add(Class.forName(className));                     }                      catch (ClassNotFoundException e) {                         throw new RuntimeException("ClassNotFoundException loading " + className);                     }                 }             }         } catch (IOException e) {             throw new RuntimeException(pkgname + " (" + directory + ") does not appear to be a valid package", e);         }     }     return classes; }