How to find annotated methods in a given package?

Question

I have a simple marker annotation for methods (similar to the first example in Item 35 in Effective Java (2nd ed)):

/**  * Marker annotation for methods that are called from installer's   * validation scripts etc.   */ @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface InstallerMethod { } 

Then, in a given package (say com.acme.installer), which has a few subpackages containing some 20 classes, I'd like to find all methods that are annotated with it. (Because I'd like to do some checks regarding all the annotated methods in a unit test.)

What (if any) is the easiest way to do this? Preferably without adding new 3rd party libraries or frameworks.

Edit: to clarify, obviously method.isAnnotationPresent(InstallerMethod.class) will be the way to check if a method has the annotation - but this problem includes finding all the methods.

Answer 1

If you want to implement it yourself, these methods will find all the classes in a given package:

/**  * Scans all classes accessible from the context class loader which belong  * to the given package and subpackages.  *   * @param packageName  *            The base package  * @return The classes  * @throws ClassNotFoundException  * @throws IOException  */ private Iterable<Class> getClasses(String packageName) throws ClassNotFoundException, IOException {     ClassLoader classLoader = Thread.currentThread().getContextClassLoader();     String path = packageName.replace('.', '/');     Enumeration<URL> resources = classLoader.getResources(path);     List<File> dirs = new ArrayList<File>();     while (resources.hasMoreElements())     {         URL resource = resources.nextElement();         URI uri = new URI(resource.toString());         dirs.add(new File(uri.getPath()));     }     List<Class> classes = new ArrayList<Class>();     for (File directory : dirs)     {         classes.addAll(findClasses(directory, packageName));     }      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  * @throws ClassNotFoundException  */ private 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())         {             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; } 

Then you can just filter on those classes with the given annotation:

for (Method method : testClass.getMethods()) {     if (method.isAnnotationPresent(InstallerMethod.class))     {         // do something     } } 

Answer 2

You should probably take a look at the open source Reflections library. With it you can easily achieve what you want with few lines of code:

Reflections reflections = new Reflections(      new ConfigurationBuilder().setUrls(      ClasspathHelper.forPackage( "com.acme.installer" ) ).setScanners(     new MethodAnnotationsScanner() ) ); Set<Method> methods = reflections.getMethodsAnnotatedWith(InstallerMethod.class);