README2.md

Annotation classpath scanning

This is the part 2 of the implementation of an injector, it reuses the code written for the part 1 (only in the tests).

The idea of the classpath scanning is to find all classes of a package annotated with some pre-registered annotation types, and auto-automatically execute the action corresponding to the annotation on those classes.

Here is an example, let suppose we have an annotation Component defined like this

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Component {
}

Then we have two classes Serviceand Dependency annotated with @Component defined as such

@Component
public class Service {
   private final Dependency dependency;

   @Inject
   public Service(Dependency dependency) {
      this.dependency = Objects.requireNonNull(dependency);
   }

   public Dependency getDependency() {
      return dependency;
   }
}

@Component
public class Dependency {
}

An annotation scanner is a class that will scan all the classes of the package, here all the classes of the package containing the class Application and for each class annotated with @Component execute the action, here, register the class as provider class in the registry.

So when calling lookupInstance on the registry, the registry knows how to create a Serviceand a Dependency.

public class Application {
  public static void main(String[] args) {
    var registry = new InjectorRegistry();
    var scanner  = new AnnotationScanner();
    scanner.addAction(Component.class, registry::registerProviderClass);
    scanner.scanClassPathPackageForAnnotations(Application.class);
    var service = registry.lookupInstance(Service.class);
  }
}

What do I need to implement it ?

For the implementation, ClassLoader.getResources() with as parameter a package name with the dots ('.') replaced by slash ('/') returns all the URLs of the folders containing the classes (you can have more than one folder, by example one folder in src and one folder in test).

Class.forName(name, /initialize=/ false, classloader) loads a Classwithout initializing the class (without running its static block).

Those two methods are already available in the class Utils2.java with the exceptions correctly managed.

Let's implement it

1. First, we want to implement a method findAllJavaFilesInFolder(path) (not public) that takes a Path and returns the name of all the Java class in the folder. A Java class is file with a filename that ends with ".class". The name of a Java class is the name of the file without the extension ".class". Implement the method findAllJavaFilesInFolder and check that the tests in the nested class "Q1" all pass.

2. Then we want to implement a method findAllClasses(packageName, classloader) (not public) that return a list of the classes (the Class<?>) contained in the package packageName loaded by the classloader taken as argument. Implement the method findAllClasses and check that the tests in the nested class "Q2" all pass.

   Note: Utils2.getResources(packageName.replace('.', '/'), classLoader) returns the URLs of the folders that contains the class files of a package. From a URL, you can get a URI and you can construct a Path from a URI. Utils2.loadClass(packageName + '.' + className, classLoader) loads a class from the qualified name of a class (the name with '.' in it).

3. We now want to add the method addAction(annotationClass, action) that take annotationClass the class of an annotation and action a function that takes a class and return void. addAction register the action for the annotation class and only one action can be registered for an annotation class Implement the method addAction and check that the tests in the nested class "Q3" all pass.

4. We want to implement the method scanClassPathPackageForAnnotations(class) that takes a class as parameter, find the corresponding package, load all the class from the folders corresponding to the package, find all annotations of the classes and run the action each time corresponding to the annotation classes. Implement the method scanClassPathPackageForAnnotations and check that the tests in the nested class "Q4" all pass.

   Note: there is a method getPackageName() to find the name of a package of a class and a method getClassLoader to get the classloader of a class.