explicitly configure extension points for an extension (#265)

pf4j/src/main/java/org/pf4j/Extension.java

@@ -34,4 +34,17 @@ public @interface Extension {
 
     int ordinal() default 0;
 
+    /**
+     * An array of extension points, that are implemented by this extension.
+     * This explicit configuration overrides the automatic detection of extension points in the
+     * {@link org.pf4j.processor.ExtensionAnnotationProcessor}.
+     * <p>
+     * In case your extension is directly derived from an extension point this attribute is NOT required.
+     * But under certain <a href="https://github.com/pf4j/pf4j/issues/264">more complex scenarios</a> it
+     * might be useful to explicitly set the extension points for an extension.
+     *
+     * @return classes of extension points, that are implemented by this extension
+     */
+    Class<? extends ExtensionPoint>[] points() default {};
+
 }

pf4j/src/main/java/org/pf4j/processor/ExtensionAnnotationProcessor.java

@@ -17,11 +17,13 @@ package org.pf4j.processor;
 
 import org.pf4j.Extension;
 import org.pf4j.ExtensionPoint;
+import org.pf4j.util.ClassUtils;
 
 import javax.annotation.processing.AbstractProcessor;
 import javax.annotation.processing.ProcessingEnvironment;
 import javax.annotation.processing.RoundEnvironment;
 import javax.lang.model.SourceVersion;
+import javax.lang.model.element.AnnotationValue;
 import javax.lang.model.element.Element;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.type.DeclaredType;
@@ -168,6 +170,20 @@ public class ExtensionAnnotationProcessor extends AbstractProcessor {
     private List<TypeElement> findExtensionPoints(TypeElement extensionElement) {
         List<TypeElement> extensionPointElements = new ArrayList<>();
 
+        // use extension points, that were explicitly set in the extension annotation
+        AnnotationValue annotatedExtensionPoints = ClassUtils.getAnnotationValue(extensionElement, Extension.class, "points");
+        List<? extends AnnotationValue> extensionPointClasses = (annotatedExtensionPoints != null) ?
+            (List<? extends AnnotationValue>) annotatedExtensionPoints.getValue() :
+            null;
+        if (extensionPointClasses != null && !extensionPointClasses.isEmpty()) {
+            for (AnnotationValue extensionPointClass : extensionPointClasses) {
+                String extensionPointClassName = extensionPointClass.getValue().toString();
+                TypeElement extensionPointElement = processingEnv.getElementUtils().getTypeElement(extensionPointClassName);
+                extensionPointElements.add(extensionPointElement);
+            }
+        }
+        // detect extension points automatically, if they are not explicitly configured (default behaviour)
+        else {
             // search in interfaces
             for (TypeMirror item : extensionElement.getInterfaces()) {
                 boolean isExtensionPoint = processingEnv.getTypeUtils().isSubtype(item, getExtensionPointType());
@@ -186,6 +202,7 @@ public class ExtensionAnnotationProcessor extends AbstractProcessor {
                     extensionPointElements.add(extensionPointElement);
                 }
             }
+        }
 
         return extensionPointElements;
     }
@@ -228,5 +245,4 @@ public class ExtensionAnnotationProcessor extends AbstractProcessor {
 
         return storage;
     }
-
 }

pf4j/src/main/java/org/pf4j/util/ClassUtils.java

@@ -15,9 +15,14 @@
  */
 package org.pf4j.util;
 
+import javax.lang.model.element.AnnotationMirror;
+import javax.lang.model.element.AnnotationValue;
+import javax.lang.model.element.ExecutableElement;
+import javax.lang.model.element.TypeElement;
 import java.lang.reflect.Modifier;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Map;
 
 /**
  * @author Decebal Suiu
@@ -72,6 +77,60 @@ public class ClassUtils {
     }
     */
 
+    /**
+     * Get a certain annotation of a {@link TypeElement}.
+     * See <a href="https://stackoverflow.com/a/10167558">stackoverflow.com</a> for more information.
+     *
+     * @param typeElement the type element, that contains the requested annotation
+     * @param annotationClass the class of the requested annotation
+     * @return the requested annotation or null, if no annotation of the provided class was found
+     * @throws NullPointerException if <code>typeElement</code> or <code>annotationClass</code> is null
+     */
+    public static AnnotationMirror getAnnotationMirror(TypeElement typeElement, Class<?> annotationClass) {
+        String annotationClassName = annotationClass.getName();
+        for (AnnotationMirror m : typeElement.getAnnotationMirrors()) {
+            if (m.getAnnotationType().toString().equals(annotationClassName)) {
+                return m;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Get a certain parameter of an {@link AnnotationMirror}.
+     * See <a href="https://stackoverflow.com/a/10167558">stackoverflow.com</a> for more information.
+     *
+     * @param annotationMirror the annotation, that contains the requested parameter
+     * @param annotationParameter the name of the requested annotation parameter
+     * @return the requested parameter or null, if no parameter of the provided name was found
+     * @throws NullPointerException if <code>annotationMirror</code> is null
+     */
+    public static AnnotationValue getAnnotationValue(AnnotationMirror annotationMirror, String annotationParameter) {
+        for (Map.Entry<? extends ExecutableElement, ? extends AnnotationValue> entry : annotationMirror.getElementValues().entrySet()) {
+            if (entry.getKey().getSimpleName().toString().equals(annotationParameter)) {
+                return entry.getValue();
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Get a certain annotation parameter of a {@link TypeElement}.
+     * See <a href="https://stackoverflow.com/a/10167558">stackoverflow.com</a> for more information.
+     *
+     * @param typeElement the type element, that contains the requested annotation
+     * @param annotationClass the class of the requested annotation
+     * @param annotationParameter the name of the requested annotation parameter
+     * @return the requested parameter or null, if no annotation for the provided class was found or no annotation parameter was found
+     * @throws NullPointerException if <code>typeElement</code> or <code>annotationClass</code> is null
+     */
+    public static AnnotationValue getAnnotationValue(TypeElement typeElement, Class<?> annotationClass, String annotationParameter) {
+        AnnotationMirror annotationMirror = getAnnotationMirror(typeElement, annotationClass);
+        return (annotationMirror != null) ?
+            getAnnotationValue(annotationMirror, annotationParameter) :
+            null;
+    }
+
     /**
      * Uses {@link Class#getSimpleName()} to convert from {@link Class} to {@link String}.
      *