Inherit event handlers

When registering an event listener, Event Bus recursively walks the
entire inheritance tree and looks for event handlers.

README.md

@@ -221,6 +221,12 @@ The same applies when an exception event handler throws an exception.
 
 To avoid this, system events never cause system events and instead just issue a warning to the logger.
 
+## Inheritance
+
+When a superclass or an interface of an event listener defines event handlers, they will be detected and registered by Event Bus, even if they are `private`.
+If an event handler is overridden by the listener, the `@Event` annotation of the overridden method is automatically considered present on the overriding method.
+If the overridden method contains an implementation, it is ignored as expected.
+
 ## Debugging
 
 In more complex setups, taking a look at the event handler execution order can be helpful for debugging.

core/src/main/java/dev/kske/eventbus/core/EventBus.java

@@ -2,10 +2,12 @@ package dev.kske.eventbus.core;
 
 import java.lang.System.Logger;
 import java.lang.System.Logger.Level;
-import java.lang.reflect.InvocationTargetException;
+import java.lang.annotation.Annotation;
+import java.lang.reflect.*;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Consumer;
+import java.util.stream.Collectors;
 
 import dev.kske.eventbus.core.handler.*;
 
@@ -14,9 +16,8 @@ import dev.kske.eventbus.core.handler.*;
  * <p>
  * A singleton instance of this class can be lazily created and acquired using the
  * {@link EventBus#getInstance()} method.
- * <p>
- * This is a thread-safe implementation.
  *
+ * @implNote This is a thread-safe implementation.
  * @author Kai S. K. Engelbart
  * @since 0.0.1
  * @see Event
@@ -237,7 +238,7 @@ public final class EventBus {
 			priority = listener.getClass().getAnnotation(Priority.class).value();
 
 		registeredListeners.add(listener);
-		for (var method : listener.getClass().getDeclaredMethods()) {
+		for (var method : getHandlerMethods(listener.getClass())) {
 			Event annotation = method.getAnnotation(Event.class);
 
 			// Skip methods without annotations
@@ -257,6 +258,45 @@ public final class EventBus {
 				listener.getClass().getName());
 	}
 
+	/**
+	 * Searches for event handling methods declared inside the inheritance hierarchy of an event
+	 * listener.
+	 *
+	 * @param listenerClass the class to inspect
+	 * @return all event handling methods defined for the given listener
+	 * @since 1.3.0
+	 */
+	private Set<Method> getHandlerMethods(Class<?> listenerClass) {
+
+		// Get methods declared by the listener
+		Set<Method> methods = getMethodsAnnotatedWith(listenerClass, Event.class);
+
+		// Recursively add superclass handlers
+		if (listenerClass.getSuperclass() != null)
+			methods.addAll(getHandlerMethods(listenerClass.getSuperclass()));
+
+		// Recursively add interface handlers
+		for (Class<?> iClass : listenerClass.getInterfaces())
+			methods.addAll(getHandlerMethods(iClass));
+
+		return methods;
+	}
+
+	/**
+	 * Searches for declared methods with a specific annotation inside a class.
+	 *
+	 * @param enclosingClass  the class to inspect
+	 * @param annotationClass the annotation to look for
+	 * @return all methods matching the search criteria
+	 * @since 1.3.0
+	 */
+	private Set<Method> getMethodsAnnotatedWith(Class<?> enclosingClass,
+		Class<? extends Annotation> annotationClass) {
+		return Arrays.stream(enclosingClass.getDeclaredMethods())
+			.filter(m -> m.isAnnotationPresent(annotationClass))
+			.collect(Collectors.toSet());
+	}
+
 	/**
 	 * Registers a callback listener, which is a consumer that is invoked when an event occurs. The
 	 * listener is not polymorphic and has the {@link #DEFAULT_PRIORITY}.

core/src/main/java/dev/kske/eventbus/core/Polymorphic.java

@@ -18,6 +18,7 @@ import java.lang.annotation.*;
  * @see Event
  */
 @Documented
+@Inherited
 @Retention(RUNTIME)
 @Target({ METHOD, TYPE })
 public @interface Polymorphic {

core/src/main/java/dev/kske/eventbus/core/Priority.java

@@ -21,6 +21,7 @@ import java.lang.annotation.*;
  * @see Event
  */
 @Documented
+@Inherited
 @Retention(RUNTIME)
 @Target({ METHOD, TYPE })
 public @interface Priority {

core/src/test/java/dev/kske/eventbus/core/InheritanceTest.java

@@ -0,0 +1,40 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.assertSame;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests whether event handlers correctly work in the context of an inheritance hierarchy.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.3.0
+ */
+class InheritanceTest extends SimpleEventListenerBase implements SimpleEventListenerInterface {
+
+	EventBus bus = new EventBus();
+
+	@Test
+	void test() {
+		bus.registerListener(this);
+		var event = new SimpleEvent();
+
+		bus.dispatch(event);
+		assertSame(4, event.getCounter());
+	}
+
+	@Override
+	void onSimpleEventAbstractHandler(SimpleEvent event) {
+		event.increment();
+	}
+
+	@Override
+	public void onSimpleEventInterfaceHandler(SimpleEvent event) {
+		event.increment();
+	}
+
+	@Event
+	private void onSimpleEventPrivate(SimpleEvent event) {
+		event.increment();
+	}
+}

core/src/test/java/dev/kske/eventbus/core/SimpleEvent.java

@@ -1,9 +1,31 @@
 package dev.kske.eventbus.core;
 
 /**
- * A simple event for testing purposes.
+ * A simple event for testing purposes. The event contains a counter that is supposed to be
+ * incremented when the event is processed by a handler. That way it is possible to test whether all
+ * handlers that were supposed to be invoked were in fact invoked.
  *
  * @author Kai S. K. Engelbart
  * @since 0.0.1
  */
-class SimpleEvent {}
+class SimpleEvent {
+
+	private int counter;
+
+	@Override
+	public String toString() {
+		return String.format("SimpleEvent[%d]", counter);
+	}
+
+	void increment() {
+		++counter;
+	}
+
+	int getCounter() {
+		return counter;
+	}
+
+	void reset() {
+		counter = 0;
+	}
+}

core/src/test/java/dev/kske/eventbus/core/SimpleEventListenerBase.java

@@ -0,0 +1,20 @@
+package dev.kske.eventbus.core;
+
+/**
+ * An abstract class defining a package-private and a private handler for {@link SimpleEvent}.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.3.0
+ */
+abstract class SimpleEventListenerBase {
+
+	@Event
+	void onSimpleEventAbstractHandler(SimpleEvent event) {
+		event.increment();
+	}
+
+	@Event
+	private void onSimpleEventPrivate(SimpleEvent event) {
+		event.increment();
+	}
+}

core/src/test/java/dev/kske/eventbus/core/SimpleEventListenerInterface.java

@@ -0,0 +1,13 @@
+package dev.kske.eventbus.core;
+
+/**
+ * An interface defining a single handler for {@link SimpleEvent}.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.3.0
+ */
+interface SimpleEventListenerInterface {
+
+	@Event
+	void onSimpleEventInterfaceHandler(SimpleEvent event);
+}