Merge pull request 'Handler Caching' (#37) from f/handler-caching into develop

Reviewed-on: https://git.kske.dev/zdm/event-bus/pulls/37
Reviewed-by: delvh <leon@kske.dev>

Jenkinsfile

@@ -0,0 +1,41 @@
+pipeline {
+	agent any
+
+	options {
+		ansiColor('xterm')
+	}
+
+	stages {
+		stage('Build') {
+			steps {
+				sh 'mvn -DskipTests clean package'
+			}
+		}
+		stage('Test') {
+			steps {
+				sh 'mvn test'
+			}
+			post {
+				always {
+					junit '*/target/surefire-reports/*.xml'
+					publishCoverage adapters: [jacocoAdapter(mergeToOneReport: true, path: '*/target/site/jacoco/jacoco.xml')]
+				}
+			}
+		}
+		stage('SonarQube Analysis') {
+			when {
+				branch 'develop'
+			}
+			steps {
+				withSonarQubeEnv('KSKE SonarQube') {
+					sh 'mvn sonar:sonar'
+				}
+			}
+		}
+	}
+	post {
+		success {
+			archiveArtifacts artifacts: '*/target/event-bus-*.jar'
+		}
+	}
+}

README.md

@@ -45,9 +45,6 @@ public class SimpleEventListener {
 }
 ```
 
-In this case, an event bus is created and used locally.
-In a more sophisticated example the class would acquire an external event bus that is used by multiple classes.
-
 Note that creating static event handlers like this
 
 ```java
@@ -57,6 +54,33 @@ private static void onSimpleEvent(SimpleEvent event) { ... }
 
 is technically possible, however you would still have to create an instance of the event listener to register it at an event bus.
 
+## Installation
+
+Event Bus is available in Maven Central.
+To include it inside your project, just add the following dependency to your `pom.xml`:
+
+```xml
+<dependencies>
+    <dependency>
+        <groupId>dev.kske</groupId>
+        <artifactId>event-bus-core</artifactId>
+        <version>1.2.0</version>
+    </dependency>
+</dependencies>
+```
+
+Then, require the Event Bus Core module in your `module-info.java`:
+
+```java
+requires dev.kske.eventbus.core;
+```
+
+If you intend to use event handlers that are inaccessible to Event Bus by means of Java language access control, make sure to allow reflective access to your package for Event Bus:
+
+```java
+opens my.package to dev.kske.eventbus.core;
+```
+
 ## Polymorphic Event Handlers
 
 On certain occasions it's practical for an event handler to accept both events of the specified type, as well as subclasses of that event.
@@ -97,6 +121,18 @@ private void onSimpleEvent() {
 
 Make sure that you **do not** both declare a parameter and specify the event type in the annotation, as this would be ambiguous.
 
+## Callback listeners
+
+While defining event handlers as annotated methods is rather simple and readable, sometimes a more flexible approach is required.
+For this reason, there are callback event handlers that allow the registration of an "inline" event listener consisting of just one handler in the form of a consumer:
+
+```java
+EventBus.getInstance().registerListener(SimpleEvent.class, e -> System.out.println("Received " + e));
+```
+
+The event type has to be defined explicitly, with the priority and polymorphism parameters being optional.
+If you intend to remove the listener later, remember to keep a reference to it, as you would have to clear the entire event bus if you didn't.
+
 ## Listener-Level Properties
 
 When defining a dedicated event listener that, for example, performs pre- or post-processing, all event handlers will probably have the same non-standard priority.
@@ -147,14 +183,37 @@ private void onDeadEvent(DeadEvent deadEvent) { ... }
 ### Detecting Exceptions Thrown by Event Handlers
 
 When an event handler throws an exception, an exception event is dispatched that wraps the original event.
-A exception handler is declared as follows:
+An exception handler is declared as follows:
 
 ```java
 private void onExceptionEvent(ExceptionEvent ExceptionEvent) { ... }
 ```
-
 Both system events reference the event bus that caused them and a warning is logged if they are unhandled.
 
+#### Yeeting Exceptions Out of an Event Handler
+
+In some cases, a warning about an `Exception` that was thrown in an event handler is not enough, stays unnoticed, or an exception should be catched explicitly.
+Event Bus explicitly dispatches no `ExceptionEvent` when an `ExceptionWrapper` exception is thrown and instead simply rethrows it.
+`ExceptionWrapper` is an unchecked exception that (as the name says) simply wraps an exception that caused it.
+This means the following is possible and results in a normal program exit:
+```java
+@Event(String.class)
+void onString() {
+	throw new ExceptionWrapper(new RuntimeException("I failed!"));
+}
+
+void helloStackTrace() {
+	EventBus.getInstance().registerListener(this);
+	try {
+		EventBus.getInstance().dispatch("A string!");
+		System.exit(-1);
+	} catch(ExceptionWrapper e) {
+		e.getCause().printStackTrace();
+		System.exit(0);
+	}
+}
+```
+
 ### What About Endless Recursion Caused By Dead Events and Exception Events?
 
 As one might imagine, an unhandled dead event would theoretically lead to an endless recursion.
@@ -162,32 +221,25 @@ 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.
 
-## Installation
+## Inheritance
 
-Event Bus is available in Maven Central.
-To include it inside your project, just add the following dependency to your `pom.xml`:
+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 already contains an implementation in the superclass, the superclass implementation is ignored as expected.
 
-```xml
-<dependencies>
-    <dependency>
-        <groupId>dev.kske</groupId>
-        <artifactId>event-bus-core</artifactId>
-        <version>1.1.0</version>
-    </dependency>
-</dependencies>
-```
+The `@Priority` and `@Polymorphic` annotations are inherited both on a class and on a method level.
+If the priority or polymorphism has to be redefined on an inherited handler, the `@Event` annotation has to be added explicitly.
 
-Then, require the Event Bus Core module in your `module-info.java`:
+## Debugging
+
+In more complex setups, taking a look at the event handler execution order can be helpful for debugging.
+Event Bus offers a method for this purpose which can be used as follows:
 
 ```java
-requires dev.kske.eventbus.core;
+System.out.println(EventBus.getInstance().debugExecutionOrder(SimpleEvent.class));
 ```
 
-If you intend to use event handlers that are inaccessible to Event Bus by means of Java language access control, make sure to allow reflective access from your module:
-
-```java
-opens my.module to dev.kske.eventbus.core;
-```
+Then, the execution order can be inspected in the console.
 
 ## Compile-Time Error Checking with Event Bus Proc
 
@@ -208,7 +260,7 @@ When using Maven, it can be registered using the Maven Compiler Plugin:
             <annotationProcessorPath>
                 <groupId>dev.kske</groupId>
                 <artifactId>event-bus-proc</artifactId>
-                <version>1.1.0</version>
+                <version>1.2.0</version>
             </annotationProcessorPath>
         </annotationProcessorPaths>
     </configuration>

core/pom.xml

@@ -0,0 +1,66 @@
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+
+	<artifactId>event-bus-core</artifactId>
+	<name>Event Bus Core</name>
+
+	<parent>
+		<groupId>dev.kske</groupId>
+		<artifactId>event-bus</artifactId>
+		<version>1.2.0</version>
+	</parent>
+
+	<dependencies>
+		<dependency>
+			<groupId>org.junit.jupiter</groupId>
+			<artifactId>junit-jupiter-api</artifactId>
+			<version>5.8.1</version>
+			<scope>test</scope>
+		</dependency>
+	</dependencies>
+
+	<build>
+
+		<!-- Disable resource folder -->
+		<resources />
+
+		<plugins>
+
+			<!-- Run unit tests -->
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-surefire-plugin</artifactId>
+				<version>3.0.0-M5</version>
+				<configuration>
+					<argLine>
+						${argLine}
+						--add-opens dev.kske.eventbus.core/dev.kske.eventbus.core=ALL-UNNAMED
+						--add-opens dev.kske.eventbus.core/dev.kske.eventbus.core.handler=ALL-UNNAMED
+					</argLine>
+				</configuration>
+			</plugin>
+
+			<!-- Generate coverage report -->
+			<plugin>
+				<groupId>org.jacoco</groupId>
+				<artifactId>jacoco-maven-plugin</artifactId>
+				<version>0.8.7</version>
+				<executions>
+					<execution>
+						<goals>
+							<goal>prepare-agent</goal>
+						</goals>
+					</execution>
+					<execution>
+						<id>report</id>
+						<phase>test</phase>
+						<goals>
+							<goal>report</goal>
+						</goals>
+					</execution>
+				</executions>
+			</plugin>
+
+		</plugins>
+	</build>
+</project>

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

@@ -22,7 +22,7 @@ import java.lang.annotation.*;
 public @interface Event {
 
 	/**
-	 * Defines the event type the handler listens to. If this value is set, the handler is not
+	 * Defines the event type the handler listens for. If this value is set, the handler is not
 	 * allowed to declare parameters.
 	 * <p>
 	 * This is useful when the event handler does not utilize the event instance.
@@ -30,13 +30,5 @@ public @interface Event {
 	 * @return the event type accepted by the handler
 	 * @since 1.0.0
 	 */
-	Class<?> value() default USE_PARAMETER.class;
-
-	/**
-	 * Signifies that the event type the handler listens to is determined by the type of its only
-	 * parameter.
-	 *
-	 * @since 0.0.3
-	 */
-	static final class USE_PARAMETER {}
+	Class<?> value() default void.class;
 }

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

@@ -0,0 +1,503 @@
+package dev.kske.eventbus.core;
+
+import java.lang.System.Logger;
+import java.lang.System.Logger.Level;
+import java.lang.annotation.Annotation;
+import java.lang.reflect.*;
+import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Consumer;
+
+import dev.kske.eventbus.core.handler.*;
+
+/**
+ * Event listeners can be registered at an event bus to be notified when an event is dispatched.
+ * <p>
+ * A singleton instance of this class can be lazily created and acquired using the
+ * {@link EventBus#getInstance()} method.
+ *
+ * @implNote This is a thread-safe implementation.
+ * @author Kai S. K. Engelbart
+ * @since 0.0.1
+ * @see Event
+ */
+public final class EventBus {
+
+	/**
+	 * Holds the state of the dispatching process on one thread.
+	 *
+	 * @since 0.1.0
+	 */
+	private static final class DispatchState {
+
+		/**
+		 * Indicates that the last event handler invoked has called {@link EventBus#cancel}. In that
+		 * case, the event is not dispatched further.
+		 *
+		 * @since 0.1.0
+		 */
+		boolean isCancelled;
+
+		/**
+		 * Is incremented when {@link EventBus#dispatch(Object)} is invoked and decremented when it
+		 * finishes. This allows keeping track of nested dispatches.
+		 *
+		 * @since 1.2.0
+		 */
+		int nestingCount;
+	}
+
+	/**
+	 * The priority assigned to every event handler without an explicitly defined priority.
+	 *
+	 * @since 1.1.0
+	 * @see Priority
+	 */
+	public static final int DEFAULT_PRIORITY = 100;
+
+	private static final EventBus singletonInstance = new EventBus();
+
+	private static final Logger logger = System.getLogger(EventBus.class.getName());
+
+	/**
+	 * Compares event handlers based on priority, but uses hash codes for equal priorities.
+	 *
+	 * @implNote As the priority comparator by itself is not consistent with equals (two handlers
+	 *           with the same priority are not necessarily equal, but would have a comparison
+	 *           result of 0), the hash code is used for the fallback comparison. This way,
+	 *           consistency with equals is restored.
+	 * @since 1.2.0
+	 */
+	private static final Comparator<EventHandler> byPriority =
+		Comparator.comparingInt(EventHandler::getPriority).reversed()
+			.thenComparingInt(EventHandler::hashCode);
+
+	/**
+	 * Returns the default event bus, which is a statically initialized singleton instance.
+	 *
+	 * @return the default event bus
+	 * @since 0.0.2
+	 */
+	public static EventBus getInstance() {
+		return singletonInstance;
+	}
+
+	/**
+	 * Event handler bindings (target class to handlers registered for that class), does not contain
+	 * other (polymorphic) handlers.
+	 *
+	 * @since 0.0.1
+	 */
+	private final Map<Class<?>, TreeSet<EventHandler>> bindings = new ConcurrentHashMap<>();
+
+	/**
+	 * A cache mapping an event class to all handlers the event should be dispatched to. This
+	 * includes polymorphic handlers that don't reference the event class explicitly. If an event
+	 * class is not contained inside this cache, the {@link #bindings} have to be traversed manually
+	 * in search of applicable handlers.
+	 *
+	 * @since 1.3.0
+	 */
+	private final Map<Class<?>, TreeSet<EventHandler>> bindingCache = new ConcurrentHashMap<>();
+
+	/**
+	 * Stores all registered event listeners (which declare event handlers) and prevents them from
+	 * being garbage collected.
+	 *
+	 * @since 0.0.1
+	 */
+	private final Set<Object> registeredListeners = ConcurrentHashMap.newKeySet();
+
+	/**
+	 * The current event dispatching state, local to each thread.
+	 *
+	 * @since 0.1.0
+	 */
+	private final ThreadLocal<DispatchState> dispatchState =
+		ThreadLocal.withInitial(DispatchState::new);
+
+	/**
+	 * Dispatches an event to all event handlers registered for it in descending order of their
+	 * priority.
+	 *
+	 * @param event the event to dispatch
+	 * @throws EventBusException    if an event handler isn't accessible or has an invalid signature
+	 * @throws ExceptionWrapper     if it is thrown by an event handler
+	 * @throws NullPointerException if the specified event is {@code null}
+	 * @since 0.0.1
+	 */
+	public void dispatch(Object event) {
+		Objects.requireNonNull(event);
+		logger.log(Level.INFO, "Dispatching event {0}", event);
+
+		// Look up dispatch state
+		var state = dispatchState.get();
+
+		// Increment nesting count (becomes > 1 during nested dispatches)
+		++state.nestingCount;
+
+		Iterator<EventHandler> handlers = getHandlersFor(event.getClass()).iterator();
+		if (handlers.hasNext()) {
+			while (handlers.hasNext())
+				if (state.isCancelled) {
+					logger.log(Level.INFO, "Cancelled dispatching event {0}", event);
+					state.isCancelled = false;
+					break;
+				} else {
+					try {
+						handlers.next().execute(event);
+					} catch (InvocationTargetException e) {
+						if (e.getCause() instanceof Error)
+
+							// Transparently pass error to the caller
+							throw (Error) e.getCause();
+						else if (e.getCause() instanceof ExceptionWrapper)
+
+							// Transparently pass exception wrapper to the caller
+							throw (ExceptionWrapper) e.getCause();
+						else if (event instanceof DeadEvent || event instanceof ExceptionEvent)
+
+							// Warn about system event not being handled
+							logger.log(Level.WARNING, event + " not handled due to exception", e);
+						else
+
+							// Dispatch exception event
+							dispatch(new ExceptionEvent(this, event, e.getCause()));
+					}
+				}
+		} else if (event instanceof DeadEvent || event instanceof ExceptionEvent) {
+
+			// Warn about the dead event not being handled
+			logger.log(Level.WARNING, "{0} not handled", event);
+		} else {
+
+			// Dispatch dead event
+			dispatch(new DeadEvent(this, event));
+		}
+
+		// Decrement nesting count (becomes 0 when all dispatches on the thread are finished)
+		--state.nestingCount;
+
+		logger.log(Level.DEBUG, "Finished dispatching event {0}", event);
+	}
+
+	/**
+	 * Searches for the event handlers bound to an event class. This includes polymorphic handlers
+	 * that are bound to a supertype of the event class.
+	 *
+	 * @implNote If the given event type was requested in the past, the handlers are retrieved from
+	 *           the {@link #bindingCache}. If not, the entire {@link #bindings} are traversed in
+	 *           search of polymorphic handlers compatible with the event type.
+	 * @param eventType the event type to use for the search
+	 * @return a navigable set containing the applicable handlers in descending order of priority
+	 * @since 1.2.0
+	 */
+	private NavigableSet<EventHandler> getHandlersFor(Class<?> eventType) {
+		return bindingCache.computeIfAbsent(eventType, k -> {
+
+			// Get handlers defined for the event class
+			TreeSet<EventHandler> handlers =
+				bindings.getOrDefault(eventType, new TreeSet<>(byPriority));
+
+			// Get polymorphic handlers
+			for (var binding : bindings.entrySet())
+				if (binding.getKey().isAssignableFrom(eventType))
+					for (var handler : binding.getValue())
+						if (handler.isPolymorphic())
+							handlers.add(handler);
+
+			return handlers;
+		});
+	}
+
+	/**
+	 * Cancels an event that is currently dispatched from inside an event handler.
+	 *
+	 * @throws EventBusException if the calling thread is not an active dispatching thread
+	 * @since 0.1.0
+	 */
+	public void cancel() {
+		var state = dispatchState.get();
+		if (state.nestingCount > 0 && !state.isCancelled)
+			state.isCancelled = true;
+		else
+			throw new EventBusException("Calling thread not an active dispatching thread!");
+	}
+
+	/**
+	 * Registers an event listener at this event bus.
+	 *
+	 * @param listener the listener to register
+	 * @throws EventBusException    if the listener is already registered or a declared event
+	 *                              handler does not comply with the specification
+	 * @throws NullPointerException if the specified listener is {@code null}
+	 * @since 0.0.1
+	 * @see Event
+	 */
+	public void registerListener(Object listener) {
+		Objects.requireNonNull(listener);
+		if (registeredListeners.contains(listener))
+			throw new EventBusException(listener + " already registered!");
+		logger.log(Level.INFO, "Registering event listener {0}", listener.getClass().getName());
+		boolean handlerBound = false;
+
+		// Predefined handler polymorphism
+		boolean polymorphic = false;
+		if (listener.getClass().isAnnotationPresent(Polymorphic.class))
+			polymorphic = listener.getClass().getAnnotation(Polymorphic.class).value();
+
+		// Predefined handler priority
+		int priority = DEFAULT_PRIORITY;
+		if (listener.getClass().isAnnotationPresent(Priority.class))
+			priority = listener.getClass().getAnnotation(Priority.class).value();
+
+		registeredListeners.add(listener);
+		for (var method : getHandlerMethods(listener.getClass())) {
+			Event annotation = method.getAnnotation(Event.class);
+
+			// Skip methods without annotations
+			if (annotation == null)
+				continue;
+
+			// Initialize and bind the handler
+			bindHandler(
+				new ReflectiveEventHandler(listener, method, annotation, polymorphic, priority));
+			handlerBound = true;
+		}
+
+		if (!handlerBound)
+			logger.log(
+				Level.WARNING,
+				"No event handlers bound for event listener {0}",
+				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
+		Class<?> superClass = listenerClass.getSuperclass();
+		if (superClass != null && superClass != Object.class)
+			methods.addAll(getHandlerMethods(superClass));
+
+		// 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) {
+		var methods = new HashSet<Method>();
+		for (var method : enclosingClass.getDeclaredMethods())
+			if (method.isAnnotationPresent(annotationClass))
+				methods.add(method);
+
+		return methods;
+	}
+
+	/**
+	 * 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}.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener) {
+		registerListener(eventType, eventListener, false, DEFAULT_PRIORITY);
+	}
+
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs. The
+	 * listener has the {@link #DEFAULT_PRIORITY}.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @param polymorphic   whether the listener is also invoked for subtypes of the event type
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener,
+		boolean polymorphic) {
+		registerListener(eventType, eventListener, polymorphic, DEFAULT_PRIORITY);
+	}
+
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs. The
+	 * listener is not polymorphic.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @param priority      the priority to assign to the listener
+	 * @since 1.2.0
+	 * @see #registerListener(Class, Consumer, boolean, int)
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener, int priority) {
+		registerListener(eventType, eventListener, false, priority);
+	}
+
+	/**
+	 * Registers a callback listener, which is a consumer that is invoked when an event occurs.
+	 *
+	 * @param <E>           the event type the listener listens for
+	 * @param eventType     the event type the listener listens for
+	 * @param eventListener the callback that is invoked when an event occurs
+	 * @param polymorphic   whether the listener is also invoked for subtypes of the event type
+	 * @param priority      the priority to assign to the listener
+	 * @since 1.2.0
+	 */
+	public <E> void registerListener(Class<E> eventType, Consumer<E> eventListener,
+		boolean polymorphic,
+		int priority) {
+		Objects.requireNonNull(eventListener);
+		if (registeredListeners.contains(eventListener))
+			throw new EventBusException(eventListener + " already registered!");
+		logger.log(Level.INFO, "Registering callback event listener {0}",
+			eventListener.getClass().getName());
+
+		registeredListeners.add(eventListener);
+		bindHandler(new CallbackEventHandler(eventType, eventListener, polymorphic, priority));
+	}
+
+	/**
+	 * Inserts a new handler into the {@link #bindings} map. Additionally, the handler is placed
+	 * inside the {@link #bindingCache} where applicable.
+	 *
+	 * @param handler the handler to bind
+	 * @since 1.2.0
+	 */
+	private void bindHandler(EventHandler handler) {
+
+		// Bind handler
+		logger.log(Level.DEBUG, "Binding event handler {0}", handler);
+		bindings.computeIfAbsent(handler.getEventType(), k -> new TreeSet<>(byPriority))
+			.add(handler);
+
+		// Insert handler into cache
+		bindingCache.computeIfAbsent(handler.getEventType(), k -> new TreeSet<>(byPriority))
+			.add(handler);
+
+		// Handler is polymorphic => insert where applicable
+		if (handler.isPolymorphic())
+			for (var binding : bindingCache.entrySet())
+				if (binding.getKey().isAssignableFrom(handler.getEventType()))
+					binding.getValue().add(handler);
+	}
+
+	/**
+	 * Removes a specific listener from this event bus.
+	 *
+	 * @param listener the listener to remove
+	 * @since 0.0.1
+	 */
+	public void removeListener(Object listener) {
+		Objects.requireNonNull(listener);
+		logger.log(Level.INFO, "Removing event listener {0}", listener.getClass().getName());
+
+		// Remove bindings from binding map
+		for (var binding : bindings.values()) {
+			var it = binding.iterator();
+			while (it.hasNext()) {
+				var handler = it.next();
+				if (handler.getListener().equals(listener)) {
+					logger.log(Level.DEBUG, "Unbinding event handler {0}", handler);
+					it.remove();
+				}
+			}
+		}
+
+		// Remove bindings from cache
+		for (var binding : bindingCache.values()) {
+			var it = binding.iterator();
+			while (it.hasNext()) {
+				var handler = it.next();
+				if (handler.getListener().equals(listener)) {
+					logger.log(Level.TRACE, "Removing event handler {0} from cache", handler);
+					it.remove();
+				}
+			}
+		}
+
+		// Remove the listener itself
+		registeredListeners.remove(listener);
+	}
+
+	/**
+	 * Removes all event listeners from this event bus.
+	 *
+	 * @since 0.0.1
+	 */
+	public void clearListeners() {
+		logger.log(Level.INFO, "Clearing event listeners");
+		bindings.clear();
+		bindingCache.clear();
+		registeredListeners.clear();
+	}
+
+	/**
+	 * Generates a string describing the event handlers that would be executed for a specific event
+	 * type, in order and without actually executing them.
+	 *
+	 * @apiNote Using this method is only recommended for debugging purposes, as the output depends
+	 *          on implementation internals which may be subject to change.
+	 * @implNote Nested dispatches are not accounted for, as this would require actually executing
+	 *           the handlers.
+	 * @param eventType the event type to generate the execution order for
+	 * @return a human-readable event handler list suitable for debugging purposes
+	 * @since 1.2.0
+	 */
+	public String debugExecutionOrder(Class<?> eventType) {
+		var	handlers	= getHandlersFor(eventType);
+		var	sj			= new StringJoiner("\n");
+
+		// Output header line
+		sj.add(String.format("Event handler execution order for %s (%d handler(s)):", eventType,
+			handlers.size()));
+		sj.add(
+			"==========================================================================================");
+
+		// Individual handlers
+		for (var handler : handlers)
+			sj.add(handler.toString());
+
+		// Bottom line
+		sj.add(
+			"==========================================================================================");
+
+		return sj.toString();
+	}
+
+	/**
+	 * Provides an unmodifiable view of the event listeners registered at this event bus.
+	 *
+	 * @return all registered event listeners
+	 * @since 0.0.1
+	 */
+	public Set<Object> getRegisteredListeners() {
+		return Collections.unmodifiableSet(registeredListeners);
+	}
+}

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

@@ -0,0 +1,39 @@
+package dev.kske.eventbus.core;
+
+/**
+ * This unchecked exception is specific to the event bus and can be thrown under the following
+ * circumstances:
+ * <ul>
+ * <li>An event handler throws an exception (which is stored as the cause)</li>
+ * <li>An event listener with an invalid event handler is registered</li>
+ * <li>{@link EventBus#cancel()} is invoked from outside an active dispatch thread</li>
+ * </ul>
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.1
+ */
+public final class EventBusException extends RuntimeException {
+
+	private static final long serialVersionUID = 7254445250300604449L;
+
+	/**
+	 * Creates a new event bus exception.
+	 *
+	 * @param message the message to display
+	 * @param cause   the cause of this exception
+	 * @since 0.0.1
+	 */
+	public EventBusException(String message, Throwable cause) {
+		super(message, cause);
+	}
+
+	/**
+	 * Creates a new event bus exception.
+	 *
+	 * @param message the message to display
+	 * @since 0.0.1
+	 */
+	public EventBusException(String message) {
+		super(message);
+	}
+}

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

@@ -0,0 +1,24 @@
+package dev.kske.eventbus.core;
+
+/**
+ * This unchecked exception acts as a wrapper for an arbitrary exception to prevent an
+ * {@link ExceptionEvent} from being dispatched. Instead, the wrapped exception is rethrown by
+ * {@link EventBus#dispatch(Object)}.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.1
+ */
+public final class ExceptionWrapper extends RuntimeException {
+
+	private static final long serialVersionUID = -2016681140617308788L;
+
+	/**
+	 * Creates a new exception wrapper.
+	 *
+	 * @param cause the exception to wrap
+	 * @since 1.2.1
+	 */
+	public ExceptionWrapper(Exception cause) {
+		super(cause);
+	}
+}

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/main/java/dev/kske/eventbus/core/handler/CallbackEventHandler.java

@@ -0,0 +1,73 @@
+package dev.kske.eventbus.core.handler;
+
+import java.lang.reflect.InvocationTargetException;
+import java.util.function.Consumer;
+
+/**
+ * An event handler wrapping a callback method.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ */
+public final class CallbackEventHandler implements EventHandler {
+
+	private final Class<?>			eventType;
+	private final Consumer<Object>	callback;
+	private final boolean			polymorphic;
+	private final int				priority;
+
+	/**
+	 * Constructs a callback event handler.
+	 *
+	 * @param <E>         the event type of the handler
+	 * @param eventType   the event type of the handler
+	 * @param callback    the callback method to execute when the handler is invoked
+	 * @param polymorphic whether the handler is polymorphic
+	 * @param priority    the priority of the handler
+	 * @since 1.2.0
+	 */
+	@SuppressWarnings("unchecked")
+	public <E> CallbackEventHandler(Class<E> eventType, Consumer<E> callback, boolean polymorphic,
+		int priority) {
+		this.eventType		= eventType;
+		this.callback		= (Consumer<Object>) callback;
+		this.polymorphic	= polymorphic;
+		this.priority		= priority;
+	}
+
+	@Override
+	public void execute(Object event) throws InvocationTargetException {
+		try {
+			callback.accept(event);
+		} catch (RuntimeException e) {
+			throw new InvocationTargetException(e, "Callback event handler failed!");
+		}
+	}
+
+	@Override
+	public String toString() {
+		return String.format(
+			"CallbackEventHandler[eventType=%s, polymorphic=%b, priority=%d]",
+			eventType, polymorphic, priority);
+	}
+
+	@Override
+	public Consumer<?> getListener() {
+		return callback;
+	}
+
+	@Override
+	public Class<?> getEventType() {
+		return eventType;
+	}
+
+	@Override
+	public int getPriority() {
+		return priority;
+	}
+
+	@Override
+	public boolean isPolymorphic() {
+		return polymorphic;
+	}
+}

core/src/main/java/dev/kske/eventbus/core/handler/EventHandler.java

@@ -0,0 +1,53 @@
+package dev.kske.eventbus.core.handler;
+
+import java.lang.reflect.InvocationTargetException;
+
+import dev.kske.eventbus.core.*;
+
+/**
+ * Internal representation of an event handling method.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ * @see EventBus
+ */
+public interface EventHandler {
+
+	/**
+	 * Executes the event handler.
+	 *
+	 * @param event the event used as the method parameter
+	 * @throws EventBusException         if the event handler isn't accessible or has an invalid
+	 *                                   signature
+	 * @throws InvocationTargetException if the handler throws an exception
+	 * @throws EventBusException         if the handler has the wrong signature or is inaccessible
+	 * @since 1.2.0
+	 */
+	void execute(Object event) throws EventBusException, InvocationTargetException;
+
+	/**
+	 * @return the listener containing this handler
+	 * @since 1.2.0
+	 */
+	Object getListener();
+
+	/**
+	 * @return the event type this handler listens for
+	 * @since 1.2.0
+	 */
+	Class<?> getEventType();
+
+	/**
+	 * @return the priority of this handler
+	 * @since 1.2.0
+	 * @see Priority
+	 */
+	int getPriority();
+
+	/**
+	 * @return whether this handler also accepts subtypes of the event type
+	 * @since 1.2.0
+	 * @see Polymorphic
+	 */
+	boolean isPolymorphic();
+}

core/src/main/java/dev/kske/eventbus/core/handler/ReflectiveEventHandler.java

@@ -0,0 +1,105 @@
+package dev.kske.eventbus.core.handler;
+
+import java.lang.reflect.*;
+
+import dev.kske.eventbus.core.*;
+
+/**
+ * An event handler wrapping a method annotated with {@link Event} and executing it using
+ * reflection.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ */
+public final class ReflectiveEventHandler implements EventHandler {
+
+	private final Object	listener;
+	private final Method	method;
+	private final Class<?>	eventType;
+	private final boolean	useParameter;
+	private final boolean	polymorphic;
+	private final int		priority;
+
+	/**
+	 * Constructs a reflective event handler.
+	 *
+	 * @param listener        the listener containing the handler
+	 * @param method          the handler method
+	 * @param annotation      the event annotation
+	 * @param defPolymorphism the predefined polymorphism (default or listener-level)
+	 * @param defPriority     the predefined priority (default or listener-level)
+	 * @throws EventBusException if the method or the annotation do not comply with the
+	 *                           specification
+	 * @since 1.2.0
+	 */
+	public ReflectiveEventHandler(Object listener, Method method, Event annotation,
+		boolean defPolymorphism, int defPriority) throws EventBusException {
+		this.listener	= listener;
+		this.method		= method;
+		useParameter	= annotation.value() == void.class;
+
+		// Check handler signature
+		if (method.getParameterCount() == 0 && useParameter)
+			throw new EventBusException(method + " does not define an event type!");
+
+		if (method.getParameterCount() == 1 && !useParameter)
+			throw new EventBusException(method + " defines an ambiguous event type!");
+
+		if (method.getParameterCount() > 1)
+			throw new EventBusException(method + " defines more than one parameter!");
+
+		// Determine handler properties
+		eventType	= useParameter ? method.getParameterTypes()[0] : annotation.value();
+		polymorphic	= method.isAnnotationPresent(Polymorphic.class)
+			? method.getAnnotation(Polymorphic.class).value()
+			: defPolymorphism;
+		priority	= method.isAnnotationPresent(Priority.class)
+			? method.getAnnotation(Priority.class).value()
+			: defPriority;
+
+		// Try to allow access if the method is not accessible
+		if (!method.canAccess(Modifier.isStatic(method.getModifiers()) ? null : listener))
+			method.setAccessible(true);
+	}
+
+	@Override
+	public void execute(Object event) throws EventBusException, InvocationTargetException {
+		try {
+			if (useParameter)
+				method.invoke(getListener(), event);
+			else
+				method.invoke(getListener());
+		} catch (IllegalArgumentException e) {
+			throw new EventBusException("Event handler rejected target / argument!", e);
+		} catch (IllegalAccessException e) {
+			throw new EventBusException("Event handler is not accessible!", e);
+		}
+	}
+
+	@Override
+	public String toString() {
+		return String.format(
+			"ReflectiveEventHandler[eventType=%s, polymorphic=%b, priority=%d, method=%s, useParameter=%b]",
+			eventType, polymorphic, priority, method, useParameter);
+	}
+
+	@Override
+	public Object getListener() {
+		return listener;
+	}
+
+	@Override
+	public Class<?> getEventType() {
+		return eventType;
+	}
+
+	@Override
+	public int getPriority() {
+		return priority;
+	}
+
+	@Override
+	public boolean isPolymorphic() {
+		return polymorphic;
+	}
+}

core/src/main/java/dev/kske/eventbus/core/handler/package-info.java

@@ -0,0 +1,8 @@
+/**
+ * Contains the internal representation of event handling methods.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ * @see dev.kske.eventbus.core.handler.EventHandler
+ */
+package dev.kske.eventbus.core.handler;

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

@@ -0,0 +1,88 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.*;
+
+/**
+ * Tests the dispatching mechanism of the event bus.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.1
+ */
+@Polymorphic
+@Priority(150)
+class DispatchTest {
+
+	EventBus bus;
+
+	/**
+	 * Constructs an event bus and registers this test instance as an event listener.
+	 *
+	 * @since 0.0.1
+	 */
+	@BeforeEach
+	void registerListener() {
+		bus = new EventBus();
+		bus.registerListener(this);
+		bus.registerListener(SimpleEvent.class, e -> {
+			e.increment();
+			assertEquals(3, e.getCounter());
+		});
+	}
+
+	/**
+	 * Tests {@link EventBus#dispatch(Object)} with multiple handler priorities, a polymorphic
+	 * handler and a static handler.
+	 *
+	 * @since 0.0.1
+	 */
+	@Test
+	void testDispatch() {
+		bus.dispatch(new SimpleEventSub());
+		bus.dispatch(new SimpleEvent());
+	}
+
+	/**
+	 * Tests {@link EventBus#debugExecutionOrder(Class)} based on the currently registered handlers.
+	 *
+	 * @since 1.2.0
+	 */
+	@Test
+	void testDebugExecutionOrder() {
+		assertEquals(
+			"Event handler execution order for class dev.kske.eventbus.core.SimpleEvent (3 handler(s)):\n"
+				+ "==========================================================================================\n"
+				+ "ReflectiveEventHandler[eventType=class dev.kske.eventbus.core.SimpleEvent, polymorphic=true, priority=200, method=void dev.kske.eventbus.core.DispatchTest.onSimpleEventFirst(dev.kske.eventbus.core.SimpleEvent), useParameter=true]\n"
+				+ "ReflectiveEventHandler[eventType=class dev.kske.eventbus.core.SimpleEvent, polymorphic=false, priority=150, method=static void dev.kske.eventbus.core.DispatchTest.onSimpleEventSecond(dev.kske.eventbus.core.SimpleEvent), useParameter=true]\n"
+				+ "CallbackEventHandler[eventType=class dev.kske.eventbus.core.SimpleEvent, polymorphic=false, priority=100]\n"
+				+ "==========================================================================================",
+			bus.debugExecutionOrder(SimpleEvent.class));
+	}
+
+	/**
+	 * Tests whether the handlers bound to an event type are correct when retrieved from the binding
+	 * cache. On the second call of {@link EventBus#debugExecutionOrder(Class)} the cache is used.
+	 *
+	 * @since 1.3.0
+	 */
+	@Test
+	void testBindingCache() {
+		assertEquals(bus.debugExecutionOrder(SimpleEventSub.class),
+			bus.debugExecutionOrder(SimpleEventSub.class));
+	}
+
+	@Event
+	@Priority(200)
+	void onSimpleEventFirst(SimpleEvent event) {
+		event.increment();
+		assertTrue(event.getCounter() == 1 || event.getCounter() == 2);
+	}
+
+	@Event
+	@Polymorphic(false)
+	static void onSimpleEventSecond(SimpleEvent event) {
+		event.increment();
+		assertEquals(2, event.getCounter());
+	}
+}

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

@@ -0,0 +1,33 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests the behavior of the event bus when an {@link ExceptionWrapper} is thrown.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.1
+ */
+class ExceptionWrapperTest {
+
+	EventBus	bus		= new EventBus();
+	String		event	= "This event will cause an exception";
+
+	/**
+	 * Tests transparent rethrowing of an exception wrapper by {@link EventBus#dispatch(Object)}.
+	 *
+	 * @since 1.2.1
+	 */
+	@Test
+	void testExceptionWrapper() {
+		bus.registerListener(this);
+		assertThrows(ExceptionWrapper.class, () -> bus.dispatch(event));
+	}
+
+	@Event(String.class)
+	void onString() {
+		throw new ExceptionWrapper(new RuntimeException("I failed!"));
+	}
+}

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

@@ -0,0 +1,43 @@
+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. The
+ * effect of handler priorities is also accounted for.
+ *
+ * @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(3, event.getCounter());
+	}
+
+	@Override
+	void onSimpleEventAbstractHandler(SimpleEvent event) {
+		assertSame(1, event.getCounter());
+	}
+
+	@Override
+	public void onSimpleEventInterfaceHandler(SimpleEvent event) {
+		event.increment();
+	}
+
+	@Event
+	@Priority(250)
+	private void onSimpleEventPrivate(SimpleEvent event) {
+		assertSame(0, event.getCounter());
+		event.increment();
+	}
+}

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

@@ -0,0 +1,73 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.*;
+
+/**
+ * Tests nested event dispatches.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.2.0
+ */
+class NestedTest {
+
+	EventBus	bus;
+	boolean		nestedHit;
+
+	/**
+	 * Constructs an event bus and registers this test instance as an event listener.
+	 *
+	 * @since 1.2.0
+	 */
+	@BeforeEach
+	void registerListener() {
+		bus = new EventBus();
+		bus.registerListener(this);
+	}
+
+	/**
+	 * Dispatches a simple event, which should in turn cause a string to be dispatched as a nested
+	 * event. If the corresponding handler sets {@link #nestedHit} to {@code true}, the test is
+	 * successful.
+	 *
+	 * @since 1.2.0
+	 */
+	@Test
+	void testNestedDispatch() {
+		bus.dispatch(new SimpleEvent());
+		assertTrue(nestedHit);
+	}
+
+	/**
+	 * Dispatches a string as a nested event and cancels the current dispatch afterwards.
+	 *
+	 * @since 1.2.0
+	 */
+	@Event(SimpleEvent.class)
+	void onSimpleEvent() {
+		bus.dispatch("Nested event");
+		bus.cancel();
+	}
+
+	/**
+	 * Sets {@link #nestedHit} to {@code true} indicating that nested dispatches work.
+	 *
+	 * @since 1.2.0
+	 */
+	@Event(String.class)
+	void onString() {
+		nestedHit = true;
+	}
+
+	/**
+	 * Fails the test if an exception is caused during the dispatch.
+	 *
+	 * @param e the event containing the exception
+	 * @since 1.2.0
+	 */
+	@Event
+	void onException(ExceptionEvent e) {
+		fail("Exception during dispatch", e.getCause());
+	}
+}

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

@@ -0,0 +1,31 @@
+package dev.kske.eventbus.core;
+
+/**
+ * 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 {
+
+	private int counter;
+
+	@Override
+	public String toString() {
+		return String.format("%s[%d]", getClass().getSimpleName(), counter);
+	}
+
+	void increment() {
+		++counter;
+	}
+
+	int getCounter() {
+		return counter;
+	}
+
+	void reset() {
+		counter = 0;
+	}
+}

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

@@ -0,0 +1,25 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * An abstract class defining a package-private and a private handler for {@link SimpleEvent}.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.3.0
+ */
+@Priority(200)
+abstract class SimpleEventListenerBase {
+
+	@Event
+	void onSimpleEventAbstractHandler(SimpleEvent event) {
+		fail("This handler should not be invoked");
+	}
+
+	@Event
+	@Priority(150)
+	private void onSimpleEventPrivate(SimpleEvent event) {
+		assertSame(1, event.getCounter());
+		event.increment();
+	}
+}

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

@@ -0,0 +1,14 @@
+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 {
+
+	@Priority(120)
+	@Event
+	void onSimpleEventInterfaceHandler(SimpleEvent event);
+}

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

@@ -6,4 +6,4 @@ package dev.kske.eventbus.core;
  * @author Kai S. K. Engelbart
  * @since 0.0.4
  */
-public class SimpleEventSub extends SimpleEvent {}
+class SimpleEventSub extends SimpleEvent {}

event-bus-core/pom.xml

@@ -1,30 +0,0 @@
-<project xmlns="http://maven.apache.org/POM/4.0.0"
-	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
-	<modelVersion>4.0.0</modelVersion>
-
-	<artifactId>event-bus-core</artifactId>
-	<name>Event Bus Core</name>
-
-	<parent>
-		<groupId>dev.kske</groupId>
-		<artifactId>event-bus</artifactId>
-		<version>1.1.0</version>
-	</parent>
-
-	<dependencies>
-		<dependency>
-			<groupId>org.junit.jupiter</groupId>
-			<artifactId>junit-jupiter-api</artifactId>
-			<version>5.6.2</version>
-			<scope>test</scope>
-		</dependency>
-	</dependencies>
-
-	<build>
-
-		<!-- Disable resource folder -->
-		<resources />
-
-	</build>
-</project>

event-bus-core/src/main/java/dev/kske/eventbus/core/EventBus.java

@@ -1,257 +0,0 @@
-package dev.kske.eventbus.core;
-
-import java.lang.System.Logger;
-import java.lang.System.Logger.Level;
-import java.lang.reflect.InvocationTargetException;
-import java.util.*;
-import java.util.concurrent.ConcurrentHashMap;
-
-/**
- * Event listeners can be registered at an event bus to be notified when an event is dispatched.
- * <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.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see Event
- */
-public final class EventBus {
-
-	/**
-	 * Holds the state of the dispatching process on one thread.
-	 *
-	 * @since 0.1.0
-	 */
-	private static final class DispatchState {
-
-		boolean isDispatching, isCancelled;
-	}
-
-	/**
-	 * The priority assigned to every event handler without an explicitly defined priority.
-	 *
-	 * @since 1.1.0
-	 * @see Priority
-	 */
-	public static final int DEFAULT_PRIORITY = 100;
-
-	private static volatile EventBus singletonInstance;
-
-	private static final Logger logger = System.getLogger(EventBus.class.getName());
-
-	/**
-	 * Produces a singleton instance of the event bus. It is lazily initialized on the first call.
-	 *
-	 * @return a singleton instance of the event bus.
-	 * @since 0.0.2
-	 */
-	public static EventBus getInstance() {
-		EventBus instance = singletonInstance;
-		if (instance == null)
-			synchronized (EventBus.class) {
-				if ((instance = singletonInstance) == null) {
-					logger.log(Level.DEBUG, "Initializing singleton event bus instance");
-					instance = singletonInstance = new EventBus();
-				}
-			}
-		return instance;
-	}
-
-	private final Map<Class<?>, TreeSet<EventHandler>>	bindings			=
-		new ConcurrentHashMap<>();
-	private final Set<Object>							registeredListeners	=
-		ConcurrentHashMap.newKeySet();
-	private final ThreadLocal<DispatchState>			dispatchState		=
-		ThreadLocal.withInitial(DispatchState::new);
-
-	/**
-	 * Dispatches an event to all event handlers registered for it in descending order of their
-	 * priority.
-	 *
-	 * @param event the event to dispatch
-	 * @throws EventBusException if an event handler isn't accessible or has an invalid signature
-	 * @since 0.0.1
-	 */
-	public void dispatch(Object event) throws EventBusException {
-		Objects.requireNonNull(event);
-		logger.log(Level.INFO, "Dispatching event {0}", event);
-
-		// Set dispatch state
-		var state = dispatchState.get();
-		state.isDispatching = true;
-
-		Iterator<EventHandler> handlers = getHandlersFor(event.getClass());
-		if (handlers.hasNext()) {
-			while (handlers.hasNext())
-				if (state.isCancelled) {
-					logger.log(Level.INFO, "Cancelled dispatching event {0}", event);
-					state.isCancelled = false;
-					break;
-				} else {
-					try {
-						handlers.next().execute(event);
-					} catch (InvocationTargetException e) {
-						if (event instanceof DeadEvent || event instanceof ExceptionEvent)
-
-							// Warn about system event not being handled
-							logger.log(Level.WARNING, event + " not handled due to exception", e);
-						else if (e.getCause() instanceof Error)
-
-							// Transparently pass error to the caller
-							throw (Error) e.getCause();
-						else
-
-							// Dispatch exception event
-							dispatch(new ExceptionEvent(this, event, e.getCause()));
-					}
-				}
-		} else if (event instanceof DeadEvent || event instanceof ExceptionEvent) {
-
-			// Warn about the dead event not being handled
-			logger.log(Level.WARNING, "{0} not handled", event);
-		} else {
-
-			// Dispatch dead event
-			dispatch(new DeadEvent(this, event));
-		}
-
-		// Reset dispatch state
-		state.isDispatching = false;
-
-		logger.log(Level.DEBUG, "Finished dispatching event {0}", event);
-	}
-
-	/**
-	 * Searches for the event handlers bound to an event class. This includes polymorphic handlers
-	 * that are bound to a supertype of the event class.
-	 *
-	 * @param eventClass the event class to use for the search
-	 * @return an iterator over the applicable handlers in descending order of priority
-	 * @since 0.0.1
-	 */
-	private Iterator<EventHandler> getHandlersFor(Class<?> eventClass) {
-
-		// Get handlers defined for the event class
-		TreeSet<EventHandler> handlers = bindings.getOrDefault(eventClass, new TreeSet<>());
-
-		// Get polymorphic handlers
-		for (var binding : bindings.entrySet())
-			if (binding.getKey().isAssignableFrom(eventClass))
-				for (var handler : binding.getValue())
-					if (handler.isPolymorphic())
-						handlers.add(handler);
-
-		return handlers.iterator();
-	}
-
-	/**
-	 * Cancels an event that is currently dispatched from inside an event handler.
-	 *
-	 * @throws EventBusException if the calling thread is not an active dispatching thread
-	 * @since 0.1.0
-	 */
-	public void cancel() {
-		var state = dispatchState.get();
-		if (state.isDispatching && !state.isCancelled)
-			state.isCancelled = true;
-		else
-			throw new EventBusException("Calling thread not an active dispatching thread!");
-	}
-
-	/**
-	 * Registers an event listener at this event bus.
-	 *
-	 * @param listener the listener to register
-	 * @throws EventBusException if the listener is already registered or a declared event handler
-	 *                           does not comply with the specification
-	 * @since 0.0.1
-	 * @see Event
-	 */
-	public void registerListener(Object listener) throws EventBusException {
-		Objects.requireNonNull(listener);
-		if (registeredListeners.contains(listener))
-			throw new EventBusException(listener + " already registered!");
-		logger.log(Level.INFO, "Registering event listener {0}", listener.getClass().getName());
-		boolean handlerBound = false;
-
-		// Predefined handler polymorphism
-		boolean polymorphic = false;
-		if (listener.getClass().isAnnotationPresent(Polymorphic.class))
-			polymorphic = listener.getClass().getAnnotation(Polymorphic.class).value();
-
-		// Predefined handler priority
-		int priority = DEFAULT_PRIORITY;
-		if (listener.getClass().isAnnotationPresent(Priority.class))
-			priority = listener.getClass().getAnnotation(Priority.class).value();
-
-		registeredListeners.add(listener);
-		for (var method : listener.getClass().getDeclaredMethods()) {
-			Event annotation = method.getAnnotation(Event.class);
-
-			// Skip methods without annotations
-			if (annotation == null)
-				continue;
-
-			// Initialize and bind the handler
-			var handler = new EventHandler(listener, method, annotation, polymorphic, priority);
-			bindings.putIfAbsent(handler.getEventType(), new TreeSet<>());
-			logger.log(Level.DEBUG, "Binding event handler {0}", handler);
-			bindings.get(handler.getEventType())
-				.add(handler);
-			handlerBound = true;
-		}
-
-		if (!handlerBound)
-			logger.log(
-				Level.WARNING,
-				"No event handlers bound for event listener {0}",
-				listener.getClass().getName());
-	}
-
-	/**
-	 * Removes a specific listener from this event bus.
-	 *
-	 * @param listener the listener to remove
-	 * @since 0.0.1
-	 */
-	public void removeListener(Object listener) {
-		Objects.requireNonNull(listener);
-		logger.log(Level.INFO, "Removing event listener {0}", listener.getClass().getName());
-
-		for (var binding : bindings.values()) {
-			var it = binding.iterator();
-			while (it.hasNext()) {
-				var handler = it.next();
-				if (handler.getListener() == listener) {
-					logger.log(Level.DEBUG, "Unbinding event handler {0}", handler);
-					it.remove();
-				}
-			}
-		}
-		registeredListeners.remove(listener);
-	}
-
-	/**
-	 * Removes all event listeners from this event bus.
-	 *
-	 * @since 0.0.1
-	 */
-	public void clearListeners() {
-		logger.log(Level.INFO, "Clearing event listeners");
-		bindings.clear();
-		registeredListeners.clear();
-	}
-
-	/**
-	 * Provides an unmodifiable view of the event listeners registered at this event bus.
-	 *
-	 * @return all registered event listeners
-	 * @since 0.0.1
-	 */
-	public Set<Object> getRegisteredListeners() {
-		return Collections.unmodifiableSet(registeredListeners);
-	}
-}

event-bus-core/src/main/java/dev/kske/eventbus/core/EventBusException.java

@@ -1,33 +0,0 @@
-package dev.kske.eventbus.core;
-
-/**
- * This runtime exception is thrown when an event bus error occurs. This can
- * either occur while registering event listeners with invalid handlers, or when
- * an event handler throws an exception.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- */
-public class EventBusException extends RuntimeException {
-
-	private static final long serialVersionUID = 1L;
-
-	/**
-	 * Creates a new event bus exception.
-	 *
-	 * @param message the message to display
-	 * @param cause   the cause of this exception
-	 */
-	public EventBusException(String message, Throwable cause) {
-		super(message, cause);
-	}
-
-	/**
-	 * Creates a new event bus exception.
-	 *
-	 * @param message the message to display
-	 */
-	public EventBusException(String message) {
-		super(message);
-	}
-}

event-bus-core/src/main/java/dev/kske/eventbus/core/EventHandler.java

@@ -1,135 +0,0 @@
-package dev.kske.eventbus.core;
-
-import java.lang.reflect.*;
-
-import dev.kske.eventbus.core.Event.USE_PARAMETER;
-
-/**
- * Internal representation of an event handling method.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see EventBus
- */
-final class EventHandler implements Comparable<EventHandler> {
-
-	private final Object	listener;
-	private final Method	method;
-	private final Class<?>	eventType;
-	private final boolean	useParameter;
-	private final boolean	polymorphic;
-	private final int		priority;
-
-	/**
-	 * Constructs an event handler.
-	 *
-	 * @param listener        the listener containing the handler
-	 * @param method          the handler method
-	 * @param annotation      the event annotation
-	 * @param defPolymorphism the predefined polymorphism (default or listener-level)
-	 * @param defPriority     the predefined priority (default or listener-level)
-	 * @throws EventBusException if the method or the annotation do not comply with the
-	 *                           specification
-	 * @since 0.0.1
-	 */
-	EventHandler(Object listener, Method method, Event annotation, boolean defPolymorphism,
-		int defPriority) throws EventBusException {
-		this.listener	= listener;
-		this.method		= method;
-		useParameter	= annotation.value() == USE_PARAMETER.class;
-
-		// Check handler signature
-		if (method.getParameterCount() == 0 && useParameter)
-			throw new EventBusException(method + " does not define an event type!");
-
-		if (method.getParameterCount() == 1 && !useParameter)
-			throw new EventBusException(method + " defines an ambiguous event type!");
-
-		if (method.getParameterCount() > 1)
-			throw new EventBusException(method + " defines more than one parameter!");
-
-		// Determine handler properties
-		eventType	= useParameter ? method.getParameterTypes()[0] : annotation.value();
-		polymorphic	= method.isAnnotationPresent(Polymorphic.class)
-			? method.getAnnotation(Polymorphic.class).value()
-			: defPolymorphism;
-		priority	= method.isAnnotationPresent(Priority.class)
-			? method.getAnnotation(Priority.class).value()
-			: defPriority;
-
-		// Allow access if the method is non-public
-		method.setAccessible(true);
-	}
-
-	/**
-	 * Compares this to another event handler based on priority. In case of equal priority a
-	 * non-zero value based on hash codes is returned.
-	 * <p>
-	 * This is used to retrieve event handlers in descending order of priority from a tree set.
-	 *
-	 * @since 0.0.1
-	 */
-	@Override
-	public int compareTo(EventHandler other) {
-		int priority = other.priority - this.priority;
-		if (priority == 0)
-			priority = listener.hashCode() - other.listener.hashCode();
-		return priority == 0 ? hashCode() - other.hashCode() : priority;
-	}
-
-	@Override
-	public String toString() {
-		return String.format(
-			"EventHandler[method=%s, eventType=%s, useParameter=%b, polymorphic=%b, priority=%d]",
-			method, eventType, useParameter, polymorphic, priority);
-	}
-
-	/**
-	 * Executes the event handler.
-	 *
-	 * @param event the event used as the method parameter
-	 * @throws EventBusException         if the event handler isn't accessible or has an invalid
-	 *                                   signature
-	 * @throws InvocationTargetException if the handler throws an exception
-	 * @throws EventBusException         if the handler has the wrong signature or is inaccessible
-	 * @since 0.0.1
-	 */
-	void execute(Object event) throws EventBusException, InvocationTargetException {
-		try {
-			if (useParameter)
-				method.invoke(listener, event);
-			else
-				method.invoke(listener);
-		} catch (IllegalArgumentException e) {
-			throw new EventBusException("Event handler rejected target / argument!", e);
-		} catch (IllegalAccessException e) {
-			throw new EventBusException("Event handler is not accessible!", e);
-		}
-	}
-
-	/**
-	 * @return the listener containing this handler
-	 * @since 0.0.1
-	 */
-	Object getListener() { return listener; }
-
-	/**
-	 * @return the event type this handler listens for
-	 * @since 0.0.3
-	 */
-	Class<?> getEventType() { return eventType; }
-
-	/**
-	 * @return the priority of this handler
-	 * @since 0.0.1
-	 * @see Priority
-	 */
-	int getPriority() { return priority; }
-
-	/**
-	 * @return whether this handler is polymorphic
-	 * @since 1.0.0
-	 * @see Polymorphic
-	 */
-	boolean isPolymorphic() { return polymorphic; }
-}

event-bus-core/src/test/java/dev/kske/eventbus/core/DispatchTest.java

@@ -1,64 +0,0 @@
-package dev.kske.eventbus.core;
-
-import static org.junit.jupiter.api.Assertions.*;
-
-import org.junit.jupiter.api.*;
-
-/**
- * Tests the dispatching mechanism of the event bus.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- */
-@Polymorphic
-@Priority(150)
-class DispatchTest {
-
-	EventBus	bus;
-	static int	hits;
-
-	/**
-	 * Constructs an event bus and registers this test instance as an event listener.
-	 *
-	 * @since 0.0.1
-	 */
-	@BeforeEach
-	void registerListener() {
-		bus = new EventBus();
-		bus.registerListener(this);
-	}
-
-	/**
-	 * Tests {@link EventBus#dispatch(Object)} with multiple handler priorities, a polymorphic
-	 * handler and a static handler.
-	 *
-	 * @since 0.0.1
-	 */
-	@Test
-	void testDispatch() {
-		bus.dispatch(new SimpleEventSub());
-		bus.dispatch(new SimpleEvent());
-	}
-
-	@Event(SimpleEvent.class)
-	@Priority(200)
-	void onSimpleEventFirst() {
-		++hits;
-		assertTrue(hits == 1 || hits == 2);
-	}
-
-	@Event(SimpleEvent.class)
-	@Polymorphic(false)
-	static void onSimpleEventSecond() {
-		++hits;
-		assertEquals(3, hits);
-	}
-
-	@Event
-	@Polymorphic(false)
-	@Priority(100)
-	void onSimpleEventThird(SimpleEvent event) {
-		++hits;
-		assertEquals(4, hits);
-	}
-}

event-bus-core/src/test/java/dev/kske/eventbus/core/SimpleEvent.java

@@ -1,9 +0,0 @@
-package dev.kske.eventbus.core;
-
-/**
- * A simple event for testing purposes.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- */
-public class SimpleEvent {}

pom.xml

@@ -5,16 +5,16 @@
 
 	<groupId>dev.kske</groupId>
 	<artifactId>event-bus</artifactId>
-	<version>1.1.0</version>
+	<version>1.2.0</version>
 	<packaging>pom</packaging>
 
 	<name>Event Bus</name>
-	<description>An event handling framework for Java utilizing annotations.</description>
+	<description>An event handling library for Java utilizing annotations.</description>
 	<url>https://git.kske.dev/kske/event-bus</url>
 
 	<modules>
-		<module>event-bus-core</module>
-		<module>event-bus-proc</module>
+		<module>core</module>
+		<module>proc</module>
 	</modules>
 
 	<licenses>
@@ -120,6 +120,28 @@
 						</goals>
 					</execution>
 				</executions>
+
+				<!-- Support JDK-style Javadoc tags -->
+				<configuration>
+					<tags>
+						<tag>
+							<name>apiNote</name>
+							<placement>a</placement>
+							<head>API Note:</head>
+						</tag>
+						<tag>
+							<name>implSpec</name>
+							<placement>a</placement>
+							<head>Implementation Requirements:</head>
+						</tag>
+						<tag>
+							<name>implNote</name>
+							<placement>a</placement>
+							<head>Implementation Note:</head>
+						</tag>
+					</tags>
+				</configuration>
+
 			</plugin>
 
 			<!-- GPG sign JAR -->

proc/.classpath

@@ -16,17 +16,17 @@
 			<attribute name="maven.pomderived" value="true"/>
 		</attributes>
 	</classpathentry>
-	<classpathentry kind="src" output="target/test-classes" path="home/kske/git/event-bus/event-bus-ap">
+	<classpathentry excluding="**" kind="src" output="target/classes" path="src/main/resources">
+		<attributes>
+			<attribute name="maven.pomderived" value="true"/>
+		</attributes>
+	</classpathentry>
+	<classpathentry kind="src" output="target/test-classes" path="src/test/java">
 		<attributes>
 			<attribute name="optional" value="true"/>
 			<attribute name="maven.pomderived" value="true"/>
 			<attribute name="test" value="true"/>
 		</attributes>
 	</classpathentry>
-	<classpathentry excluding="**" kind="src" output="target/classes" path="src/main/resources">
-		<attributes>
-			<attribute name="maven.pomderived" value="true"/>
-		</attributes>
-	</classpathentry>
 	<classpathentry kind="output" path="target/classes"/>
 </classpath>

proc/pom.xml

@@ -11,7 +11,7 @@
 	<parent>
 		<groupId>dev.kske</groupId>
 		<artifactId>event-bus</artifactId>
-		<version>1.1.0</version>
+		<version>1.2.0</version>
 	</parent>
 
 	<dependencies>
@@ -24,9 +24,6 @@
 
 	<build>
 
-		<!-- Disable test folder -->
-		<testSourceDirectory />
-
 		<plugins>
 
 			<!-- Prevent annotation processing error during compilation -->

proc/src/main/java/dev/kske/eventbus/proc/EventProcessor.java

@@ -47,13 +47,12 @@ public class EventProcessor extends AbstractProcessor {
 
 				// Task failed successfully
 				eventType		= e.getTypeMirror();
-				useParameter	= processingEnv.getTypeUtils().isSameType(eventType,
-					getTypeMirror(Event.USE_PARAMETER.class));
+				useParameter	= eventType.getKind() == TypeKind.VOID;
 			}
 
 			// Check handler signature
 			boolean pass = false;
-			if (useParameter && eventHandler.getParameters().size() == 0)
+			if (useParameter && eventHandler.getParameters().isEmpty())
 				error(eventHandler, "The method or the annotation must define the event type");
 			else if (!useParameter && eventHandler.getParameters().size() == 1)
 				error(eventHandler,
@@ -100,14 +99,15 @@ public class EventProcessor extends AbstractProcessor {
 			boolean		hasListenerPriority	= listenerPriority != null;
 
 			// Effective polymorphism
-			boolean polymorphic =
+			boolean	polymorphic				=
 				hasListenerPolymorphic ? listenerPolymorphic.value() : defPolymorphic;
 			boolean	hasHandlerPolymorphic	= eventHandler.getAnnotation(Polymorphic.class) != null;
 			if (hasHandlerPolymorphic)
 				polymorphic = eventHandler.getAnnotation(Polymorphic.class).value();
 
 			// Effective priority
-			int priority = hasListenerPriority ? listenerPriority.value() : defPriority;
+			int		priority			=
+				hasListenerPriority ? listenerPriority.value() : defPriority;
 			boolean	hasHandlerPriority	= eventHandler.getAnnotation(Priority.class) != null;
 			if (hasHandlerPriority)
 				priority = eventHandler.getAnnotation(Priority.class).value();
@@ -137,14 +137,6 @@ public class EventProcessor extends AbstractProcessor {
 		}
 	}
 
-	private TypeMirror getTypeMirror(Class<?> clazz) {
-		return getTypeElement(clazz).asType();
-	}
-
-	private TypeElement getTypeElement(Class<?> clazz) {
-		return processingEnv.getElementUtils().getTypeElement(clazz.getCanonicalName());
-	}
-
 	private void warning(Element e, String msg, Object... args) {
 		processingEnv.getMessager().printMessage(Kind.WARNING, String.format(msg, args), e);
 	}

proc/src/main/java/module-info.java

@@ -5,7 +5,7 @@
  * @author Kai S. K. Engelbart
  * @since 1.0.0
  */
-module dev.kske.eventbus.ap {
+module dev.kske.eventbus.proc {
 
 	requires java.compiler;
 	requires dev.kske.eventbus.core;