Remove obsolete paragraph from README

Reviewed-on: https://git.kske.dev/kske/event-bus/pulls/15
Reviewed-by: delvh <leon@kske.dev>

.gitignore

@@ -1,2 +1,8 @@
-/target/
+# Maven build directories
-/.settings/
+target/
+
+# Dependency reduced POM from Maven Shade Plugin
+dependency-reduced-pom.xml
+
+# Eclipse settings directories
+.settings/

.project

@@ -5,11 +5,6 @@
 	<projects>
 	</projects>
 	<buildSpec>
-		<buildCommand>
-			<name>org.eclipse.jdt.core.javabuilder</name>
-			<arguments>
-			</arguments>
-		</buildCommand>
 		<buildCommand>
 			<name>org.eclipse.m2e.core.maven2Builder</name>
 			<arguments>
@@ -17,7 +12,6 @@
 		</buildCommand>
 	</buildSpec>
 	<natures>
-		<nature>org.eclipse.jdt.core.javanature</nature>
 		<nature>org.eclipse.m2e.core.maven2Nature</nature>
 	</natures>
 </projectDescription>

CODE_OF_CONDUCT.md

@@ -0,0 +1,5 @@
+# Contributor Code of Conduct
+
+This project adheres to No Code of Conduct.  We are all adults.  We accept anyone's contributions.  Nothing else matters.
+
+For more information please visit the [No Code of Conduct](https://nocodeofconduct.com) homepage.

LICENSE

@@ -1,19 +1,13 @@
-MIT License Copyright (c) 2020 Kai S. K. Engelbart
+Copyright 2021 Kai S. K. Engelbart
 
-Permission is hereby granted, free of charge, to any person obtaining a copy
+Licensed under the Apache License, Version 2.0 (the "License");
-of this software and associated documentation files (the "Software"), to deal
+you may not use this file except in compliance with the License.
-in the Software without restriction, including without limitation the rights
+You may obtain a copy of the License at
-to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
-copies of the Software, and to permit persons to whom the Software is furnished
-to do so, subject to the following conditions:
 
-The above copyright notice and this permission notice (including the next
+    https://www.apache.org/licenses/LICENSE-2.0
-paragraph) shall be included in all copies or substantial portions of the
-Software.
 
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+Unless required by applicable law or agreed to in writing, software
-IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+distributed under the License is distributed on an "AS IS" BASIS,
-FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+See the License for the specific language governing permissions and
-WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF
+limitations under the License.
-OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

@@ -3,33 +3,31 @@
 ## Introduction
 
 This library allows passing events between different objects without them having a direct reference to each other.
-Any class can be made an event by implementing the `IEvent` interface.
+Any object can serve as an event.
 
-Using an instance of the `EventBus` class, an instant of the event class can be dispatched.
+Using an instance of the `EventBus` class, an instance of the event class can be dispatched.
 This means that it will be forwarded to all listeners registered for it at the event bus.
 
 In addition, a singleton instance of the event bus is provided by the `EventBus#getInstance()` method.
 
 To listen to events, register event handling methods using the `Event` annotation.
-For this to work, the method must have a return type of `void` and declare a single parameter of the desired event type.
+For this to work, the method must declare a single parameter of the desired event type.
-Additionally, the class containing the method must implement the `EventListener` interface.
+Alternatively, a parameter-less event handler can be declared as shown [below](#parameter-less-event-handlers).
 
 ## A Simple Example
 
-Lets look at a simple example: we declare the empty class `SimpleEvent` that implements `IEvent` and can thus be used as an event.
+Lets look at a simple example: we declare the empty class `SimpleEvent` whose objects can be used as events.
 
 ```java
-import dev.kske.eventbus.IEvent;
+public class SimpleEvent {}
-
-public class SimpleEvent implements IEvent {}
 ```
 
 Next, an event listener for the `SimpleEvent` is declared:
 
 ```java
-import dev.kske.eventbus.*;
+import dev.kske.eventbus.core.*;
 
-public class SimpleEventListener implements EventListener {
+public class SimpleEventListener {
 
     public SimpleEventListener() {
 
@@ -47,26 +45,171 @@ public class SimpleEventListener implements EventListener {
 }
 ```
 
-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
+@Event
+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.
+
+## 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.
+To include subtypes for an event handler, use the `@Polymorphic` annotation in addition to `@Event`:
+
+```java
+@Event
+@Polymorphic
+private void onSimpleEvent(SimpleEvent event) { ... }
+```
+
+## Event Handler Execution Order
+
+Sometimes when using multiple handlers for one event, it might be useful to define in which order they will be executed.
+Event Bus assigns a priority to every handler, which is `100` by default, but can be explicitly set using the `@Priority` annotation in addition to `@Event`:
+
+```java
+@Event
+@Priority(250)
+private void onSimpleEvent(SimpleEvent event) { ... }
+```
+
+**Important:**
+Events are dispatched to handlers in descending order of their priority.
+The execution order is undefined for handlers with the same priority.
+
+## Parameter-Less Event Handlers
+
+In some cases an event handler is not interested in the dispatched event instance.
+To avoid declaring a useless parameter just to specify the event type of the handler, there is an alternative:
+
+```java
+@Event(SimpleEvent.class)
+private void onSimpleEvent() {
+	System.out.println("SimpleEvent received!");
+}
+```
+
+Make sure that you **do not** both declare a parameter and specify the event type in the annotation, as this would be ambiguous.
+
+## 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.
+Instead of defining that priority for each handler, it can be defined at the listener level by annotating the listener itself.
+
+The same applies to polymorphism.
+
+## Event Consumption
+
+In some cases it might be useful to stop the propagation of an event.
+Event Bus makes this possible with event consumption:
+
+```java
+@Event(SimpleEvent.class)
+@Priority(100)
+private void onSimpleEvent() {
+	EventBus.getInstance().cancel();
+}
+
+@Event(SimpleEvent.class)
+@Priority(50)
+private void onSimpleEvent2() {
+	System.out.println("Will not be printed!");
+}
+```
+
+In this example, the second method will not be executed as it has a lower priority and the event will not be propagated after consumption.
+This applies to all event handlers that would have been executed after the one consuming the event.
+
+**Important:**
+Avoid cancelling events while using multiple event handlers with the same priority.
+As event handlers are ordered by priority, it is not defined which of them will be executed after the event has been consumed.
+
+## System Events
+
+To accommodate for special circumstances in an event distribution, system events have been introduced.
+At the moment, there are two system events, which are explained in this section.
+
+### Detecting Unhandled Events
+
+When an event is dispatched but not delivered to any handler, a dead event is dispatched that wraps the original event.
+You can declare a dead event handler to respond to this situation:
+
+```java
+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:
+
+```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.
+
+### 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.
+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
 
-Event Bus is currently hosted at [kske.dev](https://kske.dev).
+Event Bus is available in Maven Central.
-To include it inside your project, just add the Maven repository and the dependency to your `pom.xml`:
+To include it inside your project, just add the following dependency to your `pom.xml`:
 
 ```xml
-<repositories>
-	<repository>
-		<id>kske-repo</id>
-		<url>https://kske.dev/maven-repo</url>
-	</repository>
-</repositories>
-
 <dependencies>
     <dependency>
         <groupId>dev.kske</groupId>
-        <artifactId>event-bus</artifactId>
+        <artifactId>event-bus-core</artifactId>
-        <version>0.0.2</version>
+        <version>1.1.0</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 from your module:
+
+```java
+opens my.module to dev.kske.eventbus.core;
+```
+
+## Compile-Time Error Checking with Event Bus Proc
+
+To assist you with writing event listeners, the Event Bus Proc (Annotation Processor) module enforces correct usage of the `@Event` annotation during compile time.
+This reduces difficult-to-debug bugs that occur during runtime to compile-time errors which can be easily fixed.
+
+The event annotation processor detects invalid event handlers and event type issues with more to come in future versions.
+
+When using Maven, it can be registered using the Maven Compiler Plugin:
+
+```xml
+<plugin>
+    <groupId>org.apache.maven.plugins</groupId>
+    <artifactId>maven-compiler-plugin</artifactId>
+    <version>3.8.1</version>
+    <configuration>
+        <annotationProcessorPaths>
+            <annotationProcessorPath>
+                <groupId>dev.kske</groupId>
+                <artifactId>event-bus-proc</artifactId>
+                <version>1.1.0</version>
+            </annotationProcessorPath>
+        </annotationProcessorPaths>
+    </configuration>
+</plugin>
+```
+
+Alternatively, a JAR file containing the processor is offered with each release for the use within IDEs and environments without Maven support.

event-bus-core/.project

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>event-bus-core</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+		<buildCommand>
+			<name>org.eclipse.m2e.core.maven2Builder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.eclipse.jdt.core.javanature</nature>
+		<nature>org.eclipse.m2e.core.maven2Nature</nature>
+	</natures>
+</projectDescription>

event-bus-core/pom.xml

@@ -0,0 +1,30 @@
+<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/DeadEvent.java

@@ -0,0 +1,37 @@
+package dev.kske.eventbus.core;
+
+/**
+ * Wraps an event that was dispatched but for which no handler has been bound.
+ * <p>
+ * Handling dead events is useful as it can identify a poorly configured event distribution.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.1.0
+ */
+public final class DeadEvent {
+
+	private final EventBus	eventBus;
+	private final Object	event;
+
+	DeadEvent(EventBus eventBus, Object event) {
+		this.eventBus	= eventBus;
+		this.event		= event;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("DeadEvent[eventBus=%s, event=%s]", eventBus, event);
+	}
+
+	/**
+	 * @return the event bus that dispatched this event
+	 * @since 1.1.0
+	 */
+	public EventBus getEventBus() { return eventBus; }
+
+	/**
+	 * @return the event that could not be delivered
+	 * @since 1.1.0
+	 */
+	public Object getEvent() { return event; }
+}

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

@@ -0,0 +1,42 @@
+package dev.kske.eventbus.core;
+
+import static java.lang.annotation.ElementType.METHOD;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.*;
+
+/**
+ * Indicates that a method is an event handler.
+ * <p>
+ * To be successfully used as such, the method has to specify the event type by either declaring one
+ * parameter of that type or setting the annotation value to the corresponding class.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.1
+ * @see Polymorphic
+ * @see Priority
+ */
+@Documented
+@Retention(RUNTIME)
+@Target(METHOD)
+public @interface Event {
+
+	/**
+	 * Defines the event type the handler listens to. 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.
+	 *
+	 * @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 {}
+}

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

@@ -0,0 +1,257 @@
+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

@@ -0,0 +1,33 @@
+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

@@ -0,0 +1,135 @@
+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/main/java/dev/kske/eventbus/core/ExceptionEvent.java

@@ -0,0 +1,47 @@
+package dev.kske.eventbus.core;
+
+/**
+ * Wraps an event that was dispatched but caused an exception in one of its handlers.
+ * <p>
+ * Handling exception events is useful as it allows the creation of a centralized exception handling
+ * mechanism for unexpected exceptions.
+ * 
+ * @author Kai S. K. Engelbart
+ * @since 1.1.0
+ */
+public final class ExceptionEvent {
+
+	private final EventBus	eventBus;
+	private final Object	event;
+	private final Throwable	cause;
+
+	ExceptionEvent(EventBus eventBus, Object event, Throwable cause) {
+		this.eventBus	= eventBus;
+		this.event		= event;
+		this.cause		= cause;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("ExceptionEvent[eventBus=%s, event=%s, cause=%s]", eventBus, event,
+			cause);
+	}
+
+	/**
+	 * @return the event bus that dispatched this event
+	 * @since 1.1.0
+	 */
+	public EventBus getEventBus() { return eventBus; }
+
+	/**
+	 * @return the event that could not be handled because of an exception
+	 * @since 1.1.0
+	 */
+	public Object getEvent() { return event; }
+
+	/**
+	 * @return the exception that was thrown while handling the event
+	 * @since 1.1.0
+	 */
+	public Throwable getCause() { return cause; }
+}

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

@@ -0,0 +1,30 @@
+package dev.kske.eventbus.core;
+
+import static java.lang.annotation.ElementType.*;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.*;
+
+/**
+ * Allows an event handler to receive events that are subtypes of the declared event type.
+ * <p>
+ * When used on a type, the value applies to all event handlers declared within that type that don't
+ * define a value on their own.
+ * <p>
+ * This is useful when defining an event handler for an interface or an abstract class.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ * @see Event
+ */
+@Documented
+@Retention(RUNTIME)
+@Target({ METHOD, TYPE })
+public @interface Polymorphic {
+
+	/**
+	 * @return whether the event handler is polymorphic
+	 * @since 1.1.0
+	 */
+	boolean value() default true;
+}

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

@@ -0,0 +1,33 @@
+package dev.kske.eventbus.core;
+
+import static java.lang.annotation.ElementType.*;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+import java.lang.annotation.*;
+
+/**
+ * Defines the priority of an event handler. Handlers are executed in descending order of their
+ * priority.
+ * <p>
+ * When used on a type, the value applies to all event handlers declared within that type that don't
+ * define a value on their own.
+ * <p>
+ * Handlers without this annotation have the default priority of 100.
+ * <p>
+ * The execution order of handlers with the same priority is undefined.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ * @see Event
+ */
+@Documented
+@Retention(RUNTIME)
+@Target({ METHOD, TYPE })
+public @interface Priority {
+
+	/**
+	 * @return the priority of the event handler
+	 * @since 1.0.0
+	 */
+	int value();
+}

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

@@ -0,0 +1,9 @@
+/**
+ * Contains the public API and implementation of the Event Bus library.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.1
+ * @see dev.kske.eventbus.core.Event
+ * @see dev.kske.eventbus.core.EventBus
+ */
+package dev.kske.eventbus.core;

event-bus-core/src/main/java/module-info.java

@@ -0,0 +1,12 @@
+/**
+ * Contains the public API and implementation of the Event Bus library.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.3
+ * @see dev.kske.eventbus.core.Event
+ * @see dev.kske.eventbus.core.EventBus
+ */
+module dev.kske.eventbus.core {
+
+	exports dev.kske.eventbus.core;
+}

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

@@ -0,0 +1,54 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.*;
+
+/**
+ * Tests the event cancellation mechanism of the event bus.
+ *
+ * @author Kai S. K. Engelbart
+ * @author Leon Hofmeister
+ * @since 0.1.0
+ */
+class CancelTest {
+
+	EventBus	bus;
+	int			hits;
+
+	/**
+	 * Constructs an event bus and registers this test instance as an event listener.
+	 *
+	 * @since 0.1.0
+	 */
+	@BeforeEach
+	void registerListener() {
+		bus = new EventBus();
+		bus.registerListener(this);
+	}
+
+	/**
+	 * Tests {@link EventBus#cancel()} with two event handlers, of which the first cancels the
+	 * event.
+	 *
+	 * @since 0.1.0
+	 */
+	@Test
+	void testCancellation() {
+		bus.dispatch(new SimpleEvent());
+		assertEquals(1, hits);
+	}
+
+	@Event(SimpleEvent.class)
+	@Priority(100)
+	void onSimpleFirst() {
+		++hits;
+		bus.cancel();
+	}
+
+	@Event(SimpleEvent.class)
+	@Priority(50)
+	void onSimpleSecond() {
+		++hits;
+	}
+}

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

@@ -0,0 +1,49 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests the dispatching of a dead event if an event could not be delivered.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.1.0
+ */
+class DeadTest {
+
+	EventBus	bus		= new EventBus();
+	String		event	= "This event has no handler";
+	boolean		deadEventHandled;
+
+	/**
+	 * Tests dead event delivery.
+	 * 
+	 * @since 1.1.0
+	 */
+	@Test
+	void testDeadEvent() {
+		bus.registerListener(this);
+		bus.dispatch(event);
+		assertTrue(deadEventHandled);
+		bus.removeListener(this);
+	}
+
+	/**
+	 * Tests how the event bus reacts to an unhandled dead event. This should not lead to an
+	 * exception or an endless recursion and should be logged instead.
+	 * 
+	 * @since 1.1.0
+	 */
+	@Test
+	void testUnhandledDeadEvent() {
+		bus.dispatch(event);
+	}
+
+	@Event
+	void onDeadEvent(DeadEvent deadEvent) {
+		assertEquals(bus, deadEvent.getEventBus());
+		assertEquals(event, deadEvent.getEvent());
+		deadEventHandled = true;
+	}
+}

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

@@ -0,0 +1,64 @@
+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/ExceptionTest.java

@@ -0,0 +1,62 @@
+package dev.kske.eventbus.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests the dispatching of an exception event if an event handler threw an exception.
+ * 
+ * @author Kai S. K. Engelbart
+ * @since 1.1.0
+ */
+class ExceptionTest {
+
+	EventBus			bus			= new EventBus();
+	String				event		= "This event will cause an exception";
+	RuntimeException	exception	= new RuntimeException("I failed");
+	boolean				exceptionEventHandled;
+
+	/**
+	 * Tests exception event delivery.
+	 * 
+	 * @since 1.1.0
+	 */
+	@Test
+	void testExceptionEvent() {
+		bus.registerListener(this);
+		bus.registerListener(new ExceptionListener());
+		bus.dispatch(event);
+		assertTrue(exceptionEventHandled);
+		bus.clearListeners();
+	}
+
+	/**
+	 * Tests how the event bus reacts to an unhandled exception event. This should not lead to an
+	 * exception or an endless recursion and should be logged instead.
+	 * 
+	 * @since 1.1.0
+	 */
+	@Test
+	void testUnhandledExceptionEvent() {
+		bus.registerListener(this);
+		bus.dispatch(event);
+		bus.removeListener(this);
+	}
+
+	@Event(String.class)
+	void onString() {
+		throw exception;
+	}
+
+	class ExceptionListener {
+
+		@Event
+		void onExceptionEvent(ExceptionEvent exceptionEvent) {
+			assertEquals(bus, exceptionEvent.getEventBus());
+			assertEquals(event, exceptionEvent.getEvent());
+			assertEquals(exception, exceptionEvent.getCause());
+			exceptionEventHandled = true;
+		}
+	}
+}

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

@@ -1,4 +1,4 @@
-package dev.kske.eventbus;
+package dev.kske.eventbus.core;
 
 /**
  * A simple event for testing purposes.
@@ -6,4 +6,4 @@ package dev.kske.eventbus;
  * @author Kai S. K. Engelbart
  * @since 0.0.1
  */
-public class SimpleEvent implements IEvent {}
+public class SimpleEvent {}

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

@@ -0,0 +1,9 @@
+package dev.kske.eventbus.core;
+
+/**
+ * Subclass of {@link SimpleEvent} for testing purposes.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 0.0.4
+ */
+public class SimpleEventSub extends SimpleEvent {}

event-bus-proc/.classpath

@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<classpath>
+	<classpathentry kind="src" output="target/classes" path="src/main/java">
+		<attributes>
+			<attribute name="optional" value="true"/>
+			<attribute name="maven.pomderived" value="true"/>
+		</attributes>
+	</classpathentry>
+	<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/JavaSE-11">
+		<attributes>
+			<attribute name="maven.pomderived" value="true"/>
+		</attributes>
+	</classpathentry>
+	<classpathentry kind="con" path="org.eclipse.m2e.MAVEN2_CLASSPATH_CONTAINER">
+		<attributes>
+			<attribute name="maven.pomderived" value="true"/>
+		</attributes>
+	</classpathentry>
+	<classpathentry kind="src" output="target/test-classes" path="home/kske/git/event-bus/event-bus-ap">
+		<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>

event-bus-proc/.project

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>event-bus-proc</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+		<buildCommand>
+			<name>org.eclipse.m2e.core.maven2Builder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.eclipse.jdt.core.javanature</nature>
+		<nature>org.eclipse.m2e.core.maven2Nature</nature>
+	</natures>
+</projectDescription>

event-bus-proc/pom.xml

@@ -0,0 +1,75 @@
+<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-proc</artifactId>
+
+	<name>Event Bus Annotation Processor</name>
+	<description>Annotation processor checking for errors related to the @Event annotation from Event Bus.</description>
+
+	<parent>
+		<groupId>dev.kske</groupId>
+		<artifactId>event-bus</artifactId>
+		<version>1.1.0</version>
+	</parent>
+
+	<dependencies>
+		<dependency>
+			<groupId>dev.kske</groupId>
+			<artifactId>event-bus-core</artifactId>
+			<version>${project.version}</version>
+		</dependency>
+	</dependencies>
+
+	<build>
+
+		<!-- Disable test folder -->
+		<testSourceDirectory />
+
+		<plugins>
+
+			<!-- Prevent annotation processing error during compilation -->
+			<plugin>
+				<artifactId>maven-compiler-plugin</artifactId>
+				<configuration>
+					<compilerArgument>-proc:none</compilerArgument>
+				</configuration>
+			</plugin>
+
+			<!-- Include event-bus-core classes into JAR -->
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-shade-plugin</artifactId>
+				<version>3.2.4</version>
+				<executions>
+					<execution>
+						<phase>package</phase>
+						<goals>
+							<goal>shade</goal>
+						</goals>
+						<configuration>
+							<minimizeJar>true</minimizeJar>
+							<filters>
+								<filter>
+									<artifact>dev.kske:event-bus-core</artifact>
+									<excludes>
+										<exclude>META-INF/MANIFEST.MF</exclude>
+									</excludes>
+								</filter>
+								<filter>
+									<artifact>*:*</artifact>
+									<excludes>
+										<exclude>module-info.class</exclude>
+										<exclude>META-INF/maven/**</exclude>
+									</excludes>
+								</filter>
+							</filters>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+
+	</build>
+</project>

event-bus-proc/src/main/java/dev/kske/eventbus/proc/EventProcessor.java

@@ -0,0 +1,155 @@
+package dev.kske.eventbus.proc;
+
+import java.util.Set;
+
+import javax.annotation.processing.*;
+import javax.lang.model.SourceVersion;
+import javax.lang.model.element.*;
+import javax.lang.model.type.*;
+import javax.tools.Diagnostic.Kind;
+
+import dev.kske.eventbus.core.*;
+
+/**
+ * This annotation processor checks event handlers for common mistakes which can only be detected
+ * during runtime otherwise.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ */
+@SupportedAnnotationTypes("dev.kske.eventbus.core.Event")
+@SupportedSourceVersion(SourceVersion.RELEASE_11)
+public class EventProcessor extends AbstractProcessor {
+
+	@SuppressWarnings("unchecked")
+	@Override
+	public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
+		if (!roundEnv.errorRaised() && !roundEnv.processingOver())
+			processRound(
+				(Set<ExecutableElement>) roundEnv.getElementsAnnotatedWith(Event.class));
+
+		// Do not claim the processed annotations
+		return false;
+	}
+
+	private void processRound(Set<ExecutableElement> eventHandlers) {
+		for (ExecutableElement eventHandler : eventHandlers) {
+			Event		eventAnnotation	= eventHandler.getAnnotation(Event.class);
+			TypeMirror	eventType;
+
+			// Determine the event type and how it is defined
+			boolean useParameter;
+			try {
+				eventAnnotation.value();
+				throw new EventBusException(
+					"Could not determine event type of handler " + eventHandler);
+			} catch (MirroredTypeException e) {
+
+				// Task failed successfully
+				eventType		= e.getTypeMirror();
+				useParameter	= processingEnv.getTypeUtils().isSameType(eventType,
+					getTypeMirror(Event.USE_PARAMETER.class));
+			}
+
+			// Check handler signature
+			boolean pass = false;
+			if (useParameter && eventHandler.getParameters().size() == 0)
+				error(eventHandler, "The method or the annotation must define the event type");
+			else if (!useParameter && eventHandler.getParameters().size() == 1)
+				error(eventHandler,
+					"Either the method or the annotation must define the event type");
+			else if (eventHandler.getParameters().size() > 1)
+				error(eventHandler, "Method must not have more than one parameter");
+			else
+				pass = true;
+
+			// Warn the user about unused return values
+			if (useParameter && eventHandler.getReturnType().getKind() != TypeKind.VOID)
+				warning(eventHandler, "Unused return value");
+
+			// Abort checking if the handler signature is incorrect
+			if (!pass)
+				continue;
+
+			// Additional checks if parameter is used
+			if (useParameter) {
+				VariableElement paramElement = eventHandler.getParameters().get(0);
+				eventType = paramElement.asType();
+
+				// Check if parameter is object
+				// Abort checking otherwise
+				if (eventType.getKind() != TypeKind.DECLARED) {
+					error(paramElement, "Event must be an object");
+					continue;
+				}
+			}
+
+			// Get the listener containing this handler
+			TypeElement listener = (TypeElement) eventHandler.getEnclosingElement();
+
+			// Default properties
+			boolean	defPolymorphic	= false;
+			int		defPriority		= 100;
+
+			// Listener-level polymorphism
+			Polymorphic	listenerPolymorphic		= listener.getAnnotation(Polymorphic.class);
+			boolean		hasListenerPolymorphic	= listenerPolymorphic != null;
+
+			// Listener-level priority
+			Priority	listenerPriority	= listener.getAnnotation(Priority.class);
+			boolean		hasListenerPriority	= listenerPriority != null;
+
+			// Effective polymorphism
+			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;
+			boolean	hasHandlerPriority	= eventHandler.getAnnotation(Priority.class) != null;
+			if (hasHandlerPriority)
+				priority = eventHandler.getAnnotation(Priority.class).value();
+
+			// Detect useless polymorphism redefinition
+			if (hasListenerPolymorphic && hasHandlerPolymorphic
+				&& listenerPolymorphic.value() == polymorphic)
+				warning(eventHandler, "@Polymorphism is already defined at listener level");
+
+			// Detect useless priority redefinition
+			if (hasListenerPriority && hasHandlerPriority && listenerPriority.value() == priority)
+				warning(eventHandler, "@Priority is already defined at the listener level");
+
+			// Detect missing or useless @Polymorphic
+			Element eventElement = ((DeclaredType) eventType).asElement();
+
+			// Check for handlers for abstract types that aren't polymorphic
+			if (!polymorphic && (eventElement.getKind() == ElementKind.INTERFACE
+				|| eventElement.getModifiers().contains(Modifier.ABSTRACT)))
+				warning(eventHandler,
+					"Parameter should be instantiable or handler should use @Polymorphic");
+
+			// Check for handlers for final types that are polymorphic
+			else if (polymorphic && eventElement.getModifiers().contains(Modifier.FINAL))
+				warning(eventHandler,
+					"@Polymorphic should be removed as parameter cannot be subclassed");
+		}
+	}
+
+	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);
+	}
+
+	private void error(Element e, String msg, Object... args) {
+		processingEnv.getMessager().printMessage(Kind.ERROR, String.format(msg, args), e);
+	}
+}

event-bus-proc/src/main/java/dev/kske/eventbus/proc/package-info.java

@@ -0,0 +1,7 @@
+/**
+ * Contains the Event Bus annotation processor.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ */
+package dev.kske.eventbus.proc;

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

@@ -0,0 +1,12 @@
+/**
+ * Contains an annotation processor for checking for errors related to the
+ * {@link dev.kske.eventbus.core.Event} annotation from Event Bus.
+ *
+ * @author Kai S. K. Engelbart
+ * @since 1.0.0
+ */
+module dev.kske.eventbus.ap {
+
+	requires java.compiler;
+	requires dev.kske.eventbus.core;
+}

event-bus-proc/src/main/resources/META-INF/services/javax.annotation.processing.Processor

@@ -0,0 +1 @@
+dev.kske.eventbus.proc.EventProcessor

pom.xml

@@ -5,16 +5,24 @@
 
 	<groupId>dev.kske</groupId>
 	<artifactId>event-bus</artifactId>
-	<version>0.0.2</version>
+	<version>1.1.0</version>
+	<packaging>pom</packaging>
 
 	<name>Event Bus</name>
 	<description>An event handling framework for Java utilizing annotations.</description>
-	<url>https://git.kske.dev/zdm/event-bus</url>
+	<url>https://git.kske.dev/kske/event-bus</url>
+
+	<modules>
+		<module>event-bus-core</module>
+		<module>event-bus-proc</module>
+	</modules>
 
 	<licenses>
 		<license>
-			<name>MIT License</name>
+			<name>Apache License, Version 2.0</name>
-			<url>http://www.opensource.org/licenses/mit-license.php</url>
+			<url>https://www.apache.org/licenses/LICENSE-2.0.txt</url>
+			<distribution>repo</distribution>
+			<comments>A business-friendly OSS license</comments>
 		</license>
 	</licenses>
 
@@ -32,9 +40,23 @@
 		</developer>
 	</developers>
 
+	<contributors>
+		<contributor>
+			<name>Leon Hofmeister</name>
+			<email>leon@kske.dev</email>
+			<url>https://git.kske.dev/delvh</url>
+			<roles>
+				<role>qa</role>
+				<role>tester</role>
+			</roles>
+			<timezone>Europe/Berlin</timezone>
+		</contributor>
+	</contributors>
+
 	<scm>
-		<connection>scm:git:https://git.kske.dev/zdm/event-bus.git</connection>
+		<connection>scm:git:https://git.kske.dev/kske/event-bus.git</connection>
-		<developerConnection>scm:git:ssh:git@git.kske.dev:zdm/event-bus.git</developerConnection>
+		<developerConnection>scm:git:ssh://git@git.kske.dev:420/kske/event-bus.git</developerConnection>
+		<url>https://git.kske.dev/kske/event-bus</url>
 	</scm>
 
 	<properties>
@@ -44,13 +66,37 @@
 		<maven.compiler.target>11</maven.compiler.target>
 	</properties>
 
+	<!-- Configure deployment to OSSRH -->
+	<distributionManagement>
+		<snapshotRepository>
+			<id>ossrh</id>
+			<url>https://oss.sonatype.org/content/repositories/snapshots</url>
+		</snapshotRepository>
+		<repository>
+			<id>ossrh</id>
+			<url>https://oss.sonatype.org/service/local/staging/deploy/maven2/</url>
+		</repository>
+	</distributionManagement>
+
 	<build>
-		<resources />
+
+		<!-- Disable test resource folder -->
 		<testResources />
+
 		<plugins>
+
+			<!-- Support Java 9 modules -->
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-compiler-plugin</artifactId>
+				<version>3.8.1</version>
+			</plugin>
+
+			<!-- Attach sources to JAR -->
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-source-plugin</artifactId>
+				<version>3.2.1</version>
 				<executions>
 					<execution>
 						<id>attach-sources</id>
@@ -60,9 +106,12 @@
 					</execution>
 				</executions>
 			</plugin>
+
+			<!-- Attach Javadoc to JAR -->
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-javadoc-plugin</artifactId>
+				<version>3.2.0</version>
 				<executions>
 					<execution>
 						<id>attach-javadocs</id>
@@ -72,15 +121,23 @@
 					</execution>
 				</executions>
 			</plugin>
+
+			<!-- GPG sign JAR -->
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-gpg-plugin</artifactId>
+				<version>1.6</version>
+				<executions>
+					<execution>
+						<id>sign-artifacts</id>
+						<phase>verify</phase>
+						<goals>
+							<goal>sign</goal>
+						</goals>
+					</execution>
+				</executions>
+			</plugin>
+
 		</plugins>
 	</build>
-
-	<dependencies>
-		<dependency>
-			<groupId>org.junit.jupiter</groupId>
-			<artifactId>junit-jupiter-api</artifactId>
-			<version>5.6.2</version>
-			<scope>test</scope>
-		</dependency>
-	</dependencies>
 </project>

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

@@ -1,34 +0,0 @@
-package dev.kske.eventbus;
-
-import static java.lang.annotation.ElementType.METHOD;
-import static java.lang.annotation.RetentionPolicy.RUNTIME;
-
-import java.lang.annotation.*;
-
-/**
- * Indicates that a method is an event handler. To be successfully used as such, the method has to
- * comply with the following specifications:
- * <ul>
- * <li>Declared inside a class that implements {@link EventListener}</li>
- * <li>One parameter of a type that implements {@link IEvent}</li>
- * <li>Return type of {@code void}</li>
- * </ul>
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- */
-@Documented
-@Retention(RUNTIME)
-@Target(METHOD)
-public @interface Event {
-
-	/**
-	 * Defines the priority of the event handler. Handlers are executed in descending order of their
-	 * priority.
-	 * <p>
-	 * The execution order of handlers with the same priority is undefined.
-	 *
-	 * @since 0.0.1
-	 */
-	int priority() default 100;
-}

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

@@ -1,137 +0,0 @@
-package dev.kske.eventbus;
-
-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 {
-
-	private static EventBus singletonInstance;
-
-	/**
-	 * 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() {
-		if (singletonInstance == null)
-			singletonInstance = new EventBus();
-		return singletonInstance;
-	}
-
-	private final Map<Class<? extends IEvent>, Collection<EventHandler>> bindings
-		= new ConcurrentHashMap<>();
-	private final Set<EventListener> registeredListeners = ConcurrentHashMap.newKeySet();
-
-	/**
-	 * Dispatches an event to all event handlers registered for it in descending order of their
-	 * priority.
-	 *
-	 * @param event the event to dispatch
-	 * @since 0.0.1
-	 */
-	public void dispatch(IEvent event) {
-		getHandlersFor(event.getClass()).forEach(handler -> handler.execute(event));
-	}
-
-	/**
-	 * Searches for the event handlers bound to an event class.
-	 *
-	 * @param eventClass the event class to use for the search
-	 * @return all event handlers registered for the event class
-	 * @since 0.0.1
-	 */
-	private List<EventHandler> getHandlersFor(Class<? extends IEvent> eventClass) {
-		return bindings.containsKey(eventClass) ? new ArrayList<>(bindings.get(eventClass))
-			: new ArrayList<>();
-	}
-
-	/**
-	 * 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 to the specification
-	 * @since 0.0.1
-	 * @see Event
-	 */
-	public void registerListener(EventListener listener) throws EventBusException {
-		if (registeredListeners.contains(listener))
-			throw new EventBusException(listener + " already registered!");
-
-		registeredListeners.add(listener);
-		for (var method : listener.getClass().getDeclaredMethods()) {
-			Event annotation = method.getAnnotation(Event.class);
-
-			// Skip methods without annotations
-			if (annotation == null)
-				continue;
-
-			// Check for correct method signature and return type
-			if (method.getParameterCount() != 1)
-				throw new EventBusException(method + " does not have an argument count of 1!");
-
-			if (!method.getReturnType().equals(void.class))
-				throw new EventBusException(method + " does not have a return type of void!");
-
-			var param = method.getParameterTypes()[0];
-			if (!IEvent.class.isAssignableFrom(param))
-				throw new EventBusException(param + " is not of type IEvent!");
-
-			@SuppressWarnings("unchecked")
-			var realParam = (Class<? extends IEvent>) param;
-			if (!bindings.containsKey(realParam))
-				bindings.put(realParam, new TreeSet<>());
-
-			bindings.get(realParam).add(new EventHandler(listener, method, annotation));
-		}
-	}
-
-	/**
-	 * Removes a specific listener from this event bus.
-	 *
-	 * @param listener the listener to remove
-	 * @since 0.0.1
-	 */
-	public void removeListener(EventListener listener) {
-		for (var binding : bindings.values()) {
-			var it = binding.iterator();
-			while (it.hasNext())
-				if (it.next().getListener() == listener)
-					it.remove();
-		}
-		registeredListeners.remove(listener);
-	}
-
-	/**
-	 * Removes all event listeners from this event bus.
-	 *
-	 * @since 0.0.1
-	 */
-	public void clearListeners() {
-		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<EventListener> getRegisteredListeners() {
-		return Collections.unmodifiableSet(registeredListeners);
-	}
-}

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

@@ -1,21 +0,0 @@
-package dev.kske.eventbus;
-
-/**
- * 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;
-
-	public EventBusException(String message, Throwable cause) {
-		super(message, cause);
-	}
-
-	public EventBusException(String message) {
-		super(message);
-	}
-}

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

@@ -1,85 +0,0 @@
-package dev.kske.eventbus;
-
-import java.lang.reflect.*;
-
-/**
- * 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 EventListener listener;
-	private final Method method;
-	private final Event annotation;
-
-	/**
-	 * Constructs an event handler.
-	 *
-	 * @param listener   the listener containing the handler
-	 * @param method     the handler method
-	 * @param annotation the event annotation
-	 * @since 0.0.1
-	 */
-	EventHandler(EventListener listener, Method method, Event annotation) {
-		this.listener = listener;
-		this.method = method;
-		this.annotation = annotation;
-		method.setAccessible(true);
-	}
-
-	/**
-	 * Compares this to another event handler based on {@link Event#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 the correct order from a tree set.
-	 *
-	 * @since 0.0.1
-	 */
-	@Override
-	public int compareTo(EventHandler other) {
-		int priority = other.annotation.priority() - annotation.priority();
-		if (priority == 0)
-			priority = listener.hashCode() - other.listener.hashCode();
-		return priority == 0 ? hashCode() - other.hashCode() : priority;
-	}
-
-	/**
-	 * Executes the event handler.
-	 *
-	 * @param event the event used as the method parameter
-	 * @throws EventBusException if the handler throws an exception
-	 * @since 0.0.1
-	 */
-	void execute(IEvent event) throws EventBusException {
-		try {
-			method.invoke(listener, event);
-		} catch (
-			IllegalAccessException
-			| IllegalArgumentException
-			| InvocationTargetException e
-		) {
-			throw new EventBusException("Failed to invoke event handler!", e);
-		}
-	}
-
-	/**
-	 * @return the listener containing this handler
-	 * @since 0.0.1
-	 */
-	EventListener getListener() { return listener; }
-
-	/**
-	 * @return the event annotation
-	 * @since 0.0.1
-	 */
-	Event getAnnotation() { return annotation; }
-
-	/**
-	 * @return the priority of the event annotation
-	 * @since 0.0.1
-	 */
-	int getPriority() { return annotation.priority(); }
-}

src/main/java/dev/kske/eventbus/EventListener.java

@@ -1,12 +0,0 @@
-package dev.kske.eventbus;
-
-/**
- * Marker interface for event listeners. Event listeners can contain event handling methods to which
- * events can be dispatched.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see Event
- * @see EventBus
- */
-public interface EventListener {}

src/main/java/dev/kske/eventbus/IEvent.java

@@ -1,12 +0,0 @@
-package dev.kske.eventbus;
-
-/**
- * Marker interface for event objects. Event objects can be used as event handler parameters and
- * thus can be dispatched to the event bus.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see Event
- * @see EventBus
- */
-public interface IEvent {}

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

@@ -1,9 +0,0 @@
-/**
- * Contains the public API and implementation of the event bus library.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- * @see dev.kske.eventbus.Event
- * @see dev.kske.eventbus.EventBus
- */
-package dev.kske.eventbus;

src/test/java/dev/kske/eventbus/EventBusTest.java

@@ -1,38 +0,0 @@
-package dev.kske.eventbus;
-
-import static org.junit.jupiter.api.Assertions.assertEquals;
-
-import org.junit.jupiter.api.*;
-
-/**
- * Tests the of the event bus library.
- *
- * @author Kai S. K. Engelbart
- * @since 0.0.1
- */
-class EventBusTest implements EventListener {
-
-	int hits;
-
-	@BeforeEach
-	public void registerListener() {
-		EventBus.getInstance().registerListener(this);
-	}
-
-	@Test
-	void testDispatch() {
-		EventBus.getInstance().dispatch(new SimpleEvent());
-	}
-
-	@Event(priority = 50)
-	private void onSimpleEventSecond(SimpleEvent event) {
-		++hits;
-		assertEquals(2, hits);
-	}
-
-	@Event(priority = 150)
-	private void onSimpleEventFirst(SimpleEvent event) {
-		++hits;
-		assertEquals(1, hits);
-	}
-}