Add ExceptionEvent

An exception event wraps an event that caused an exception inside of an
event handler while being dispatched and is then dispatched to dedicated
handlers.

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

@@ -2,6 +2,7 @@ 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;
 
@@ -63,6 +64,7 @@ public final class EventBus {
 	 * 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) {
@@ -81,16 +83,27 @@ public final class EventBus {
 					state.isCancelled = false;
 					break;
 				} else {
-					handlers.next().execute(event);
+					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", e);
+						else
+
+							// Dispatch exception event
+							dispatch(new ExceptionEvent(this, event, e.getCause()));
+					}
 				}
-		} else if (!(event instanceof DeadEvent)) {
-			
-			// Dispatch dead event
-			dispatch(new DeadEvent(this, event));
-		} else {
-			
+		} 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

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

@@ -91,17 +91,21 @@ final class EventHandler implements Comparable<EventHandler> {
 	 * Executes the event handler.
 	 *
 	 * @param event the event used as the method parameter
-	 * @throws EventBusException if the handler throws an exception
+	 * @throws EventBusException         if the event handler isn't accessible or has an invalid
+	 *                                   signature
+	 * @throws InvocationTargetException if the handler throws an exception
 	 * @since 0.0.1
 	 */
-	void execute(Object event) throws EventBusException {
+	void execute(Object event) throws EventBusException, InvocationTargetException {
 		try {
 			if (useParameter)
 				method.invoke(listener, event);
 			else
 				method.invoke(listener);
-		} catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
-			throw new EventBusException("Failed to invoke event handler!", e);
+		} catch (IllegalArgumentException e) {
+			throw new EventBusException("Event handler rejected target / argument!", e);
+		} catch (IllegalAccessException e) {
+			throw new EventBusException("Event handler is not accessible!", e);
 		}
 	}
 

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/test/java/dev/kske/eventbus/core/DeadTest.java

@@ -31,7 +31,7 @@ class DeadTest {
 
 	/**
 	 * Tests how the event bus reacts to an unhandled dead event. This should not lead to an
-	 * exception or endless recursion and instead be logged.
+	 * exception or an endless recursion and should be logged instead.
 	 * 
 	 * @since 1.1.0
 	 */

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;
+		}
+	}
+}