Merge pull request 'Transparently Propagate Event Handler Errors' (#14) from b/error-passthrough into develop

Reviewed-on: https://git.kske.dev/kske/event-bus/pulls/14
Reviewed-by: delvh <leon@kske.dev>
Reviewed-by: DieGurke <maxi@kske.dev>

README.md

@@ -123,6 +123,38 @@ This applies to all event handlers that would have been executed after the one c
 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 available in Maven Central.

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/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;
 
@@ -63,9 +64,10 @@ 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) {
+	public void dispatch(Object event) throws EventBusException {
 		Objects.requireNonNull(event);
 		logger.log(Level.INFO, "Dispatching event {0}", event);
 
@@ -73,14 +75,40 @@ public final class EventBus {
 		var state = dispatchState.get();
 		state.isDispatching = true;
 
-		for (var handler : getHandlersFor(event.getClass()))
-			if (state.isCancelled) {
-				logger.log(Level.INFO, "Cancelled dispatching event {0}", event);
-				state.isCancelled = false;
-				break;
-			} else {
-				handler.execute(event);
-			}
+		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;
@@ -89,25 +117,26 @@ public final class EventBus {
 	}
 
 	/**
-	 * Searches for the event handlers bound to an event class.
+	 * 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 all event handlers registered for the event class
+	 * @return an iterator over the applicable handlers in descending order of priority
 	 * @since 0.0.1
 	 */
-	private List<EventHandler> getHandlersFor(Class<?> eventClass) {
+	private Iterator<EventHandler> getHandlersFor(Class<?> eventClass) {
 
 		// Get handlers defined for the event class
-		Set<EventHandler> handlers = bindings.getOrDefault(eventClass, new TreeSet<>());
+		TreeSet<EventHandler> handlers = bindings.getOrDefault(eventClass, new TreeSet<>());
 
-		// Get subtype handlers
+		// 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 new ArrayList<>(handlers);
+		return handlers.iterator();
 	}
 
 	/**

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

@@ -68,7 +68,7 @@ final class EventHandler implements Comparable<EventHandler> {
 	 * 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 order of descending priority from a tree set.
+	 * This is used to retrieve event handlers in descending order of priority from a tree set.
 	 *
 	 * @since 0.0.1
 	 */
@@ -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

@@ -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

@@ -27,8 +27,8 @@ class DispatchTest {
 	}
 
 	/**
-	 * Tests {@link EventBus#dispatch(Object)} with multiple handler priorities, a subtype handler
-	 * and a static handler.
+	 * Tests {@link EventBus#dispatch(Object)} with multiple handler priorities, a polymorphic
+	 * handler and a static handler.
 	 *
 	 * @since 0.0.1
 	 */

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

@@ -63,6 +63,10 @@ public class EventProcessor extends AbstractProcessor {
 			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;
@@ -80,14 +84,20 @@ public class EventProcessor extends AbstractProcessor {
 				}
 			}
 
+			// Detect missing or useless @Polymorphic
+			boolean	polymorphic		= eventHandler.getAnnotation(Polymorphic.class) != null;
+			Element	eventElement	= ((DeclaredType) eventType).asElement();
+
 			// Check for handlers for abstract types that aren't polymorphic
-			Element eventElement = ((DeclaredType) eventType).asElement();
-			if (eventHandler.getAnnotation(Polymorphic.class) == null
-				&& (eventElement.getKind() == ElementKind.INTERFACE
-					|| eventElement.getModifiers().contains(Modifier.ABSTRACT))) {
+			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");
 		}
 	}