Add simple annotation processor, generate shaded processor JAR

event-bus-ap/pom.xml

@@ -27,5 +27,31 @@
 		<!-- Disable test folder -->
 		<testSourceDirectory />
 
+		<plugins>
+			<plugin>
+				<artifactId>maven-compiler-plugin</artifactId>
+				<configuration>
+					<compilerArgument>-proc:none</compilerArgument>
+				</configuration>
+			</plugin>
+
+			<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>
+						</configuration>
+					</execution>
+				</executions>
+			</plugin>
+		</plugins>
+
 	</build>
 </project>

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

@@ -0,0 +1,78 @@
+package dev.kske.eventbus.ap;
+
+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 {
+
+	@Override
+	public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
+		for (Element annotatedElement : roundEnv.getElementsAnnotatedWith(Event.class)) {
+			ExecutableElement	method			= (ExecutableElement) annotatedElement;
+			Event				eventAnnotation	= method.getAnnotation(Event.class);
+			boolean				useParameter	= false;
+
+			// Determine how the event type is defined
+			try {
+				eventAnnotation.eventType();
+			} catch (MirroredTypeException e) {
+
+				// Task failed successfully
+				useParameter = processingEnv.getTypeUtils().isSameType(e.getTypeMirror(),
+					processingEnv.getElementUtils()
+						.getTypeElement(Event.USE_PARAMETER.class.getCanonicalName()).asType());
+			}
+
+			// Check for correct method signature and return type
+			if (method.getParameters().size() == 0 && useParameter)
+				error(method, "The method or the annotation should define the event type");
+
+			if (method.getParameters().size() == 1 && !useParameter)
+				error(method, "Either the method or the annotation should define the event type");
+
+			if (method.getParameters().size() > 1)
+				error(method, "Method should not have more than one parameter");
+
+			if (!method.getReturnType().getKind().equals(TypeKind.VOID))
+				error(method, "Method needs a return type of void");
+
+			// Check event type
+			var paramType = ((ExecutableType) method.asType()).getParameterTypes().get(0);
+			if (useParameter && !processingEnv.getTypeUtils().isAssignable(paramType,
+				processingEnv.getElementUtils()
+					.getTypeElement(IEvent.class.getCanonicalName()).asType()))
+				error(method.getParameters().get(0), "Parameter should implement IEvent");
+
+			// Check listener for interface implementation
+			if (!((TypeElement) method.getEnclosingElement()).getInterfaces().contains(processingEnv
+				.getElementUtils().getTypeElement(EventListener.class.getCanonicalName()).asType()))
+				warning(method.getEnclosingElement(),
+					"Class should implement EventListener interface");
+		}
+		return true;
+	}
+
+	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-ap/src/main/java/dev/kske/eventbus/ap/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.ap;

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

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