Add error message for @Listener methods.

src/ap/java/org/spongepowered/plugin/processor/ListenerProcessor.java

@@ -0,0 +1,104 @@
+/*
+ * This file is part of SpongeAPI, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) SpongePowered <https://www.spongepowered.org>
+ * Copyright (c) contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * 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 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
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package org.spongepowered.plugin.processor;
+
+import static javax.tools.Diagnostic.Kind.ERROR;
+
+import org.spongepowered.api.event.Event;
+import org.spongepowered.api.event.Listener;
+
+import java.util.List;
+import java.util.Set;
+
+import javax.annotation.processing.AbstractProcessor;
+import javax.annotation.processing.Messager;
+import javax.annotation.processing.RoundEnvironment;
+import javax.annotation.processing.SupportedAnnotationTypes;
+import javax.annotation.processing.SupportedSourceVersion;
+import javax.lang.model.SourceVersion;
+import javax.lang.model.element.Element;
+import javax.lang.model.element.ElementKind;
+import javax.lang.model.element.ExecutableElement;
+import javax.lang.model.element.Modifier;
+import javax.lang.model.element.TypeElement;
+import javax.lang.model.element.VariableElement;
+import javax.lang.model.type.TypeKind;
+import javax.lang.model.type.TypeMirror;
+import javax.lang.model.util.Elements;
+import javax.lang.model.util.Types;
+import javax.tools.Diagnostic;
+
+@SupportedAnnotationTypes(ListenerProcessor.LISTENER_ANNOTATION_CLASS)
+@SupportedSourceVersion(SourceVersion.RELEASE_8)
+public class ListenerProcessor extends AbstractProcessor {
+
+    static final String LISTENER_ANNOTATION_CLASS = "org.spongepowered.api.event.Listener";
+    private static final String EVENT_CLASS = Event.class.getName();
+
+    @Override
+    public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
+        if (ProcessorUtils.contains(annotations, Listener.class)) {
+            for (Element e : roundEnv.getElementsAnnotatedWith(Listener.class)) {
+                if (e.getKind() != ElementKind.METHOD) {
+                    processingEnv.getMessager().printMessage(ERROR, "Invalid element of type " + e.getKind() + " annotated with @Listener", e);
+                    continue;
+                }
+                ExecutableElement method = (ExecutableElement) e;
+
+                Messager msg = this.processingEnv.getMessager();
+                if (method.getModifiers().contains(Modifier.STATIC)) {
+                    msg.printMessage(Diagnostic.Kind.ERROR, "method must not be static", method);
+                }
+                if (!method.getModifiers().contains(Modifier.PUBLIC)) {
+                    msg.printMessage(Diagnostic.Kind.ERROR, "method must be public", method);
+                }
+                if (method.getModifiers().contains(Modifier.ABSTRACT)) {
+                    msg.printMessage(Diagnostic.Kind.ERROR, "method must not be abstract", method);
+                }
+                if (method.getEnclosingElement().getKind().isInterface()) {
+                    msg.printMessage(Diagnostic.Kind.ERROR, "interfaces cannot declare listeners", method);
+                }
+                if (method.getReturnType().getKind() != TypeKind.VOID) {
+                    msg.printMessage(Diagnostic.Kind.ERROR, "method must return void", method);
+                }
+                List<? extends VariableElement> parameters = method.getParameters();
+                if (parameters.isEmpty() || !isTypeSubclass(parameters.get(0), EVENT_CLASS)) {
+                    msg.printMessage(Diagnostic.Kind.ERROR, "method must have an Event as its first parameter", method);
+                }
+            }
+        }
+
+        return false;
+    }
+
+    private boolean isTypeSubclass(Element typedElement, String subclass) {
+        Elements elements = processingEnv.getElementUtils();
+        Types types = processingEnv.getTypeUtils();
+
+        TypeMirror event = types.getDeclaredType(elements.getTypeElement(subclass));
+        return types.isAssignable(typedElement.asType(), event);
+    }
+
+}

src/ap/java/org/spongepowered/plugin/processor/PluginProcessor.java

@@ -39,7 +39,6 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
-import java.util.Collection;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -118,7 +117,7 @@ public boolean process(Set<? extends TypeElement> annotations, RoundEnvironment
             return false;
         }
 
-        if (!contains(annotations, Plugin.class)) {
+        if (!ProcessorUtils.contains(annotations, Plugin.class)) {
             return false;
         }
 
@@ -200,19 +199,4 @@ private Messager getMessager() {
         return this.processingEnv.getMessager();
     }
 
-    private static boolean contains(Collection<? extends TypeElement> elements, Class<?> clazz) {
-        if (elements.isEmpty()) {
-            return false;
-        }
-
-        final String name = clazz.getName();
-        for (TypeElement element : elements) {
-            if (element.getQualifiedName().contentEquals(name)) {
-                return true;
-            }
-        }
-
-        return false;
-    }
-
 }

src/ap/java/org/spongepowered/plugin/processor/ProcessorUtils.java

@@ -0,0 +1,51 @@
+/*
+ * This file is part of SpongeAPI, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) SpongePowered <https://www.spongepowered.org>
+ * Copyright (c) contributors
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * 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 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
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package org.spongepowered.plugin.processor;
+
+import java.util.Collection;
+
+import javax.lang.model.element.TypeElement;
+
+final class ProcessorUtils {
+
+    private ProcessorUtils() {
+    }
+
+    static boolean contains(Collection<? extends TypeElement> elements, Class<?> clazz) {
+        if (elements.isEmpty()) {
+            return false;
+        }
+
+        final String name = clazz.getName();
+        for (TypeElement element : elements) {
+            if (element.getQualifiedName().contentEquals(name)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+
+}

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

@@ -1 +1,2 @@
 org.spongepowered.plugin.processor.PluginProcessor
+org.spongepowered.plugin.processor.ListenerProcessor