Remove the restriction that listener methods must be public (#2376)

Author: zml2008

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

@@ -57,7 +57,7 @@
 import javax.lang.model.util.Types;
 
 @SupportedAnnotationTypes(ListenerProcessor.LISTENER_ANNOTATION_CLASS)
-@SupportedSourceVersion(SourceVersion.RELEASE_8)
+@SupportedSourceVersion(SourceVersion.RELEASE_16)
 public class ListenerProcessor extends AbstractProcessor {
 
     static final String LISTENER_ANNOTATION_CLASS = "org.spongepowered.api.event.Listener";
@@ -86,24 +86,21 @@ public boolean process(final Set<? extends TypeElement> annotations, final Round
                 final ExecutableElement method = (ExecutableElement) e;
 
                 if (method.getModifiers().contains(Modifier.STATIC)) {
-                    this.error("method must not be static", method);
-                }
-                if (!method.getModifiers().contains(Modifier.PUBLIC)) {
-                    this.error("method must be public", method);
+                    this.error("Event listener method must not be static", method);
                 }
                 if (method.getModifiers().contains(Modifier.ABSTRACT)) {
-                    this.error("method must not be abstract", method);
+                    this.error("Event listener method must not be abstract", method);
                 }
                 if (method.getEnclosingElement().getKind().isInterface()) {
                     this.error("interfaces cannot declare listeners", method);
                 }
                 if (method.getReturnType().getKind() != TypeKind.VOID) {
-                    this.error("method must return void", method);
+                    this.error("Event listener method must return void", method);
                 }
                 final List<? extends VariableElement> parameters = method.getParameters();
                 final DeclaredType eventType;
                 if (parameters.isEmpty() || !this.isTypeSubclass(parameters.get(0), ListenerProcessor.EVENT_CLASS)) {
-                    this.error("method must have an Event as its first parameter", method);
+                    this.error("Event listener method must have an Event as its first parameter", method);
                     eventType = null;
                 } else {
                     eventType = (DeclaredType) parameters.get(0).asType();

src/main/java/org/spongepowered/api/event/EventManager.java

@@ -36,13 +36,11 @@ public interface EventManager {
      * Registers {@link Event} methods annotated with @{@link Listener} in the
      * specified object.
      *
-     * <p>Only methods that are public will be registered and the class must be
-     * public as well.</p>
+     * <p>This will not include any methods declared in supertypes, but will
+     * test for private and package-private listener methods.</p>
      *
-     * @param plugin The plugin instance
+     * @param plugin The plugin container
      * @param obj The object
-     * @throws IllegalArgumentException Thrown if {@code plugin} is not a plugin
-     *         instance
      */
     void registerListeners(PluginContainer plugin, Object obj);
 

src/main/java/org/spongepowered/api/event/Listener.java

@@ -32,9 +32,6 @@
 
 /**
  * Used to annotate a method as an {@link EventListener}.
- *
- * <p>The method being targeted must be public and must be in a class that is
- * also public.</p>
  */
 @Retention(RUNTIME)
 @Target(METHOD)