Service injector - implement stricter requirements for PostConstruct methods

- Implement similar restrictions as prescribed by the PostConstruct documentation:
   - Class may have at most one method annotated with PostConstruct
   - PostConstruct method must return void
- Javadoc: replace mentions of injector construction where any injection method was meant

src/main/java/fr/xephi/authme/initialization/AuthMeServiceInitializer.java

@@ -144,8 +144,8 @@ public class AuthMeServiceInitializer {
     }
 
     /**
-     * Instantiates the given class by locating an @Inject constructor and retrieving
-     * or instantiating its parameters.
+     * Instantiates the given class by locating its @Inject elements and retrieving
+     * or instantiating the required instances.
      *
      * @param clazz the class to instantiate
      * @param traversedClasses collection of classes already traversed
@@ -164,13 +164,13 @@ public class AuthMeServiceInitializer {
         validateInjectionHasNoCircularDependencies(injection.getDependencies(), traversedClasses);
         Object[] dependencies = resolveDependencies(injection, traversedClasses);
         T object = injection.instantiateWith(dependencies);
-        executePostConstructMethods(object);
+        executePostConstructMethod(object);
         return object;
     }
 
     /**
-     * Resolves the dependencies for the given constructor, i.e. returns a collection that satisfy
-     * the constructor's parameter types by retrieving elements or instantiating them where necessary.
+     * Resolves the dependencies for the given class instantiation, i.e. returns a collection that satisfy
+     * the class' dependencies by retrieving elements or instantiating them where necessary.
      *
      * @param injection the injection parameters
      * @param traversedClasses collection of traversed classes
@@ -247,22 +247,21 @@ public class AuthMeServiceInitializer {
             + "allowed packages. It must be provided explicitly or the package must be passed to the constructor.");
     }
 
-    private static void executePostConstructMethods(Object object) {
-        for (Method method : object.getClass().getDeclaredMethods()) {
-            if (method.isAnnotationPresent(PostConstruct.class)) {
-                if (method.getParameterTypes().length == 0 && !Modifier.isStatic(method.getModifiers())) {
+    /**
+     * Executes an object's method annotated with {@link PostConstruct} if present.
+     * Throws an exception if there are multiple such methods, or if the method is static.
+     *
+     * @param object the object to execute the post construct method for
+     */
+    private static void executePostConstructMethod(Object object) {
+        Method postConstructMethod = getAndValidatePostConstructMethod(object.getClass());
+        if (postConstructMethod != null) {
             try {
-                        method.setAccessible(true);
-                        method.invoke(object);
+                postConstructMethod.setAccessible(true);
+                postConstructMethod.invoke(object);
             } catch (IllegalAccessException | InvocationTargetException e) {
                 throw new UnsupportedOperationException(e);
             }
-                } else {
-                    throw new IllegalStateException(String.format("@PostConstruct methods may not be static or have "
-                        + " any parameters. Method '%s' of class '%s' is either static or has parameters",
-                        method.getName(), object.getClass().getSimpleName()));
-                }
-            }
         }
     }
 
@@ -272,6 +271,31 @@ public class AuthMeServiceInitializer {
         }
     }
 
+    /**
+     * Validate and locate the given class' post construct method. Returns {@code null} if none present.
+     *
+     * @param clazz the class to search
+     * @return post construct method, or null
+     */
+    private static Method getAndValidatePostConstructMethod(Class<?> clazz) {
+        Method postConstructMethod = null;
+        for (Method method : clazz.getDeclaredMethods()) {
+            if (method.isAnnotationPresent(PostConstruct.class)) {
+                if (postConstructMethod != null) {
+                    throw new IllegalStateException("Multiple methods with @PostConstruct on " + clazz);
+                } else if (method.getParameterTypes().length > 0 || Modifier.isStatic(method.getModifiers())) {
+                    throw new IllegalStateException("@PostConstruct method may not be static or have any parameters. "
+                        + "Invalid method in " + clazz);
+                } else if (method.getReturnType() != void.class) {
+                    throw new IllegalStateException("@PostConstruct method must be void. Offending class: " + clazz);
+                } else {
+                    postConstructMethod = method;
+                }
+            }
+        }
+        return postConstructMethod;
+    }
+
     @SafeVarargs
     private static <T> Injection<T> firstNotNull(Provider<? extends Injection<T>>... providers) {
         for (Provider<? extends Injection<T>> provider : providers) {

src/main/java/fr/xephi/authme/permission/PermissionsManager.java

@@ -96,11 +96,9 @@ public class PermissionsManager implements PermissionsService {
 
     /**
      * Setup and hook into the permissions systems.
-     *
-     * @return The detected permissions system.
      */
     @PostConstruct
-    public PermissionsSystemType setup() {
+    public void setup() {
         // Force-unhook from current hooked permissions systems
         unhook();
 
@@ -177,7 +175,7 @@ public class PermissionsManager implements PermissionsService {
                 ConsoleLogger.info("Hooked into " + type.getName() + "!");
 
                 // Return the used permissions system type
-                return type;
+                return;
 
             } catch (Exception ex) {
                 // An error occurred, show a warning message
@@ -187,7 +185,6 @@ public class PermissionsManager implements PermissionsService {
 
         // No recognized permissions system found, show a message and return
         ConsoleLogger.info("No supported permissions system found! Permissions are disabled!");
-        return null;
     }
 
     /**

src/test/java/fr/xephi/authme/initialization/AuthMeServiceInitializerTest.java

@@ -166,7 +166,7 @@ public class AuthMeServiceInitializerTest {
         PostConstructTestClass testClass = initializer.get(PostConstructTestClass.class);
 
         // then
-        assertThat(testClass.werePostConstructsCalled(), equalTo(true));
+        assertThat(testClass.wasPostConstructCalled(), equalTo(true));
         assertThat(testClass.getBetaManager(), not(nullValue()));
     }
 
@@ -188,6 +188,18 @@ public class AuthMeServiceInitializerTest {
         initializer.get(InvalidPostConstruct.ThrowsException.class);
     }
 
+    @Test(expected = RuntimeException.class)
+    public void shouldThrowForMultiplePostConstructMethods() {
+        // given / when / then
+        initializer.get(InvalidPostConstruct.MultiplePostConstructs.class);
+    }
+
+    @Test(expected = RuntimeException.class)
+    public void shouldThrowForPostConstructNotReturningVoid() {
+        // given / when / then
+        initializer.get(InvalidPostConstruct.NotVoidReturnType.class);
+    }
+
     @Test(expected = RuntimeException.class)
     public void shouldThrowForAbstractNonRegisteredDependency() {
         // given / when / then

src/test/java/fr/xephi/authme/initialization/samples/InvalidPostConstruct.java

@@ -9,10 +9,8 @@ import javax.inject.Inject;
 public abstract class InvalidPostConstruct {
 
     public static final class WithParams {
-        @SuppressWarnings("unused")
         @Inject
         private AlphaService alphaService;
-        @SuppressWarnings("unused")
         @Inject
         private ProvidedClass providedClass;
 
@@ -41,4 +39,28 @@ public abstract class InvalidPostConstruct {
             throw new IllegalStateException("Exception in post construct");
         }
     }
+
+    public static final class NotVoidReturnType {
+        @Inject
+        private ProvidedClass providedClass;
+
+        @PostConstruct
+        public int returnsInt() {
+            return 42;
+        }
+    }
+
+    public static final class MultiplePostConstructs {
+        @Inject
+        private ProvidedClass providedClass;
+
+        @PostConstruct
+        public void postConstruct1() {
+            // --
+        }
+        @PostConstruct
+        public void postConstruct2() {
+            // --
+        }
+    }
 }

src/test/java/fr/xephi/authme/initialization/samples/PostConstructTestClass.java

@@ -17,22 +17,15 @@ public class PostConstructTestClass implements SettingsDependent {
     @Inject
     private BetaManager betaManager;
     private boolean wasPostConstructCalled = false;
-    private boolean wasSecondPostConstructCalled = false;
     private boolean wasReloaded = false;
 
     @PostConstruct
-    protected void setFieldToTrue() {
+    public void postConstructMethod() {
         wasPostConstructCalled = true;
     }
 
-    @PostConstruct
-    public int otherPostConstructMethod() {
-        wasSecondPostConstructCalled = true;
-        return 42;
-    }
-
-    public boolean werePostConstructsCalled() {
-        return wasPostConstructCalled && wasSecondPostConstructCalled;
+    public boolean wasPostConstructCalled() {
+        return wasPostConstructCalled;
     }
 
     public BetaManager getBetaManager() {