Fix some warnings (javadoc, pom, sonar)

pf4j/src/main/java/org/pf4j/AbstractExtensionFinder.java

     protected volatile Map<String, ExtensionInfo> extensionInfos; // cache extension infos by class name
     protected Boolean checkForExtensionDependencies = null;
 
-    public AbstractExtensionFinder(PluginManager pluginManager) {
+    protected AbstractExtensionFinder(PluginManager pluginManager) {
         this.pluginManager = pluginManager;
     }
 
     public abstract Map<String, Set<String>> readClasspathStorages();
 
     @Override
-    @SuppressWarnings("unchecked")
     public <T> List<ExtensionWrapper<T>> find(Class<T> type) {
         log.debug("Finding extensions of extension point '{}'", type.getName());
         Map<String, Set<String>> entries = getEntries();

pf4j/src/main/java/org/pf4j/AbstractPluginManager.java

     /**
      * The plugins roots are supplied as comma-separated list by {@code System.getProperty("pf4j.pluginsDir", "plugins")}.
      */
-    public AbstractPluginManager() {
+    protected AbstractPluginManager() {
         initialize();
     }
 
      *
      * @param pluginsRoots the roots to search for plugins
      */
-    public AbstractPluginManager(Path... pluginsRoots) {
+    protected AbstractPluginManager(Path... pluginsRoots) {
         this(Arrays.asList(pluginsRoots));
     }
 
      *
      * @param pluginsRoots the roots to search for plugins
      */
-    public AbstractPluginManager(List<Path> pluginsRoots) {
+    protected AbstractPluginManager(List<Path> pluginsRoots) {
         this.pluginsRoots.addAll(pluginsRoots);
 
         initialize();
 
         return pluginWrapper;
     }
-    
+
     /**
      * creates the plugin wrapper. override this if you want to prevent plugins having full access to the plugin manager
-     * 
+     *
      * @return
      */
     protected PluginWrapper createPluginWrapper(PluginDescriptor pluginDescriptor, Path pluginPath, ClassLoader pluginClassLoader) {
     /**
      * Set to true to allow requires expression to be exactly x.y.z.
      * The default is false, meaning that using an exact version x.y.z will
-     * implicitly mean the same as >=x.y.z
+     * implicitly mean the same as &gt;=x.y.z
      *
      * @param exactVersionAllowed set to true or false
      */

pf4j/src/main/java/org/pf4j/ClassLoadingStrategy.java

 public class ClassLoadingStrategy {
 
     /**
-     * application(parent) -> plugin -> dependencies
+     * application(parent) -> plugin -> dependencies
      */
     public static final ClassLoadingStrategy APD = new ClassLoadingStrategy(Arrays.asList(Source.APPLICATION, Source.PLUGIN, Source.DEPENDENCIES));
 
     /**
-     * application(parent) -> dependencies -> plugin
+     * application(parent) -> dependencies -> plugin
      */
     public static final ClassLoadingStrategy ADP = new ClassLoadingStrategy(Arrays.asList(Source.APPLICATION, Source.DEPENDENCIES, Source.PLUGIN));
 
     /**
-     * plugin -> application(parent) -> dependencies
+     * plugin -> application(parent) -> dependencies
      */
     public static final ClassLoadingStrategy PAD = new ClassLoadingStrategy(Arrays.asList(Source.PLUGIN, Source.APPLICATION, Source.DEPENDENCIES));
 
     /**
-     * dependencies -> application(parent) -> plugin
+     * dependencies -> application(parent) -> plugin
      */
     public static final ClassLoadingStrategy DAP = new ClassLoadingStrategy(Arrays.asList(Source.DEPENDENCIES, Source.APPLICATION, Source.PLUGIN));
 
     /**
-     * dependencies -> plugin -> application(parent)
+     * dependencies -> plugin -> application(parent)
      */
     public static final ClassLoadingStrategy DPA = new ClassLoadingStrategy(Arrays.asList(Source.DEPENDENCIES, Source.PLUGIN, Source.APPLICATION));
 
     /**
-     * plugin -> dependencies -> application(parent)
+     * plugin -> dependencies -> application(parent)
      */
     public static final ClassLoadingStrategy PDA = new ClassLoadingStrategy(Arrays.asList(Source.PLUGIN, Source.DEPENDENCIES, Source.APPLICATION));
 

pf4j/src/main/java/org/pf4j/util/ClassUtils.java

  */
 public class ClassUtils {
 
+    private ClassUtils() {}
+
     public static List<String> getAllInterfacesNames(Class<?> aClass) {
         return toString(getAllInterfaces(aClass));
     }
         return list;
     }
 
-    /*
-    public static List<String> getAllAbstractClassesNames(Class<?> aClass) {
-        return toString(getAllInterfaces(aClass));
-    }
-
-    public static List getAllAbstractClasses(Class aClass) {
-        List<Class<?>> list = new ArrayList<>();
-
-        Class<?> superclass = aClass.getSuperclass();
-        while (superclass != null) {
-            if (Modifier.isAbstract(superclass.getModifiers())) {
-                list.add(superclass);
-            }
-            superclass = superclass.getSuperclass();
-        }
-
-        return list;
-    }
-    */
-
     /**
      * Get a certain annotation of a {@link TypeElement}.
      * See <a href="https://stackoverflow.com/a/10167558">stackoverflow.com</a> for more information.
         return null;
     }
 
-    /*
-    public static Element getAnnotationMirrorElement(TypeElement typeElement, Class<?> annotationClass) {
-        AnnotationMirror annotationMirror = getAnnotationMirror(typeElement, annotationClass);
-        return annotationMirror != null ? annotationMirror.getAnnotationType().asElement() : null;
-    }
-    */
-
     /**
      * Get a certain parameter of an {@link AnnotationMirror}.
      * See <a href="https://stackoverflow.com/a/10167558">stackoverflow.com</a> for more information.
 
     /**
      * Uses {@link Class#getSimpleName()} to convert from {@link Class} to {@link String}.
-     *
-     * @param classes
-     * @return
      */
     private static List<String> toString(List<Class<?>> classes) {
         List<String> list = new ArrayList<>();

pf4j/src/main/java/org/pf4j/util/StringUtils.java

  */
 public class StringUtils {
 
+    private StringUtils() {}
+
     public static boolean isNullOrEmpty(String str) {
         return (str == null) || str.isEmpty();
     }

pom.xml

     <licenses>
         <license>
             <name>The Apache Software License, Version 2.0</name>
-            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
+            <url>https://www.apache.org/licenses/LICENSE-2.0.txt</url>
             <distribution>repo</distribution>
         </license>
     </licenses>
         <pluginManagement>
             <plugins>
                 <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-compiler-plugin</artifactId>
                     <version>3.8.0</version>
                     <configuration>
                 </plugin>
 
                 <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-surefire-plugin</artifactId>
                     <version>2.22.1</version>
                 </plugin>
 
                 <plugin>
+                    <groupId>org.apache.maven.plugins</groupId>
                     <artifactId>maven-jar-plugin</artifactId>
                     <version>2.6</version>
                 </plugin>
             </plugin>
 
             <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-jar-plugin</artifactId>
                 <configuration>
                     <archive>