Add more javadoc

24 files changed, 386 insertions, 52 deletions

diff --git a/pf4j/src/main/java/org/pf4j/Extension.java b/pf4j/src/main/java/org/pf4j/Extension.java
index 57a435c..fec9ea3 100644
--- a/pf4j/src/main/java/org/pf4j/Extension.java
+++ b/pf4j/src/main/java/org/pf4j/Extension.java
@@ -15,15 +15,19 @@
  */
 package org.pf4j;
 
-import static java.lang.annotation.ElementType.TYPE;
-import static java.lang.annotation.RetentionPolicy.RUNTIME;
-
 import java.lang.annotation.Documented;
 import java.lang.annotation.Inherited;
 import java.lang.annotation.Retention;
 import java.lang.annotation.Target;
 
+import static java.lang.annotation.ElementType.TYPE;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
 /**
+ * An extension is a class that extends an extension point.
+ * Use this annotation to mark a class as an extension.
+ * The extension class must implement the extension point interface or extend the extension point abstract class.
+ *
  * @author Decebal Suiu
  */
 @Retention(RUNTIME)
@@ -32,6 +36,12 @@ import java.lang.annotation.Target;
 @Documented
 public @interface Extension {
 
+    /**
+     * The order of the extension.
+     * The ordinal is used to sort the extensions.
+     *
+     * @return the order of the extension
+     */
     int ordinal() default 0;
 
     /**
diff --git a/pf4j/src/main/java/org/pf4j/ExtensionDescriptor.java b/pf4j/src/main/java/org/pf4j/ExtensionDescriptor.java
index 0c27bf4..0c5c943 100644
--- a/pf4j/src/main/java/org/pf4j/ExtensionDescriptor.java
+++ b/pf4j/src/main/java/org/pf4j/ExtensionDescriptor.java
@@ -16,6 +16,10 @@
 package org.pf4j;
 
 /**
+ * Describes an extension.
+ * The extension is described by the class and the ordinal (the order of the extension).
+
+ *
  * @author Decebal Suiu
  */
 public class ExtensionDescriptor {
diff --git a/pf4j/src/main/java/org/pf4j/ExtensionFactory.java b/pf4j/src/main/java/org/pf4j/ExtensionFactory.java
index 176c6ff..62e8804 100644
--- a/pf4j/src/main/java/org/pf4j/ExtensionFactory.java
+++ b/pf4j/src/main/java/org/pf4j/ExtensionFactory.java
@@ -20,6 +20,12 @@ package org.pf4j;
  */
 public interface ExtensionFactory {
 
+    /**
+     * Creates an extension instance.
+     *
+     * @param extensionClass the extension class
+     * @return the extension instance
+     */
     <T> T create(Class<T> extensionClass);
 
 }
diff --git a/pf4j/src/main/java/org/pf4j/ExtensionFinder.java b/pf4j/src/main/java/org/pf4j/ExtensionFinder.java
index c905364..7eff7ea 100644
--- a/pf4j/src/main/java/org/pf4j/ExtensionFinder.java
+++ b/pf4j/src/main/java/org/pf4j/ExtensionFinder.java
@@ -19,6 +19,8 @@ import java.util.List;
 import java.util.Set;
 
 /**
+ * Provides the functionality for finding extensions.
+ *
  * @author Decebal Suiu
  */
 public interface ExtensionFinder {
@@ -34,7 +36,7 @@ public interface ExtensionFinder {
     <T> List<ExtensionWrapper<T>> find(Class<T> type, String pluginId);
 
     /**
-     * Retrieves a list with all extensions found for a plugin
+     * Retrieves a list with all extensions found for a plugin.
      */
     List<ExtensionWrapper> find(String pluginId);
 
diff --git a/pf4j/src/main/java/org/pf4j/ExtensionPoint.java b/pf4j/src/main/java/org/pf4j/ExtensionPoint.java
index 271047c..260289b 100644
--- a/pf4j/src/main/java/org/pf4j/ExtensionPoint.java
+++ b/pf4j/src/main/java/org/pf4j/ExtensionPoint.java
@@ -17,6 +17,10 @@ package org.pf4j;
 
 /**
  * An extension point is a formal declaration in a plugin (or in application API) where customization is allowed.
+ * It's a place where custom code can be "plugged in".
+ * <p>
+ * An extension point is defined by an interface or an abstract class.
+ * The extension point is used by the application to discover and use the custom implementations.
  *
  * @author Decebal Suiu
  */
diff --git a/pf4j/src/main/java/org/pf4j/ExtensionWrapper.java b/pf4j/src/main/java/org/pf4j/ExtensionWrapper.java
index 3899d5f..0f58e1a 100644
--- a/pf4j/src/main/java/org/pf4j/ExtensionWrapper.java
+++ b/pf4j/src/main/java/org/pf4j/ExtensionWrapper.java
@@ -17,6 +17,7 @@ package org.pf4j;
 
 /**
  * A wrapper over extension instance.
+ * It contains the extension descriptor and the extension instance.
  *
  * @author Decebal Suiu
  */
diff --git a/pf4j/src/main/java/org/pf4j/Plugin.java b/pf4j/src/main/java/org/pf4j/Plugin.java
index aaee012..2f806eb 100644
--- a/pf4j/src/main/java/org/pf4j/Plugin.java
+++ b/pf4j/src/main/java/org/pf4j/Plugin.java
@@ -21,6 +21,9 @@ import org.slf4j.LoggerFactory;
 /**
  * This class will be extended by all plugins and
  * serve as the common class between a plugin and the application.
+ * <p>
+ * Create (it's optional) a Plugin class if you are interested in plugin's lifecycle events (start, stop, ...)
+ * or you want to pass some context to the plugin.
  *
  * @author Decebal Suiu
  */
diff --git a/pf4j/src/main/java/org/pf4j/PluginAlreadyLoadedException.java b/pf4j/src/main/java/org/pf4j/PluginAlreadyLoadedException.java
index 411756c..9cc6f2a 100644
--- a/pf4j/src/main/java/org/pf4j/PluginAlreadyLoadedException.java
+++ b/pf4j/src/main/java/org/pf4j/PluginAlreadyLoadedException.java
@@ -18,6 +18,8 @@ package org.pf4j;
 import java.nio.file.Path;
 
 /**
+ * Thrown when a plugin is already loaded.
+ *
  * @author Decebal Suiu
  */
 public class PluginAlreadyLoadedException extends PluginRuntimeException {
@@ -26,7 +28,7 @@ public class PluginAlreadyLoadedException extends PluginRuntimeException {
     private final Path pluginPath;
 
     public PluginAlreadyLoadedException(String pluginId, Path pluginPath) {
-        super("Plugin '{}' already loaded with id '{}'", pluginPath, pluginId);
+        super("Plugin '{}' already loaded with id '{}'", pluginPath, pluginId);
 
         this.pluginId = pluginId;
         this.pluginPath = pluginPath;
diff --git a/pf4j/src/main/java/org/pf4j/PluginClassLoader.java b/pf4j/src/main/java/org/pf4j/PluginClassLoader.java
index d22dd55..8d0f2e6 100644
--- a/pf4j/src/main/java/org/pf4j/PluginClassLoader.java
+++ b/pf4j/src/main/java/org/pf4j/PluginClassLoader.java
@@ -30,8 +30,10 @@ import java.util.List;
 import java.util.Objects;
 
 /**
- * One instance of this class should be created by plugin manager for every available plug-in.
- * By default, this class loader is a Parent Last ClassLoader - it loads the classes from the plugin's jars
+ * One instance of this class should be created for every available plug-in.
+ * It's responsible for loading classes and resources from the plug-in.
+ * <p>
+ * By default, this {@link ClassLoader} is a Parent Last ClassLoader - it loads the classes from the plugin's jars
  * before delegating to the parent class loader.
  * Use {@link #classLoadingStrategy} to change the loading strategy.
  *
@@ -44,18 +46,20 @@ public class PluginClassLoader extends URLClassLoader {
     private static final String JAVA_PACKAGE_PREFIX = "java.";
     private static final String PLUGIN_PACKAGE_PREFIX = "org.pf4j.";
 
-    private PluginManager pluginManager;
-    private PluginDescriptor pluginDescriptor;
-    private ClassLoadingStrategy classLoadingStrategy;
+    private final PluginManager pluginManager;
+    private final PluginDescriptor pluginDescriptor;
+    private final ClassLoadingStrategy classLoadingStrategy;
 
     public PluginClassLoader(PluginManager pluginManager, PluginDescriptor pluginDescriptor, ClassLoader parent) {
         this(pluginManager, pluginDescriptor, parent, ClassLoadingStrategy.PDA);
     }
 
     /**
+     * Creates a new {@link PluginClassLoader} for the given plugin using parent first strategy.
+     *
      * @deprecated Replaced by {@link #PluginClassLoader(PluginManager, PluginDescriptor, ClassLoader, ClassLoadingStrategy)}.
      * If {@code parentFirst} is {@code true}, indicates that the parent {@link ClassLoader} should be consulted
-     * before trying to load the a class through this loader.
+     * before trying to load a class through this loader.
      */
     @Deprecated
     public PluginClassLoader(PluginManager pluginManager, PluginDescriptor pluginDescriptor, ClassLoader parent, boolean parentFirst) {
@@ -63,7 +67,12 @@ public class PluginClassLoader extends URLClassLoader {
     }
 
     /**
-     * classloading according to {@code classLoadingStrategy}
+     * Creates a new {@link PluginClassLoader} for the given plugin using the specified class loading strategy.
+     *
+     * @param pluginManager the plugin manager
+     * @param pluginDescriptor the plugin descriptor
+     * @param parent the parent class loader
+     * @param classLoadingStrategy the strategy to use for loading classes and resources
      */
     public PluginClassLoader(PluginManager pluginManager, PluginDescriptor pluginDescriptor, ClassLoader parent, ClassLoadingStrategy classLoadingStrategy) {
         super(new URL[0], parent);
@@ -73,12 +82,22 @@ public class PluginClassLoader extends URLClassLoader {
         this.classLoadingStrategy = classLoadingStrategy;
     }
 
+    /**
+     * Adds the specified URL to the search path for classes and resources.
+     *
+     * @param url the URL to be added to the search path of URLs
+     */
     @Override
     public void addURL(URL url) {
         log.debug("Add '{}'", url);
         super.addURL(url);
     }
 
+    /**
+     * Adds the specified file to the search path for classes and resources.
+     *
+     * @param file the file to be added to the search path of URLs
+     */
     public void addFile(File file) {
         try {
             addURL(file.getCanonicalFile().toURI().toURL());
@@ -89,10 +108,15 @@ public class PluginClassLoader extends URLClassLoader {
     }
 
     /**
+     * Loads the class with the specified name.
+     * <p>
      * By default, it uses a child first delegation model rather than the standard parent first.
      * If the requested class cannot be found in this class loader, the parent class loader will be consulted
      * via the standard {@link ClassLoader#loadClass(String)} mechanism.
      * Use {@link #classLoadingStrategy} to change the loading strategy.
+     *
+     * @param className the name of the class
+     * @return the loaded class
      */
     @Override
     public Class<?> loadClass(String className) throws ClassNotFoundException {
@@ -146,7 +170,8 @@ public class PluginClassLoader extends URLClassLoader {
     }
 
     /**
-     * Load the named resource from this plugin.
+     * Loads the named resource from this plugin.
+     * <p>
      * By default, this implementation checks the plugin's classpath first then delegates to the parent.
      * Use {@link #classLoadingStrategy} to change the loading strategy.
      *
@@ -181,6 +206,7 @@ public class PluginClassLoader extends URLClassLoader {
         return null;
     }
 
+
     @Override
     public Enumeration<URL> getResources(String name) throws IOException {
         List<URL> resources = new ArrayList<>();
@@ -205,6 +231,12 @@ public class PluginClassLoader extends URLClassLoader {
         return Collections.enumeration(resources);
     }
 
+    /**
+     * Loads the class with the specified name from the dependencies of the plugin.
+     *
+     * @param className the name of the class
+     * @return the loaded class
+     */
     protected Class<?> loadClassFromDependencies(String className) {
         log.trace("Search in dependencies for class '{}'", className);
         List<PluginDependency> dependencies = pluginDescriptor.getDependencies();
@@ -226,6 +258,12 @@ public class PluginClassLoader extends URLClassLoader {
         return null;
     }
 
+    /**
+     * Finds the resource with the given name in the dependencies of the plugin.
+     *
+     * @param name the name of the resource
+     * @return the URL to the resource, {@code null} if the resource was not found
+     */
     protected URL findResourceFromDependencies(String name) {
         log.trace("Search in dependencies for resource '{}'", name);
         List<PluginDependency> dependencies = pluginDescriptor.getDependencies();
@@ -246,6 +284,13 @@ public class PluginClassLoader extends URLClassLoader {
         return null;
     }
 
+    /**
+     * Finds all resources with the given name in the dependencies of the plugin.
+     *
+     * @param name the name of the resource
+     * @return an enumeration of {@link URL} objects for the resource
+     * @throws IOException if I/O errors occur
+     */
     protected Collection<URL> findResourcesFromDependencies(String name) throws IOException {
         log.trace("Search in dependencies for resources '{}'", name);
         List<URL> results = new ArrayList<>();
diff --git a/pf4j/src/main/java/org/pf4j/PluginClasspath.java b/pf4j/src/main/java/org/pf4j/PluginClasspath.java
index 62a2f90..a649a92 100644
--- a/pf4j/src/main/java/org/pf4j/PluginClasspath.java
+++ b/pf4j/src/main/java/org/pf4j/PluginClasspath.java
@@ -23,38 +23,75 @@ import java.util.Set;
 
 /**
  * The classpath of the plugin.
+ * <p>
  * It contains {@code classes} directories (directories that contain classes files)
  * and {@code jars} directories (directories that contain jars files).
+ * <p>
+ * The classpath is used to create the {@link ClassLoader} for the plugin.
  *
  * @author Decebal Suiu
  */
 public class PluginClasspath {
 
-    private Set<String> classesDirectories = new HashSet<>();
-    private Set<String> jarsDirectories = new HashSet<>();
+    private final Set<String> classesDirectories = new HashSet<>();
+    private final Set<String> jarsDirectories = new HashSet<>();
 
+    /**
+     * Get the classes directories.
+     *
+     * @return a set of directories that contain classes files
+     */
     public Set<String> getClassesDirectories() {
         return classesDirectories;
     }
 
+    /**
+     * Add classes directories.
+     *
+     * @param classesDirectories a set of directories that contain classes files
+     * @return this object for chaining
+     */
     public PluginClasspath addClassesDirectories(String... classesDirectories) {
         return addClassesDirectories(Arrays.asList(classesDirectories));
     }
 
+    /**
+     * Add classes directories.
+     *
+     * @param classesDirectories a collection of directories that contain classes files
+     * @return this object for chaining
+     */
     public PluginClasspath addClassesDirectories(Collection<String> classesDirectories) {
         this.classesDirectories.addAll(classesDirectories);
 
         return this;
     }
 
+    /**
+     * Get the jars directories.
+     *
+     * @return a set of directories that contain jars files
+     */
     public Set<String> getJarsDirectories() {
         return jarsDirectories;
     }
 
+    /**
+     * Add jars directories.
+     *
+     * @param jarsDirectories a set of directories that contain jars files
+     * @return this object for chaining
+     */
     public PluginClasspath addJarsDirectories(String... jarsDirectories) {
         return addJarsDirectories(Arrays.asList(jarsDirectories));
     }
 
+    /**
+     * Add jars directories.
+     *
+     * @param jarsDirectories a collection of directories that contain jars files
+     * @return this object for chaining
+     */
     public PluginClasspath addJarsDirectories(Collection<String> jarsDirectories) {
         this.jarsDirectories.addAll(jarsDirectories);
 
@@ -74,4 +111,5 @@ public class PluginClasspath {
     public int hashCode() {
         return Objects.hash(classesDirectories, jarsDirectories);
     }
+
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginDependency.java b/pf4j/src/main/java/org/pf4j/PluginDependency.java
index fbb081d..339fbad 100644
--- a/pf4j/src/main/java/org/pf4j/PluginDependency.java
+++ b/pf4j/src/main/java/org/pf4j/PluginDependency.java
@@ -18,13 +18,26 @@ package org.pf4j;
 import java.util.Objects;
 
 /**
+ * A plugin dependency is a dependency that the plugin has on another plugin.
+ * <p>
+ * The dependency is defined by the plugin id and the version of the plugin that is required.
+ * <p>
+ * A dependency is considered as optional, if the plugin id ends with a question mark.
+ * For example, the plugin id "my-plugin?" is considered as optional.
+ * <p>
+ * The plugin id and the version are separated by the '@' character.
+ * For example, the dependency "my-plugin@1.0.0" means that the plugin "my-plugin" with version "1.0.0" is required.
+ * If the version is not specified, then the plugin is required with any version.
+ * For example, the dependency "my-plugin" means that the plugin "my-plugin" with any version is required.
+ *
+ * @see VersionManager
  * @author Decebal Suiu
  */
 public class PluginDependency {
 
     private String pluginId;
     private String pluginVersionSupport = "*";
-    private boolean optional;
+    private final boolean optional;
 
     public PluginDependency(String dependency) {
         int index = dependency.indexOf('@');
@@ -44,14 +57,29 @@ public class PluginDependency {
         }
     }
 
+    /**
+     * Returns the unique identifier of the plugin.
+     *
+     * @return the plugin id
+     */
     public String getPluginId() {
         return pluginId;
     }
 
+    /**
+     * Returns the version of the plugin that is required.
+     *
+     * @return the version of the plugin that is required
+     */
     public String getPluginVersionSupport() {
         return pluginVersionSupport;
     }
 
+    /**
+     * Returns {@code true} if the dependency is optional, {@code false} otherwise.
+     *
+     * @return {@code true} if the dependency is optional, {@code false} otherwise
+     */
     public boolean isOptional() {
         return optional;
     }
@@ -77,4 +105,5 @@ public class PluginDependency {
     public int hashCode() {
         return Objects.hash(pluginId, pluginVersionSupport, optional);
     }
+
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginDescriptor.java b/pf4j/src/main/java/org/pf4j/PluginDescriptor.java
index 3b92eff..f3c3ccb 100644
--- a/pf4j/src/main/java/org/pf4j/PluginDescriptor.java
+++ b/pf4j/src/main/java/org/pf4j/PluginDescriptor.java
@@ -24,20 +24,65 @@ import java.util.List;
  */
 public interface PluginDescriptor {
 
+    /**
+     * The unique identifier of the plugin.
+     *
+     * @return the plugin id
+     */
     String getPluginId();
 
+    /**
+     * Returns a description of the plugin.
+     *
+     * @return the plugin description
+     */
     String getPluginDescription();
 
+    /**
+     * Returns the fully qualified class name of the plugin class.
+     * The plugin class must implement the {@link Plugin} interface.
+     *
+     * @return the plugin class
+     */
     String getPluginClass();
 
+    /**
+     * Returns the plugin version.
+     * The version must be unique for each release of the plugin.
+     * The version is used to check if the plugin is compatible with the application.
+     *
+     * @see VersionManager
+     * @return the plugin version
+     */
     String getVersion();
 
+    /**
+     * Returns the required version of the application.
+     *
+     * @return the required version of the application
+     */
     String getRequires();
 
+    /**
+     * Returns the author of the plugin.
+     *
+     * @return the author of the plugin
+     */
     String getProvider();
 
+    /**
+     * Returns the license of the plugin.
+     *
+     * @return the license of the plugin
+     */
     String getLicense();
 
+    /**
+     * Returns the dependencies of the plugin.
+     * A dependency is represented by a {@link PluginDependency} object.
+     *
+     * @return the dependencies of the plugin
+     */
     List<PluginDependency> getDependencies();
 
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginDescriptorFinder.java b/pf4j/src/main/java/org/pf4j/PluginDescriptorFinder.java
index 35185af..bff386c 100644
--- a/pf4j/src/main/java/org/pf4j/PluginDescriptorFinder.java
+++ b/pf4j/src/main/java/org/pf4j/PluginDescriptorFinder.java
@@ -19,6 +19,7 @@ import java.nio.file.Path;
 
 /**
  * Find a plugin descriptor for a plugin path.
+ * <p>
  * You can find the plugin descriptor in manifest file {@link ManifestPluginDescriptorFinder},
  * properties file {@link PropertiesPluginDescriptorFinder}, xml file,
  * java services (with {@link java.util.ServiceLoader}), etc.
@@ -28,10 +29,19 @@ import java.nio.file.Path;
 public interface PluginDescriptorFinder {
 
     /**
-     * Returns true if this finder is applicable to the given {@link Path}.
+     * Returns {@code true} if this finder is applicable to the given {@code pluginPath}.
+     * This is used to select the appropriate finder for a given plugin path.
+     *
+     * @param pluginPath the plugin path
      */
     boolean isApplicable(Path pluginPath);
 
+    /**
+     * Find the plugin descriptor for the given {@code pluginPath}.
+     *
+     * @param pluginPath the plugin path
+     * @return the plugin descriptor or {@code null} if not found
+     */
     PluginDescriptor find(Path pluginPath);
 
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginFactory.java b/pf4j/src/main/java/org/pf4j/PluginFactory.java
index 2eeb6e7..8129e64 100644
--- a/pf4j/src/main/java/org/pf4j/PluginFactory.java
+++ b/pf4j/src/main/java/org/pf4j/PluginFactory.java
@@ -16,10 +16,16 @@
 package org.pf4j;
 
 /**
- * Creates a plugin instance.
+ * It's responsible for creating a plugin instance.
  */
 public interface PluginFactory {
 
+    /**
+     * Create a plugin instance.
+     *
+     * @param pluginWrapper the plugin wrapper
+     * @return a plugin instance
+     */
     Plugin create(PluginWrapper pluginWrapper);
 
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginLoader.java b/pf4j/src/main/java/org/pf4j/PluginLoader.java
index ee98109..c66349b 100644
--- a/pf4j/src/main/java/org/pf4j/PluginLoader.java
+++ b/pf4j/src/main/java/org/pf4j/PluginLoader.java
@@ -19,19 +19,30 @@ import java.nio.file.Path;
 
 /**
  * Load all information (classes) needed by a plugin.
+ * <p>
+ * The plugin loader is responsible for creating a class loader for a plugin
+ * and loading all classes/resources needed by the plugin.
  *
  * @author Decebal Suiu
  */
 public interface PluginLoader {
 
     /**
-     * Returns true if this loader is applicable to the given {@link Path}.
+     * Returns {@code true} if this loader is applicable to the given plugin path.
+     * This is used to select the appropriate loader for a given plugin path.
      *
-     * @param pluginPath
-     * @return
+     * @param pluginPath the plugin path
+     * @return true if this loader is applicable to the given {@link Path}
      */
     boolean isApplicable(Path pluginPath);
 
+    /**
+     * Load all information (classes) needed by a plugin.
+     *
+     * @param pluginPath the plugin path
+     * @param pluginDescriptor the plugin descriptor
+     * @return the class loader for the plugin
+     */
     ClassLoader loadPlugin(Path pluginPath, PluginDescriptor pluginDescriptor);
 
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginManager.java b/pf4j/src/main/java/org/pf4j/PluginManager.java
index b717656..5fd36cf 100644
--- a/pf4j/src/main/java/org/pf4j/PluginManager.java
+++ b/pf4j/src/main/java/org/pf4j/PluginManager.java
@@ -141,6 +141,12 @@ public interface PluginManager {
      */
     boolean deletePlugin(String pluginId);
 
+    /**
+     * Retrieves the class loader for the specified plugin.
+     *
+     * @param pluginId the unique plugin identifier, specified in its metadata
+     * @return the class loader for the plugin
+     */
     ClassLoader getPluginClassLoader(String pluginId);
 
     List<Class<?>> getExtensionClasses(String pluginId);
@@ -153,6 +159,12 @@ public interface PluginManager {
 
     <T> List<T> getExtensions(Class<T> type, String pluginId);
 
+    /**
+     * Retrieves the extensions for the specified plugin.
+     *
+     * @param pluginId the unique plugin identifier, specified in its metadata
+     * @return the extensions for the plugin
+     */
     List getExtensions(String pluginId);
 
     Set<String> getExtensionClassNames(String pluginId);
@@ -165,14 +177,14 @@ public interface PluginManager {
     RuntimeMode getRuntimeMode();
 
     /**
-     * Returns {@code true} if the runtime mode is {@code RuntimeMode.DEVELOPMENT}.
+     * Returns {@code true} if the runtime mode is {@link RuntimeMode#DEVELOPMENT}.
      */
     default boolean isDevelopment() {
         return RuntimeMode.DEVELOPMENT.equals(getRuntimeMode());
     }
 
     /**
-     * Returns {@code true} if the runtime mode is not {@code RuntimeMode.DEVELOPMENT}.
+     * Returns {@code true} if the runtime mode is not {@link RuntimeMode#DEVELOPMENT}.
      */
     default boolean isNotDevelopment() {
         return !isDevelopment();
@@ -193,7 +205,7 @@ public interface PluginManager {
      * disables all version checking.
      *
      * @default 0.0.0
-     * @param version
+     * @param version the system version
      */
     void setSystemVersion(String version);
 
@@ -215,7 +227,7 @@ public interface PluginManager {
     Path getPluginsRoot();
 
     /**
-     * Gets the a read-only list of all paths of the folders where plugins are installed.
+     * Gets a read-only list of all paths of the folders where plugins are installed.
      *
      * @return Paths of plugins roots
      */
diff --git a/pf4j/src/main/java/org/pf4j/PluginRepository.java b/pf4j/src/main/java/org/pf4j/PluginRepository.java
index 6bc2356..c5f2569 100644
--- a/pf4j/src/main/java/org/pf4j/PluginRepository.java
+++ b/pf4j/src/main/java/org/pf4j/PluginRepository.java
@@ -19,7 +19,7 @@ import java.nio.file.Path;
 import java.util.List;
 
 /**
- * Directory that contains plugins. A plugin could be a {@code directory}, @code zip} or {@code jar} file.
+ * Directory that contains plugins. A plugin could be a {@code directory}, {@code zip} or {@code jar} file.
  *
  * @author Decebal Suiu
  * @author Mário Franco
diff --git a/pf4j/src/main/java/org/pf4j/PluginState.java b/pf4j/src/main/java/org/pf4j/PluginState.java
index e28fc30..989157f 100644
--- a/pf4j/src/main/java/org/pf4j/PluginState.java
+++ b/pf4j/src/main/java/org/pf4j/PluginState.java
@@ -16,6 +16,14 @@
 package org.pf4j;
 
 /**
+ * The state of a plugin.
+ * <p>
+ * Lifecycle of a plugin:
+ * <pre>
+ * CREATED -> RESOLVED -> STARTED -> STOPPED
+ * CREATED -> DISABLED
+ * CREATED -> FAILED
+ *
  * @author Decebal Suiu
  */
 public enum PluginState {
@@ -51,14 +59,14 @@ public enum PluginState {
      */
     FAILED("FAILED");
 
-    private String status;
+    private final String status;
 
-    private PluginState(String status) {
+    PluginState(String status) {
         this.status = status;
     }
 
     public boolean equals(String status) {
-    	return (status == null ? false : this.status.equalsIgnoreCase(status));
+        return (this.status.equalsIgnoreCase(status));
     }
 
     @Override
@@ -66,13 +74,20 @@ public enum PluginState {
         return status;
     }
 
+    /**
+     * Parse a string to a {@link PluginState}.
+     *
+     * @param string the string to parse
+     * @return the {@link PluginState} or null if the string is not a valid state
+     */
     public static PluginState parse(String string) {
-		for (PluginState status : values()) {
-			if (status.equals(string)) {
-				return status;
-			}
-		}
+        for (PluginState status : values()) {
+            if (status.equals(string)) {
+                return status;
+            }
+        }
+
+        return null;
+    }
 
-		return null;
-	}
 }
diff --git a/pf4j/src/main/java/org/pf4j/PluginStateEvent.java b/pf4j/src/main/java/org/pf4j/PluginStateEvent.java
index 8a3b9af..99ef91c 100644
--- a/pf4j/src/main/java/org/pf4j/PluginStateEvent.java
+++ b/pf4j/src/main/java/org/pf4j/PluginStateEvent.java
@@ -18,6 +18,12 @@ package org.pf4j;
 import java.util.EventObject;
 
 /**
+ * Event object that indicates a change in the state of a plugin.
+ * The event is propagated to all registered listeners.
+ * The event source is the {@link PluginManager} that changed the state of the plugin.
+ * The event object contains the plugin that changed its state and the old state.
+ *
+ * @see PluginStateListener
  * @author Decebal Suiu
  */
 public class PluginStateEvent extends EventObject {
@@ -32,19 +38,39 @@ public class PluginStateEvent extends EventObject {
         this.oldState = oldState;
     }
 
+    /**
+     * The object on which the Event initially occurred.
+     *
+     * @return the PluginManager that changed the state of the plugin
+     */
     @Override
     public PluginManager getSource() {
         return (PluginManager) super.getSource();
     }
 
+    /**
+     * The plugin that changed its state.
+     *
+     * @return the plugin that changed its state
+     */
     public PluginWrapper getPlugin() {
         return plugin;
     }
 
+    /**
+     * The new state of the plugin.
+     *
+     * @return the new state of the plugin
+     */
     public PluginState getPluginState() {
         return plugin.getPluginState();
     }
 
+    /**
+     * The old state of the plugin.
+     *
+     * @return the old state of the plugin
+     */
     public PluginState getOldState() {
         return oldState;
     }
diff --git a/pf4j/src/main/java/org/pf4j/PluginStateListener.java b/pf4j/src/main/java/org/pf4j/PluginStateListener.java
index ba65860..7b9a412 100644
--- a/pf4j/src/main/java/org/pf4j/PluginStateListener.java
+++ b/pf4j/src/main/java/org/pf4j/PluginStateListener.java
@@ -18,14 +18,19 @@ package org.pf4j;
 import java.util.EventListener;
 
 /**
- * PluginStateListener defines the interface for an object that listens to plugin state changes.
+ * Defines the interface for an object that listens to plugin state changes.
+ * <p>
+ * The class that is interested in processing a plugin state event implements this interface.
  *
+ * @see PluginStateEvent
  * @author Decebal Suiu
  */
 public interface PluginStateListener extends EventListener {
 
     /**
-     * Invoked when a plugin's state (for example DISABLED, STARTED) is changed.
+     * Invoked when a plugin's state (for example {@link PluginState#DISABLED}, {@link PluginState#STARTED}) is changed.
+     *
+     * @param event the plugin state event
      */
     void pluginStateChanged(PluginStateEvent event);
 
diff --git a/pf4j/src/main/java/org/pf4j/PluginStatusProvider.java b/pf4j/src/main/java/org/pf4j/PluginStatusProvider.java
index 46967b2..4fa9113 100644
--- a/pf4j/src/main/java/org/pf4j/PluginStatusProvider.java
+++ b/pf4j/src/main/java/org/pf4j/PluginStatusProvider.java
@@ -16,6 +16,12 @@
 package org.pf4j;
 
 /**
+ * Provides a way to check if a plugin is disabled and to disable/enable a plugin.
+ * <p>
+ * This is useful when you want to store the plugin status in a database or in a file.
+ *
+ * @see PluginState#DISABLED
+ *
  * @author Decebal Suiu
  * @author Mário Franco
  */
diff --git a/pf4j/src/main/java/org/pf4j/PluginWrapper.java b/pf4j/src/main/java/org/pf4j/PluginWrapper.java
index 64a14d2..21f7b25 100644
--- a/pf4j/src/main/java/org/pf4j/PluginWrapper.java
+++ b/pf4j/src/main/java/org/pf4j/PluginWrapper.java
@@ -24,13 +24,13 @@ import java.nio.file.Path;
  */
 public class PluginWrapper {
 
-    private PluginManager pluginManager;
-    private PluginDescriptor descriptor;
-    private Path pluginPath;
-    private ClassLoader pluginClassLoader;
+    private final PluginManager pluginManager;
+    private final PluginDescriptor descriptor;
+    private final Path pluginPath;
+    private final ClassLoader pluginClassLoader;
     private PluginFactory pluginFactory;
     private PluginState pluginState;
-    private RuntimeMode runtimeMode;
+    private final RuntimeMode runtimeMode;
 
     private Throwable failedException;
 
@@ -48,6 +48,8 @@ public class PluginWrapper {
 
     /**
      * Returns the plugin manager.
+     *
+     * @return the plugin manager
      */
     public PluginManager getPluginManager() {
         return pluginManager;
@@ -55,6 +57,8 @@ public class PluginWrapper {
 
     /**
      * Returns the plugin descriptor.
+     *
+     * @return the plugin descriptor
      */
     public PluginDescriptor getDescriptor() {
         return descriptor;
@@ -62,20 +66,29 @@ public class PluginWrapper {
 
     /**
      * Returns the path of this plugin.
+     *
+     * @return the path of this plugin
      */
     public Path getPluginPath() {
         return pluginPath;
     }
 
     /**
-     * Returns the plugin class loader used to load classes and resources
-     * for this plug-in. The class loader can be used to directly access
-     * plug-in resources and classes.
+     * Returns the plugin {@link ClassLoader} used to load classes and resources for this plug-in.
+     * <p>
+     * The class loader can be used to directly access plug-in resources and classes.
+     *
+     * @return the plugin class loader
      */
     public ClassLoader getPluginClassLoader() {
         return pluginClassLoader;
     }
 
+    /**
+     * Returns the plugin instance.
+     *
+     * @return the plugin instance
+     */
     public Plugin getPlugin() {
         if (plugin == null) {
             plugin = pluginFactory.create(this);
@@ -84,16 +97,26 @@ public class PluginWrapper {
         return plugin;
     }
 
+    /**
+     * Returns the plugin factory.
+     *
+     * @return the plugin factory
+     */
     public PluginState getPluginState() {
         return pluginState;
     }
 
+    /**
+     * Returns the runtime mode.
+     *
+     * @return the runtime mode
+     */
     public RuntimeMode getRuntimeMode() {
         return runtimeMode;
     }
 
     /**
-     * Shortcut
+     * Shortcut for {@code getDescriptor().getPluginId()}.
      */
     public String getPluginId() {
         return getDescriptor().getPluginId();
@@ -133,10 +156,20 @@ public class PluginWrapper {
         return "PluginWrapper [descriptor=" + descriptor + ", pluginPath=" + pluginPath + "]";
     }
 
+    /**
+     * Used internally by the framework to set the plugin factory.
+     *
+     * @param pluginState the plugin state
+     */
     public void setPluginState(PluginState pluginState) {
         this.pluginState = pluginState;
     }
 
+    /**
+     * Used internally by the framework to set the plugin factory.
+     *
+     * @param pluginFactory the plugin factory
+     */
     public void setPluginFactory(PluginFactory pluginFactory) {
         this.pluginFactory = pluginFactory;
     }
@@ -144,11 +177,18 @@ public class PluginWrapper {
     /**
      * Returns the exception with which the plugin fails to start.
      * See @{link PluginStatus#FAILED}.
+     *
+     * @return the exception with which the plugin fails to start
      */
     public Throwable getFailedException() {
         return failedException;
     }
 
+    /**
+     * Used internally by the framework to set the exception with which the plugin fails to start.
+     *
+     * @param failedException the exception with which the plugin fails to start
+     */
     public void setFailedException(Throwable failedException) {
         this.failedException = failedException;
     }
diff --git a/pf4j/src/main/java/org/pf4j/RuntimeMode.java b/pf4j/src/main/java/org/pf4j/RuntimeMode.java
index 909cc38..d3c00bd 100644
--- a/pf4j/src/main/java/org/pf4j/RuntimeMode.java
+++ b/pf4j/src/main/java/org/pf4j/RuntimeMode.java
@@ -20,6 +20,12 @@ import java.util.Map;
 import java.util.NoSuchElementException;
 
 /**
+ * The runtime mode of the PF4J application.
+ * <p>
+ * The runtime mode is used to determine the behavior of the application.
+ * For example, in development mode, the application may display detailed error messages,
+ * while in deployment mode, the application may display a generic error message.
+ *
  * @author Decebal Suiu
  */
 public enum RuntimeMode {
@@ -51,6 +57,13 @@ public enum RuntimeMode {
         return name;
     }
 
+    /**
+     * Returns the runtime mode with the specified name.
+     *
+     * @param name the name of the runtime mode
+     * @return the runtime mode with the specified name
+     * @throws NoSuchElementException if the runtime mode with the specified name is not found
+     */
     public static RuntimeMode byName(String name) {
         if (map.containsKey(name)) {
             return map.get(name);
diff --git a/pf4j/src/main/java/org/pf4j/VersionManager.java b/pf4j/src/main/java/org/pf4j/VersionManager.java
index 5caf9e2..e3e2737 100644
--- a/pf4j/src/main/java/org/pf4j/VersionManager.java
+++ b/pf4j/src/main/java/org/pf4j/VersionManager.java
@@ -19,6 +19,7 @@ import java.util.Comparator;
 
 /**
  * Manager responsible for versions of plugins.
+ * It's used to check if a version matches a constraint and to compare two versions.
  *
  * @author Decebal Suiu
  */
@@ -28,17 +29,17 @@ public interface VersionManager {
      * Check if a {@code constraint} and a {@code version} match.
      * A possible constrain can be {@code >=1.0.0 & <2.0.0}.
      *
-     * @param version
-     * @param constraint
-     * @return
+     * @param version the version to check
+     * @param constraint the constraint to check
+     * @return {@code true} if the version matches the constraint, {@code false} otherwise
      */
     boolean checkVersionConstraint(String version, String constraint);
 
     /**
      * Compare two versions. It's similar with {@link Comparator#compare(Object, Object)}.
      *
-     * @param v1
-     * @param v2
+     * @param v1 the first version to compare
+     * @param v2 the second version to compare
      */
     int compareVersions(String v1, String v2);