Kotlin plugin example added and README updated for Kotlin. (#146)

README.md

@@ -223,6 +223,13 @@ The `maven-compiler-plugin` can be configured to do this like so:
 </plugin>
 ```
 
+### Kotlin
+
+PF4J can be used in Kotlin project as well. One has to use the Kotlin annotation processing tool 
+[**kapt**](https://kotlinlang.org/docs/reference/kapt.html) for the plugin project written in Kotlin.
+The demo_gradle project contains one plugin project _plugin3_ written in Kotlin for demonstration.
+
+
 Plugin assembly
 ------------------------------
 After you developed a plugin the next step is to deploy it in your application. For this task, one option is to create a zip file with a structure described in section [How to use](https://github.com/decebals/pf4j/blob/master/README.md#how-to-use) from the beginning of the document.  

demo_gradle/plugins/plugin3/build.gradle

@@ -0,0 +1,49 @@
+buildscript {
+    ext.kotlin_version = '1.1.2-2'
+
+    repositories {
+        mavenCentral()
+    }
+    dependencies {
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+    }
+}
+
+jar {
+    baseName = 'KotlinPlugin'
+    version = '0.1.0'
+    manifest {
+        attributes 'Plugin-Class' : 'ro.fortsoft.pf4j.demo.kotlin.KotlinPlugin',
+                'Plugin-Id' : 'KotlinPlugin',
+                'Plugin-Version' : '1.0.0',
+                'Plugin-Provider' : 'Anindya Chatterjee'
+    }
+}
+
+task plugin(type: Jar) {
+    baseName = 'KotlinPlugin'
+    version = '0.1.0'
+    into('classes'){
+        with jar
+    }
+    into('lib'){
+        from configurations.compile
+    }
+    extension('zip')
+}
+assemble.dependsOn plugin
+
+apply plugin: 'kotlin'
+apply plugin: 'kotlin-kapt'
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    compileOnly project(':api')
+    kapt 'ro.fortsoft.pf4j:pf4j:1.+'
+    compile 'org.apache.commons:commons-lang3:3.0'
+    testCompile group: 'junit', name: 'junit', version: '4.+'
+    compile "org.jetbrains.kotlin:kotlin-stdlib-jre8:$kotlin_version"
+}

demo_gradle/plugins/plugin3/src/main/kotlin/ro/fortsoft/pf4j/demo/kotlin/KotlinPlugin.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright (C) 2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package ro.fortsoft.pf4j.demo.kotlin
+
+import org.apache.commons.lang3.StringUtils
+import ro.fortsoft.pf4j.Extension
+import ro.fortsoft.pf4j.Plugin
+import ro.fortsoft.pf4j.PluginWrapper
+import ro.fortsoft.pf4j.demo.api.Greeting
+
+/**
+ * A sample plugin written in Kotlin
+ *
+ * @author Anindya Chatterjee
+ */
+class KotlinPlugin(wrapper: PluginWrapper) : Plugin(wrapper) {
+
+    override fun start() {
+        println("KotlinPlugin.start()")
+        println(StringUtils.upperCase("KotlinPlugin"))
+    }
+
+    override fun stop() {
+        println("KotlinPlugin.stop()")
+    }
+}
+
+@Extension
+class KotlinGreeting : Greeting {
+    override fun getGreeting(): String {
+        return "KotlinGreetings"
+    }
+}

demo_gradle/settings.gradle

@@ -1,4 +1,6 @@
 include 'api'
 include 'app'
 include 'plugins/plugin1'
-include 'plugins/plugin2'
+include 'plugins/plugin2'
+include 'plugins/plugin3'
+