Added simple plugin support (#927)

Using org.clapper:classutil library and a very simple `Plugin` interface.

README.md

@@ -152,6 +152,17 @@ If you want to persist the data directory, you can make the volume to your host
 docker run -ti --rm -v "/path_on_host:/data" -e "JAVA_OPTS=-Declair.printToConsole" acinq/eclair
 ```
 
+## Plugins
+
+For advanced usage, Eclair supports plugins written in Scala, Java, or any JVM-compatible language.
+
+A valid plugin is a jar that contains an implementation of the [Plugin](eclair-node/src/main/scala/fr/acinq/eclair/Plugin.scala) interface.
+
+Here is how to run Eclair with plugins:
+```shell
+java -jar eclair-node-<version>-<commit_id>.jar <plugin1.jar> <plugin2.jar> <...>
+```
+
 ## Mainnet usage
 
 Following are the minimum configuration files you need to use for Bitcoin Core and Eclair.

eclair-node/pom.xml

@@ -74,6 +74,12 @@
             <artifactId>eclair-core_${scala.version.short}</artifactId>
             <version>${project.version}</version>
         </dependency>
+        <dependency>
+            <!-- for plugins -->
+            <groupId>org.clapper</groupId>
+            <artifactId>classutil_${scala.version.short}</artifactId>
+            <version>1.4.0</version>
+        </dependency>
         <dependency>
             <groupId>ch.qos.logback</groupId>
             <artifactId>logback-classic</artifactId>

eclair-node/src/main/scala/fr/acinq/eclair/Boot.scala

@@ -22,6 +22,7 @@ import akka.actor.ActorSystem
 import grizzled.slf4j.Logging
 
 import scala.concurrent.ExecutionContext
+import scala.util.{Failure, Success}
 
 /**
   * Created by PM on 25/01/2016.
@@ -31,10 +32,15 @@ object Boot extends App with Logging {
   val datadir = new File(System.getProperty("eclair.datadir", System.getProperty("user.home") + "/.eclair"))
 
   try {
+    val plugins = Plugin.loadPlugins(args.map(new File(_)))
+    plugins.foreach(plugin => logger.info(s"loaded plugin ${plugin.getClass.getSimpleName}"))
     implicit val system: ActorSystem = ActorSystem("eclair-node")
     implicit val ec: ExecutionContext = system.dispatcher
-    new Setup(datadir).bootstrap onFailure {
-      case t: Throwable => onError(t)
+    val setup = new Setup(datadir)
+    plugins.foreach(_.onSetup(setup))
+    setup.bootstrap onComplete {
+      case Success(kit) => plugins.foreach(_.onKit(kit))
+      case Failure(t) => onError(t)
     }
   } catch {
     case t: Throwable => onError(t)
@@ -47,4 +53,3 @@ object Boot extends App with Logging {
     System.exit(1)
   }
 }
-

eclair-node/src/main/scala/fr/acinq/eclair/Plugin.scala

@@ -0,0 +1,29 @@
+package fr.acinq.eclair
+
+import java.io.File
+import java.net.{URL, URLClassLoader}
+
+import org.clapper.classutil.ClassFinder
+
+trait Plugin {
+
+  def onSetup(setup: Setup): Unit
+
+  def onKit(kit: Kit): Unit
+
+}
+
+object Plugin {
+
+  def loadPlugins(jars: Seq[File]): Seq[Plugin] = {
+      val finder = ClassFinder(jars)
+      val classes = finder.getClasses
+      val urls = jars.map(f => new URL(s"file:${f.getCanonicalPath}"))
+      val loader = new URLClassLoader(urls.toArray, ClassLoader.getSystemClassLoader)
+      classes
+        .filter(_.isConcrete)
+        .filter(_.implements(classOf[Plugin].getName))
+        .map(c => Class.forName(c.name, true, loader).getDeclaredConstructor().newInstance().asInstanceOf[Plugin])
+        .toList
+    }
+}