revise current API

jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/RabitTracker.java

@@ -129,6 +129,7 @@ public class RabitTracker {
   public boolean start() {
     if (startTrackerProcess()) {
       logger.debug("Tracker started, with env=" + envs.toString());
+      System.out.println("Tracker started, with env=" + envs.toString());
       // also start a tracker logger
       Thread logger_thread = new Thread(new TrackerProcessLogger());
       logger_thread.setDaemon(true);

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/Booster.scala

@@ -119,7 +119,7 @@ class Booster private[xgboost4j](booster: JBooster) extends Serializable {
    */
   @throws(classOf[XGBoostError])
   def predict(data: DMatrix, outPutMargin: Boolean = false, treeLimit: Int = 0)
-    : Array[Array[Float]] = {
+      : Array[Array[Float]] = {
     booster.predict(data.jDMatrix, outPutMargin, treeLimit)
   }
 
@@ -178,6 +178,10 @@ class Booster private[xgboost4j](booster: JBooster) extends Serializable {
     booster.getFeatureScore(featureMap).asScala
   }
 
+  def toByteArray: Array[Byte] = {
+    booster.toByteArray
+  }
+
   /**
     *  Dispose the booster when it is no longer needed
     */