[jvm-packages] add format option when saving a model (#7940)

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

@@ -34,6 +34,7 @@ import org.apache.commons.logging.LogFactory;
  * Booster for xgboost, this is a model API that support interactive build of a XGBoost Model
  */
 public class Booster implements Serializable, KryoSerializable {
+  public static final String DEFAULT_FORMAT = "deprecated";
   private static final Log logger = LogFactory.getLog(Booster.class);
   // handle to the booster.
   private long handle = 0;
@@ -391,7 +392,22 @@ public class Booster implements Serializable, KryoSerializable {
    * @param out The output stream
    */
   public void saveModel(OutputStream out) throws XGBoostError, IOException {
-    out.write(this.toByteArray());
+    saveModel(out, DEFAULT_FORMAT);
+  }
+
+  /**
+   * Save the model to file opened as output stream.
+   * The model format is compatible with other xgboost bindings.
+   * The output stream can only save one xgboost model.
+   * This function will close the OutputStream after the save.
+   *
+   * @param out The output stream
+   * @param format The model format (ubj, json, deprecated)
+   * @throws XGBoostError
+   * @throws IOException
+   */
+  public void saveModel(OutputStream out, String format) throws XGBoostError, IOException {
+    out.write(this.toByteArray(format));
     out.close();
   }
 
@@ -643,7 +659,7 @@ public class Booster implements Serializable, KryoSerializable {
    * @throws XGBoostError native error
    */
   public byte[] toByteArray() throws XGBoostError {
-    return this.toByteArray("deprecated");
+    return this.toByteArray(DEFAULT_FORMAT);
   }
 
   /**

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

@@ -207,6 +207,7 @@ class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
   def saveModel(modelPath: String): Unit = {
     booster.saveModel(modelPath)
   }
+
   /**
     * save model to Output stream
     *
@@ -216,6 +217,18 @@ class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
   def saveModel(out: java.io.OutputStream): Unit = {
     booster.saveModel(out)
   }
+
+  /**
+   * save model to Output stream
+   * @param out output stream
+   * @param format the supported model format, (json, ubj, deprecated)
+   * @throws ml.dmlc.xgboost4j.java.XGBoostError
+   */
+  @throws(classOf[XGBoostError])
+  def saveModel(out: java.io.OutputStream, format: String): Unit = {
+    booster.saveModel(out, format)
+  }
+
   /**
    * Dump model as Array of string
    *
@@ -315,7 +328,7 @@ class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
     */
   @throws(classOf[XGBoostError])
   def toByteArray: Array[Byte] = {
-    booster.toByteArray
+    booster.toByteArray()
   }
 
   /**