[jvm-packages] Add methods operating attributes of booster in jvm package, which follow API design in python package. (#4336)

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

@@ -116,6 +116,60 @@ public class Booster implements Serializable, KryoSerializable {
     }
   }
 
+  /**
+   * Get attributes stored in the Booster as a Map.
+   *
+   * @return A map contain attribute pairs.
+   * @throws XGBoostError native error
+   */
+  public final Map<String, String> getAttrs() throws XGBoostError {
+    String[][] attrNames = new String[1][];
+    XGBoostJNI.checkCall(XGBoostJNI.XGBoosterGetAttrNames(handle, attrNames));
+    Map<String, String> attrMap = new HashMap<>();
+    for (String name: attrNames[0]) {
+      attrMap.put(name, this.getAttr(name));
+    }
+    return attrMap;
+  }
+
+  /**
+   * Get attribute from the Booster.
+   *
+   * @param key   attribute key
+   * @return attribute value
+   * @throws XGBoostError native error
+   */
+  public final String getAttr(String key) throws XGBoostError {
+    String[] attrValue = new String[1];
+    XGBoostJNI.checkCall(XGBoostJNI.XGBoosterGetAttr(handle, key, attrValue));
+    return attrValue[0];
+  }
+
+  /**
+   * Set attribute to the Booster.
+   *
+   * @param key   attribute key
+   * @param value attribute value
+   * @throws XGBoostError native error
+   */
+  public final void setAttr(String key, String value) throws XGBoostError {
+    XGBoostJNI.checkCall(XGBoostJNI.XGBoosterSetAttr(handle, key, value));
+  }
+
+  /**
+   * Set attributes to the Booster.
+   *
+   * @param attrs attributes key-value map
+   * @throws XGBoostError native error
+   */
+  public void setAttrs(Map<String, String> attrs) throws XGBoostError {
+    if (attrs != null) {
+      for (Map.Entry<String, String> entry : attrs.entrySet()) {
+        setAttr(entry.getKey(), entry.getValue());
+      }
+    }
+  }
+
   /**
    * Update the booster for one iteration.
    *

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

@@ -114,6 +114,7 @@ class XGBoostJNI {
   public final static native int XGBoosterDumpModelExWithFeatures(
     long handle, String[] feature_names, int with_stats, String format, String[][] out_strings);
 
+  public final static native int XGBoosterGetAttrNames(long handle, String[][] out_strings);
   public final static native int XGBoosterGetAttr(long handle, String key, String[] out_string);
   public final static native int XGBoosterSetAttr(long handle, String key, String value);
   public final static native int XGBoosterLoadRabitCheckpoint(long handle, int[] out_version);

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

@@ -32,6 +32,48 @@ import scala.collection.mutable
 class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
   extends Serializable  with KryoSerializable {
 
+  /**
+   * Get attributes stored in the Booster as a Map.
+   *
+   * @return A map contain attribute pairs.
+   */
+  @throws(classOf[XGBoostError])
+  def getAttrs: Map[String, String] = {
+    booster.getAttrs.asScala.toMap
+  }
+
+  /**
+   * Get attribute from the Booster.
+   *
+   * @param key   attr name
+   * @return attr value
+   */
+  @throws(classOf[XGBoostError])
+  def getAttr(key: String): String = {
+    booster.getAttr(key)
+  }
+
+  /**
+   * Set attribute to the Booster.
+   *
+   * @param key   attr name
+   * @param value attr value
+   */
+  @throws(classOf[XGBoostError])
+  def setAttr(key: String, value: String): Unit = {
+    booster.setAttr(key, value)
+  }
+
+  /**
+   * set attributes
+   *
+   * @param params attributes key-value map
+   */
+  @throws(classOf[XGBoostError])
+  def setAttrs(params: Map[String, String]): Unit = {
+    booster.setAttrs(params.asJava)
+  }
+
   /**
     * Set parameter to the Booster.
     *