diff --git a/jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/CommunicatorRobustnessSuite.scala b/jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/CommunicatorRobustnessSuite.scala
new file mode 100644
index 000000000000..579e3dd37447
--- /dev/null
+++ b/jvm-packages/xgboost4j-spark/src/test/scala/ml/dmlc/xgboost4j/scala/spark/CommunicatorRobustnessSuite.scala
@@ -0,0 +1,276 @@
+/*
+ Copyright (c) 2014-2022 by Contributors
+
+ 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 ml.dmlc.xgboost4j.scala.spark
+
+import java.util.concurrent.LinkedBlockingDeque
+
+import scala.util.Random
+
+import ml.dmlc.xgboost4j.java.{Communicator, RabitTracker => PyRabitTracker}
+import ml.dmlc.xgboost4j.java.IRabitTracker.TrackerStatus
+import ml.dmlc.xgboost4j.scala.rabit.{RabitTracker => ScalaRabitTracker}
+import ml.dmlc.xgboost4j.scala.DMatrix
+import org.scalatest.FunSuite
+
+class CommunicatorRobustnessSuite extends FunSuite with PerTest {
+
+  private def getXGBoostExecutionParams(paramMap: Map[String, Any]): XGBoostExecutionParams = {
+    val classifier = new XGBoostClassifier(paramMap)
+    val xgbParamsFactory = new XGBoostExecutionParamsFactory(classifier.MLlib2XGBoostParams, sc)
+    xgbParamsFactory.buildXGBRuntimeParams
+  }
+
+  test("Customize host ip and python exec for Rabit tracker") {
+    val hostIp = "192.168.22.111"
+    val pythonExec = "/usr/bin/python3"
+
+    val paramMap = Map(
+      "num_workers" -> numWorkers,
+      "tracker_conf" -> TrackerConf(0L, "python", hostIp))
+    val xgbExecParams = getXGBoostExecutionParams(paramMap)
+    val tracker = XGBoost.getTracker(xgbExecParams.numWorkers, xgbExecParams.trackerConf)
+    tracker match {
+      case pyTracker: PyRabitTracker =>
+        val cmd = pyTracker.getRabitTrackerCommand
+        assert(cmd.contains(hostIp))
+        assert(cmd.startsWith("python"))
+      case _ => assert(false, "expected python tracker implementation")
+    }
+
+    val paramMap1 = Map(
+      "num_workers" -> numWorkers,
+      "tracker_conf" -> TrackerConf(0L, "python", "", pythonExec))
+    val xgbExecParams1 = getXGBoostExecutionParams(paramMap1)
+    val tracker1 = XGBoost.getTracker(xgbExecParams1.numWorkers, xgbExecParams1.trackerConf)
+    tracker1 match {
+      case pyTracker: PyRabitTracker =>
+        val cmd = pyTracker.getRabitTrackerCommand
+        assert(cmd.startsWith(pythonExec))
+        assert(!cmd.contains(hostIp))
+      case _ => assert(false, "expected python tracker implementation")
+    }
+
+    val paramMap2 = Map(
+      "num_workers" -> numWorkers,
+      "tracker_conf" -> TrackerConf(0L, "python", hostIp, pythonExec))
+    val xgbExecParams2 = getXGBoostExecutionParams(paramMap2)
+    val tracker2 = XGBoost.getTracker(xgbExecParams2.numWorkers, xgbExecParams2.trackerConf)
+    tracker2 match {
+      case pyTracker: PyRabitTracker =>
+        val cmd = pyTracker.getRabitTrackerCommand
+        assert(cmd.startsWith(pythonExec))
+        assert(cmd.contains(s" --host-ip=${hostIp}"))
+      case _ => assert(false, "expected python tracker implementation")
+    }
+  }
+
+  test("training with Scala-implemented Rabit tracker") {
+    val eval = new EvalError()
+    val training = buildDataFrame(Classification.train)
+    val testDM = new DMatrix(Classification.test.iterator)
+    val paramMap = Map("eta" -> "1", "max_depth" -> "6",
+      "objective" -> "binary:logistic", "num_round" -> 5, "num_workers" -> numWorkers,
+      "tracker_conf" -> TrackerConf(60 * 60 * 1000, "scala"))
+    val model = new XGBoostClassifier(paramMap).fit(training)
+    assert(eval.eval(model._booster.predict(testDM, outPutMargin = true), testDM) < 0.1)
+  }
+
+  test("test Communicator allreduce to validate Scala-implemented Rabit tracker") {
+    val vectorLength = 100
+    val rdd = sc.parallelize(
+      (1 to numWorkers * vectorLength).toArray.map { _ => Random.nextFloat() }, numWorkers).cache()
+
+    val tracker = new ScalaRabitTracker(numWorkers)
+    tracker.start(0)
+    val trackerEnvs = tracker.getWorkerEnvs
+    val collectedAllReduceResults = new LinkedBlockingDeque[Array[Float]]()
+
+    val rawData = rdd.mapPartitions { iter =>
+      Iterator(iter.toArray)
+    }.collect()
+
+    val maxVec = (0 until vectorLength).toArray.map { j =>
+      (0 until numWorkers).toArray.map { i => rawData(i)(j) }.max
+    }
+
+    val allReduceResults = rdd.mapPartitions { iter =>
+      Communicator.init(trackerEnvs)
+      val arr = iter.toArray
+      val results = Communicator.allReduce(arr, Communicator.OpType.MAX)
+      Communicator.shutdown()
+      Iterator(results)
+    }.cache()
+
+    val sparkThread = new Thread() {
+      override def run(): Unit = {
+        allReduceResults.foreachPartition(() => _)
+        val byPartitionResults = allReduceResults.collect()
+        assert(byPartitionResults(0).length == vectorLength)
+        collectedAllReduceResults.put(byPartitionResults(0))
+      }
+    }
+    sparkThread.start()
+    assert(tracker.waitFor(0L) == 0)
+    sparkThread.join()
+
+    assert(collectedAllReduceResults.poll().sameElements(maxVec))
+  }
+
+  test("test Java RabitTracker wrapper's exception handling: it should not hang forever.") {
+    /*
+      Deliberately create new instances of SparkContext in each unit test to avoid reusing the
+      same thread pool spawned by the local mode of Spark. As these tests simulate worker crashes
+      by throwing exceptions, the crashed worker thread never calls Rabit.shutdown, and therefore
+      corrupts the internal state of the native Rabit C++ code. Calling Rabit.init() in subsequent
+      tests on a reentrant thread will crash the entire Spark application, an undesired side-effect
+      that should be avoided.
+     */
+    val rdd = sc.parallelize(1 to numWorkers, numWorkers).cache()
+
+    val tracker = new PyRabitTracker(numWorkers)
+    tracker.start(0)
+    val trackerEnvs = tracker.getWorkerEnvs
+
+    val workerCount: Int = numWorkers
+    /*
+       Simulate worker crash events by creating dummy Rabit workers, and throw exceptions in the
+       last created worker. A cascading event chain will be triggered once the RuntimeException is
+       thrown: the thread running the dummy spark job (sparkThread) catches the exception and
+       delegates it to the UnCaughtExceptionHandler, which is the Rabit tracker itself.
+
+       The Java RabitTracker class reacts to exceptions by killing the spawned process running
+       the Python tracker. If at least one Rabit worker has yet connected to the tracker before
+       it is killed, the resulted connection failure will trigger the Rabit worker to call
+       "exit(-1);" in the native C++ code, effectively ending the dummy Spark task.
+
+       In cluster (standalone or YARN) mode of Spark, tasks are run in containers and thus are
+       isolated from each other. That is, one task calling "exit(-1);" has no effect on other tasks
+       running in separate containers. However, as unit tests are run in Spark local mode, in which
+       tasks are executed by threads belonging to the same process, one thread calling "exit(-1);"
+       ultimately kills the entire process, which also happens to host the Spark driver, causing
+       the entire Spark application to crash.
+
+       To prevent unit tests from crashing, deterministic delays were introduced to make sure that
+       the exception is thrown at last, ideally after all worker connections have been established.
+       For the same reason, the Java RabitTracker class delays the killing of the Python tracker
+       process to ensure that pending worker connections are handled.
+     */
+    val dummyTasks = rdd.mapPartitions { iter =>
+      Communicator.init(trackerEnvs)
+      val index = iter.next()
+      Thread.sleep(100 + index * 10)
+      if (index == workerCount) {
+        // kill the worker by throwing an exception
+        throw new RuntimeException("Worker exception.")
+      }
+      Communicator.shutdown()
+      Iterator(index)
+    }.cache()
+
+    val sparkThread = new Thread() {
+      override def run(): Unit = {
+        // forces a Spark job.
+        dummyTasks.foreachPartition(() => _)
+      }
+    }
+
+    sparkThread.setUncaughtExceptionHandler(tracker)
+    sparkThread.start()
+    assert(tracker.waitFor(0) != 0)
+  }
+
+  test("test Scala RabitTracker's exception handling: it should not hang forever.") {
+    val rdd = sc.parallelize(1 to numWorkers, numWorkers).cache()
+
+    val tracker = new ScalaRabitTracker(numWorkers)
+    tracker.start(0)
+    val trackerEnvs = tracker.getWorkerEnvs
+
+    val workerCount: Int = numWorkers
+    val dummyTasks = rdd.mapPartitions { iter =>
+      Communicator.init(trackerEnvs)
+      val index = iter.next()
+      Thread.sleep(100 + index * 10)
+      if (index == workerCount) {
+        // kill the worker by throwing an exception
+        throw new RuntimeException("Worker exception.")
+      }
+      Communicator.shutdown()
+      Iterator(index)
+    }.cache()
+
+    val sparkThread = new Thread() {
+      override def run(): Unit = {
+        // forces a Spark job.
+        dummyTasks.foreachPartition(() => _)
+      }
+    }
+    sparkThread.setUncaughtExceptionHandler(tracker)
+    sparkThread.start()
+    assert(tracker.waitFor(0L) == TrackerStatus.FAILURE.getStatusCode)
+  }
+
+  test("test Scala RabitTracker's workerConnectionTimeout") {
+    val rdd = sc.parallelize(1 to numWorkers, numWorkers).cache()
+
+    val tracker = new ScalaRabitTracker(numWorkers)
+    tracker.start(500)
+    val trackerEnvs = tracker.getWorkerEnvs
+
+    val dummyTasks = rdd.mapPartitions { iter =>
+      val index = iter.next()
+      // simulate that the first worker cannot connect to tracker due to network issues.
+      if (index != 1) {
+        Communicator.init(trackerEnvs)
+        Thread.sleep(1000)
+        Communicator.shutdown()
+      }
+
+      Iterator(index)
+    }.cache()
+
+    val sparkThread = new Thread() {
+      override def run(): Unit = {
+        // forces a Spark job.
+        dummyTasks.foreachPartition(() => _)
+      }
+    }
+    sparkThread.setUncaughtExceptionHandler(tracker)
+    sparkThread.start()
+    // should fail due to connection timeout
+    assert(tracker.waitFor(0L) == TrackerStatus.FAILURE.getStatusCode)
+  }
+
+  test("should allow the dataframe containing communicator calls to be partially evaluated for" +
+    " multiple times (ISSUE-4406)") {
+    val paramMap = Map(
+      "eta" -> "1",
+      "max_depth" -> "6",
+      "silent" -> "1",
+      "objective" -> "binary:logistic")
+    val trainingDF = buildDataFrame(Classification.train)
+    val model = new XGBoostClassifier(paramMap ++ Array("num_round" -> 10,
+      "num_workers" -> numWorkers)).fit(trainingDF)
+    val prediction = model.transform(trainingDF)
+    // a partial evaluation of dataframe will cause rabit initialized but not shutdown in some
+    // threads
+    prediction.show()
+    // a full evaluation here will re-run init and shutdown all rabit proxy
+    // expecting no error
+    prediction.collect()
+  }
+}
diff --git a/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Communicator.java b/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Communicator.java
new file mode 100644
index 000000000000..795e7d99e8fe
--- /dev/null
+++ b/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/Communicator.java
@@ -0,0 +1,152 @@
+package ml.dmlc.xgboost4j.java;
+
+import java.io.Serializable;
+import java.nio.ByteBuffer;
+import java.nio.ByteOrder;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Collective communicator global class for synchronization.
+ *
+ * Currently the communicator API is experimental, function signatures may change in the future
+ * without notice.
+ */
+public class Communicator {
+
+  public enum OpType implements Serializable {
+    MAX(0), MIN(1), SUM(2);
+
+    private int op;
+
+    public int getOperand() {
+      return this.op;
+    }
+
+    OpType(int op) {
+      this.op = op;
+    }
+  }
+
+  public enum DataType implements Serializable {
+    INT8(0, 1), UINT8(1, 1), INT32(2, 4), UINT32(3, 4),
+    INT64(4, 8), UINT64(5, 8), FLOAT32(6, 4), FLOAT64(7, 8);
+
+    private final int enumOp;
+    private final int size;
+
+    public int getEnumOp() {
+      return this.enumOp;
+    }
+
+    public int getSize() {
+      return this.size;
+    }
+
+    DataType(int enumOp, int size) {
+      this.enumOp = enumOp;
+      this.size = size;
+    }
+  }
+
+  private static void checkCall(int ret) throws XGBoostError {
+    if (ret != 0) {
+      throw new XGBoostError(XGBoostJNI.XGBGetLastError());
+    }
+  }
+
+  // used as way to test/debug passed communicator init parameters
+  public static Map<String, String> communicatorEnvs;
+  public static List<String> mockList = new LinkedList<>();
+
+  /**
+   * Initialize the collective communicator on current working thread.
+   *
+   * @param envs The additional environment variables to pass to the communicator.
+   * @throws XGBoostError
+   */
+  public static void init(Map<String, String> envs) throws XGBoostError {
+    communicatorEnvs = envs;
+    String[] args = new String[envs.size() * 2 + mockList.size() * 2];
+    int idx = 0;
+    for (java.util.Map.Entry<String, String> e : envs.entrySet()) {
+      args[idx++] = e.getKey();
+      args[idx++] = e.getValue();
+    }
+    // pass list of rabit mock strings eg mock=0,1,0,0
+    for (String mock : mockList) {
+      args[idx++] = "mock";
+      args[idx++] = mock;
+    }
+    checkCall(XGBoostJNI.CommunicatorInit(args));
+  }
+
+  /**
+   * Shutdown the communicator in current working thread, equals to finalize.
+   *
+   * @throws XGBoostError
+   */
+  public static void shutdown() throws XGBoostError {
+    checkCall(XGBoostJNI.CommunicatorFinalize());
+  }
+
+  /**
+   * Print the message via the communicator.
+   *
+   * @param msg
+   * @throws XGBoostError
+   */
+  public static void communicatorPrint(String msg) throws XGBoostError {
+    checkCall(XGBoostJNI.CommunicatorPrint(msg));
+  }
+
+  /**
+   * get rank of current thread.
+   *
+   * @return the rank.
+   * @throws XGBoostError
+   */
+  public static int getRank() throws XGBoostError {
+    int[] out = new int[1];
+    checkCall(XGBoostJNI.CommunicatorGetRank(out));
+    return out[0];
+  }
+
+  /**
+   * get world size of current job.
+   *
+   * @return the worldsize
+   * @throws XGBoostError
+   */
+  public static int getWorldSize() throws XGBoostError {
+    int[] out = new int[1];
+    checkCall(XGBoostJNI.CommunicatorGetWorldSize(out));
+    return out[0];
+  }
+
+  /**
+   * perform Allreduce on distributed float vectors using operator op.
+   *
+   * @param elements local elements on distributed workers.
+   * @param op       operator used for Allreduce.
+   * @return All-reduced float elements according to the given operator.
+   */
+  public static float[] allReduce(float[] elements, OpType op) {
+    DataType dataType = DataType.FLOAT32;
+    ByteBuffer buffer = ByteBuffer.allocateDirect(dataType.getSize() * elements.length)
+            .order(ByteOrder.nativeOrder());
+
+    for (float el : elements) {
+      buffer.putFloat(el);
+    }
+    buffer.flip();
+
+    XGBoostJNI.CommunicatorAllreduce(buffer, elements.length, dataType.getEnumOp(),
+            op.getOperand());
+    float[] results = new float[elements.length];
+    buffer.asFloatBuffer().get(results);
+
+    return results;
+  }
+}
diff --git a/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoostJNI.java b/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoostJNI.java
index d2285af90e08..72234f526b08 100644
--- a/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoostJNI.java
+++ b/jvm-packages/xgboost4j/src/main/java/ml/dmlc/xgboost4j/java/XGBoostJNI.java
@@ -148,6 +148,17 @@ public final static native int XGBoosterDumpModelExWithFeatures(
   final static native int RabitAllreduce(ByteBuffer sendrecvbuf, int count,
                                                 int enum_dtype, int enum_op);
 
+  // communicator functions
+  public final static native int CommunicatorInit(String[] args);
+  public final static native int CommunicatorFinalize();
+  public final static native int CommunicatorPrint(String msg);
+  public final static native int CommunicatorGetRank(int[] out);
+  public final static native int CommunicatorGetWorldSize(int[] out);
+
+  // Perform Allreduce operation on data in sendrecvbuf.
+  final static native int CommunicatorAllreduce(ByteBuffer sendrecvbuf, int count,
+    int enum_dtype, int enum_op);
+
   public final static native int XGDMatrixSetInfoFromInterface(
     long handle, String field, String json);
 
diff --git a/jvm-packages/xgboost4j/src/native/xgboost4j.cpp b/jvm-packages/xgboost4j/src/native/xgboost4j.cpp
index 630040731e67..a89e0f07a341 100644
--- a/jvm-packages/xgboost4j/src/native/xgboost4j.cpp
+++ b/jvm-packages/xgboost4j/src/native/xgboost4j.cpp
@@ -977,6 +977,89 @@ JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_RabitAllreduce
   return 0;
 }
 
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorInit
+ * Signature: ([Ljava/lang/String;)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorInit
+  (JNIEnv *jenv, jclass jcls, jobjectArray jargs) {
+  xgboost::Json config{xgboost::Object{}};
+  bst_ulong len = (bst_ulong)jenv->GetArrayLength(jargs);
+  assert(len % 2 == 0);
+  for (bst_ulong i = 0; i < len / 2; ++i) {
+    jstring key = (jstring)jenv->GetObjectArrayElement(jargs, 2 * i);
+    std::string key_str(jenv->GetStringUTFChars(key, 0), jenv->GetStringLength(key));
+    jstring value = (jstring)jenv->GetObjectArrayElement(jargs, 2 * i + 1);
+    std::string value_str(jenv->GetStringUTFChars(value, 0), jenv->GetStringLength(value));
+    config[key_str] = xgboost::String(value_str);
+  }
+  std::string json_str;
+  xgboost::Json::Dump(config, &json_str);
+  JVM_CHECK_CALL(XGCommunicatorInit(json_str.c_str()));
+  return 0;
+}
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorFinalize
+ * Signature: ()I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorFinalize
+  (JNIEnv *jenv, jclass jcls) {
+  JVM_CHECK_CALL(XGCommunicatorFinalize());
+  return 0;
+}
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorPrint
+ * Signature: (Ljava/lang/String;)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorPrint
+  (JNIEnv *jenv, jclass jcls, jstring jmsg) {
+  std::string str(jenv->GetStringUTFChars(jmsg, 0),
+                  jenv->GetStringLength(jmsg));
+  JVM_CHECK_CALL(XGCommunicatorPrint(str.c_str()));
+  return 0;
+}
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorGetRank
+ * Signature: ([I)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorGetRank
+  (JNIEnv *jenv, jclass jcls, jintArray jout) {
+  jint rank = XGCommunicatorGetRank();
+  jenv->SetIntArrayRegion(jout, 0, 1, &rank);
+  return 0;
+}
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorGetWorldSize
+ * Signature: ([I)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorGetWorldSize
+  (JNIEnv *jenv, jclass jcls, jintArray jout) {
+  jint out = XGCommunicatorGetWorldSize();
+  jenv->SetIntArrayRegion(jout, 0, 1, &out);
+  return 0;
+}
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorAllreduce
+ * Signature: (Ljava/nio/ByteBuffer;III)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorAllreduce
+  (JNIEnv *jenv, jclass jcls, jobject jsendrecvbuf, jint jcount, jint jenum_dtype, jint jenum_op) {
+  void *ptr_sendrecvbuf = jenv->GetDirectBufferAddress(jsendrecvbuf);
+  JVM_CHECK_CALL(XGCommunicatorAllreduce(ptr_sendrecvbuf, (size_t) jcount, jenum_dtype, jenum_op));
+  return 0;
+}
+
 namespace xgboost {
 namespace jni {
   XGB_DLL int XGDeviceQuantileDMatrixCreateFromCallbackImpl(JNIEnv *jenv, jclass jcls,
diff --git a/jvm-packages/xgboost4j/src/native/xgboost4j.h b/jvm-packages/xgboost4j/src/native/xgboost4j.h
index 2db64a16992c..7baae983cf51 100644
--- a/jvm-packages/xgboost4j/src/native/xgboost4j.h
+++ b/jvm-packages/xgboost4j/src/native/xgboost4j.h
@@ -335,6 +335,54 @@ JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_RabitVersionNumber
 JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_RabitAllreduce
   (JNIEnv *, jclass, jobject, jint, jint, jint);
 
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorInit
+ * Signature: ([Ljava/lang/String;)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorInit
+  (JNIEnv *, jclass, jobjectArray);
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorFinalize
+ * Signature: ()I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorFinalize
+  (JNIEnv *, jclass);
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorPrint
+ * Signature: (Ljava/lang/String;)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorPrint
+  (JNIEnv *, jclass, jstring);
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorGetRank
+ * Signature: ([I)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorGetRank
+  (JNIEnv *, jclass, jintArray);
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorGetWorldSize
+ * Signature: ([I)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorGetWorldSize
+  (JNIEnv *, jclass, jintArray);
+
+/*
+ * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
+ * Method:    CommunicatorAllreduce
+ * Signature: (Ljava/nio/ByteBuffer;III)I
+ */
+JNIEXPORT jint JNICALL Java_ml_dmlc_xgboost4j_java_XGBoostJNI_CommunicatorAllreduce
+  (JNIEnv *, jclass, jobject, jint, jint, jint);
+
 /*
  * Class:     ml_dmlc_xgboost4j_java_XGBoostJNI
  * Method:    XGDMatrixSetInfoFromInterface