[jvm-packages] refine tracker (#10313)

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

jvm-packages/xgboost4j/pom.xml

@@ -53,6 +53,12 @@
           <version>${scalatest.version}</version>
           <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>com.fasterxml.jackson.core</groupId>
+            <artifactId>jackson-databind</artifactId>
+            <version>${fasterxml.jackson.version}</version>
+            <scope>provided</scope>
+        </dependency>
     </dependencies>
 
     <build>

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

@@ -7,7 +7,7 @@ import java.util.Map;
  *
  *  - start(timeout): Start the tracker awaiting for worker connections, with a given
  *  timeout value (in seconds).
- *  - workerArgs(): Return the arguments needed to initialize Rabit clients.
+ *  - getWorkerArgs(): Return the arguments needed to initialize Rabit clients.
  *  - waitFor(timeout): Wait for the task execution by the worker nodes for at most `timeout`
  *  milliseconds.
  *
@@ -21,21 +21,8 @@ import java.util.Map;
  * brokers connections between workers.
  */
 public interface ITracker extends Thread.UncaughtExceptionHandler {
-  enum TrackerStatus {
-    SUCCESS(0), INTERRUPTED(1), TIMEOUT(2), FAILURE(3);
 
-    private int statusCode;
-
-    TrackerStatus(int statusCode) {
-      this.statusCode = statusCode;
-    }
-
-    public int getStatusCode() {
-      return this.statusCode;
-    }
-  }
-
-  Map<String, Object> workerArgs() throws XGBoostError;
+  Map<String, Object> getWorkerArgs() throws XGBoostError;
 
   boolean start() throws XGBoostError;
 

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

@@ -1,3 +1,19 @@
+/*
+ Copyright (c) 2014-2024 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.java;
 
 import java.util.Map;
@@ -10,14 +26,12 @@ import org.apache.commons.logging.LogFactory;
 
 /**
  * Java implementation of the Rabit tracker to coordinate distributed workers.
- *
- * The tracker must be started on driver node before running distributed jobs.
  */
 public class RabitTracker implements ITracker {
   // Maybe per tracker logger?
   private static final Log logger = LogFactory.getLog(RabitTracker.class);
   private long handle = 0;
-  private Thread tracker_daemon;
+  private Thread trackerDaemon;
 
   public RabitTracker(int numWorkers) throws XGBoostError {
     this(numWorkers, "");
@@ -44,7 +58,7 @@ public class RabitTracker implements ITracker {
     } catch (InterruptedException ex) {
       logger.error(ex);
     } finally {
-      this.tracker_daemon.interrupt();
+      this.trackerDaemon.interrupt();
     }
   }
 
@@ -52,16 +66,14 @@ public class RabitTracker implements ITracker {
    * Get environments that can be used to pass to worker.
    * @return The environment settings.
    */
-  public Map<String, Object> workerArgs() throws XGBoostError {
+  public Map<String, Object> getWorkerArgs() throws XGBoostError {
     // fixme: timeout
     String[] args = new String[1];
     XGBoostJNI.checkCall(XGBoostJNI.TrackerWorkerArgs(this.handle, 0, args));
     ObjectMapper mapper = new ObjectMapper();
-    TypeReference<Map<String, Object>> typeRef = new TypeReference<Map<String, Object>>() {
-    };
     Map<String, Object> config;
     try {
-      config = mapper.readValue(args[0], typeRef);
+      config = mapper.readValue(args[0], new TypeReference<Map<String, Object>>() {});
     } catch (JsonProcessingException ex) {
       throw new XGBoostError("Failed to get worker arguments.", ex);
     }
@@ -74,18 +86,18 @@ public class RabitTracker implements ITracker {
 
   public boolean start() throws XGBoostError {
     XGBoostJNI.checkCall(XGBoostJNI.TrackerRun(this.handle));
-    this.tracker_daemon = new Thread(() -> {
+    this.trackerDaemon = new Thread(() -> {
       try {
-        XGBoostJNI.checkCall(XGBoostJNI.TrackerWaitFor(this.handle, 0));
+        waitFor(0);
       } catch (XGBoostError ex) {
         logger.error(ex);
         return; // exit the thread
       }
     });
-    this.tracker_daemon.setDaemon(true);
-    this.tracker_daemon.start();
+    this.trackerDaemon.setDaemon(true);
+    this.trackerDaemon.start();
 
-    return this.tracker_daemon.isAlive();
+    return this.trackerDaemon.isAlive();
   }
 
   public void waitFor(long timeout) throws XGBoostError {