First attempt at testing thread safety

jvm-packages/xgboost4j/src/test/java/ml/dmlc/xgboost4j/java/BoosterImplTest.java

@@ -317,17 +317,18 @@ public void testBoosterInplacePredict() throws  XGBoostError, IOException {
     // Create thread pool
     int n_tasks = 20;
     List<Future<Boolean>> result = new ArrayList(n_tasks);
-    ExecutorService executorService = Executors.newFixedThreadPool(5);  // Create pool of 5 threads
-
-    // Submit all the tasks
-    for (int i=0; i<n_tasks; i++) {
-      result.add(executorService.submit(new InplacePredictionTask(i, booster, testX2, test_rows, features, predicts)));
-    }
-
-    // Tell the executor service we are done
-    executorService.shutdown();
+    ExecutorService executorService = Executors.newFixedThreadPool(n_tasks);  // Create pool of 20 threads
+    CountDownLatch latch = new CountDownLatch(n_tasks); // Latch for testing thread safety
 
     try {
+      for (int i = 0; i < n_tasks; i++) {
+        int finalI = i;
+        executorService.execute(() -> {
+          result.add(executorService.submit(new InplacePredictionTask(finalI, booster, testX2, test_rows, features, predicts)));
+          latch.countDown();
+        });
+      }
+      latch.await();
       executorService.awaitTermination(10, TimeUnit.SECONDS);
 
       // Get the result from each Future returned and confirm success