Expose Batcher#sendOutstanding to provide async batch flushing

gax/src/main/java/com/google/api/gax/batching/Batcher.java

@@ -62,6 +62,14 @@ public interface Batcher<ElementT, ElementResultT> extends AutoCloseable {
    */
   void flush() throws InterruptedException;
 
+  /**
+   * Sends accumulated elements asynchronously for batching.
+   *
+   * <p>Note: This method can be invoked concurrently unlike {@link #add} and {@link #close}, which
+   * can only be called from a single user thread. Please take caution to avoid race condition.
+   */
+  void sendOutstanding();
+
   /**
    * Closes this Batcher by preventing new elements from being added and flushing the existing
    * elements.

gax/src/main/java/com/google/api/gax/batching/BatcherImpl.java

@@ -132,27 +132,23 @@ public ApiFuture<ElementResultT> add(ElementT element) {
     }
 
     if (currentOpenBatch.hasAnyThresholdReached()) {
-      sendBatch();
+      sendOutstanding();
     }
     return result;
   }
 
   /** {@inheritDoc} */
   @Override
   public void flush() throws InterruptedException {
-    sendBatch();
+    sendOutstanding();
     awaitAllOutstandingBatches();
   }
 
-  /**
-   * Sends accumulated elements asynchronously for batching.
-   *
-   * <p>Note: This method can be invoked concurrently unlike {@link #add} and {@link #close}, which
-   * can only be called from a single user thread. Please take caution to avoid race condition.
-   */
-  private void sendBatch() {
-
+  /** {@inheritDoc} */
+  @Override
+  public void sendOutstanding() {
     final Batch<ElementT, ElementResultT, RequestT, ResponseT> accumulatedBatch;
+
     synchronized (elementLock) {
       if (currentOpenBatch.isEmpty()) {
         return;
@@ -280,7 +276,7 @@ boolean hasAnyThresholdReached() {
   }
 
   /**
-   * Executes {@link #sendBatch()} on a periodic interval.
+   * Executes {@link #sendOutstanding()} on a periodic interval.
    *
    * <p>This class holds a weak reference to the Batcher instance and cancels polling if the target
    * Batcher has been garbage collected.
@@ -303,7 +299,7 @@ public void run() {
       if (batcher == null) {
         scheduledFuture.cancel(true);
       } else {
-        batcher.sendBatch();
+        batcher.sendOutstanding();
       }
     }
 

gax/src/test/java/com/google/api/gax/batching/BatcherImplTest.java

@@ -41,6 +41,7 @@
 import com.google.api.gax.rpc.ApiCallContext;
 import com.google.api.gax.rpc.UnaryCallable;
 import com.google.api.gax.rpc.testing.FakeBatchableApi.LabeledIntList;
+import com.google.api.gax.rpc.testing.FakeBatchableApi.LabeledIntSquarerCallable;
 import com.google.api.gax.rpc.testing.FakeBatchableApi.SquarerBatchingDescriptorV2;
 import java.util.ArrayList;
 import java.util.List;
@@ -55,6 +56,7 @@
 import java.util.concurrent.ScheduledFuture;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicInteger;
 import java.util.logging.Filter;
 import java.util.logging.Level;
 import java.util.logging.LogRecord;
@@ -113,6 +115,35 @@ public void testResultsAreResolvedAfterFlush() throws Exception {
     assertThat(anotherResult.isDone()).isFalse();
   }
 
+  @Test
+  public void testSendOutstanding() {
+    final AtomicInteger callableCounter = new AtomicInteger();
+
+    underTest =
+        new BatcherImpl<>(
+            SQUARER_BATCHING_DESC_V2,
+            new LabeledIntSquarerCallable() {
+              @Override
+              public ApiFuture<List<Integer>> futureCall(LabeledIntList request) {
+                callableCounter.incrementAndGet();
+                return super.futureCall(request);
+              }
+            },
+            labeledIntList,
+            batchingSettings,
+            EXECUTOR);
+
+    // Empty Batcher
+    underTest.sendOutstanding();
+    assertThat(callableCounter.get()).isEqualTo(0);
+
+    underTest.add(2);
+    underTest.add(3);
+    underTest.add(4);
+    underTest.sendOutstanding();
+    assertThat(callableCounter.get()).isEqualTo(1);
+  }
+
   /** Element results are resolved after batch is closed. */
   @Test
   public void testWhenBatcherIsClose() throws Exception {
@@ -490,7 +521,7 @@ private void testElementTriggers(BatchingSettings settings) throws Exception {
             SQUARER_BATCHING_DESC_V2, callLabeledIntSquarer, labeledIntList, settings, EXECUTOR);
     Future<Integer> result = underTest.add(4);
     assertThat(result.isDone()).isFalse();
-    // After this element is added, the batch triggers sendBatch().
+    // After this element is added, the batch triggers sendOutstanding().
     Future<Integer> anotherResult = underTest.add(5);
     // Both the elements should be resolved now.
     assertThat(result.isDone()).isTrue();