services: set the default serving state of all services to SERVING

services/src/main/java/io/grpc/services/HealthServiceImpl.java

@@ -59,6 +59,11 @@ final class HealthServiceImpl extends HealthGrpc.HealthImplBase {
   private final HashMap<String, IdentityHashMap<StreamObserver<HealthCheckResponse>, Boolean>>
       watchers = new HashMap<>();
 
+  HealthServiceImpl() {
+    // Copy of what Go and C++ do.
+    statusMap.put(HealthStatusManager.SERVICE_NAME_ALL_SERVICES, ServingStatus.SERVING);
+  }
+
   @Override
   public void check(HealthCheckRequest request,
       StreamObserver<HealthCheckResponse> responseObserver) {

services/src/main/java/io/grpc/services/HealthStatusManager.java

@@ -26,6 +26,9 @@ import io.grpc.health.v1.HealthCheckResponse.ServingStatus;
  * created in the constructor of {@code HealthStatusManager}, and it can be retrieved by the
  * {@link #getHealthService()} method.
  * The health status manager can update the health statuses of the server.
+ *
+ * <p>The default, empty-string, service name, {@link #SERVICE_NAME_ALL_SERVICES}, is initialized to
+ * {@link ServingStatus#SERVING}.
  */
 @io.grpc.ExperimentalApi("https://github.com/grpc/grpc-java/issues/4696")
 public final class HealthStatusManager {

services/src/test/java/io/grpc/services/HealthStatusManagerTest.java

@@ -133,6 +133,19 @@ public class HealthStatusManagerTest {
     assertThat(resp.getStatus()).isEqualTo(ServingStatus.NOT_SERVING);
   }
 
+  @Test
+  public void defaultIsServing() throws Exception {
+    HealthCheckRequest request =
+        HealthCheckRequest.newBuilder().setService(HealthStatusManager.SERVICE_NAME_ALL_SERVICES)
+            .build();
+
+    HealthCheckResponse response =
+        blockingStub.withDeadlineAfter(5, TimeUnit.SECONDS).check(request);
+
+    assertThat(response).isEqualTo(
+        HealthCheckResponse.newBuilder().setStatus(ServingStatus.SERVING).build());
+  }
+
   @Test
   public void getHealthService_getterReturnsTheSameHealthRefAfterUpdate() throws Exception {
     BindableService health = manager.getHealthService();