Prevent construction of container classes and reduce API

auth/src/main/java/io/grpc/auth/ClientAuthInterceptor.java

@@ -58,7 +58,7 @@ import java.util.concurrent.Executor;
  * <p> Uses the new and simplified Google auth library:
  * https://github.com/google/google-auth-library-java
  */
-public class ClientAuthInterceptor implements ClientInterceptor {
+public final class ClientAuthInterceptor implements ClientInterceptor {
 
   private final Credentials credentials;
 

benchmarks/src/generated/main/grpc/io/grpc/testing/TestServiceGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class TestServiceGrpc {
 
+  private TestServiceGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.TestService";
 
   // Static method descriptors that strictly reflect the proto.

benchmarks/src/generated/main/grpc/io/grpc/testing/WorkerGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class WorkerGrpc {
 
+  private WorkerGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.Worker";
 
   // Static method descriptors that strictly reflect the proto.

compiler/src/java_plugin/cpp/java_generator.cpp

@@ -511,6 +511,9 @@ static void PrintService(const ServiceDescriptor* service,
       "@$Generated$(\"by gRPC proto compiler\")\n"
       "public class $service_class_name$ {\n\n");
   p->Indent();
+  p->Print(
+      *vars,
+      "private $service_class_name$() {}\n\n");
 
   p->Print(
       *vars,

compiler/src/test/golden/TestService.java.txt

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class TestServiceGrpc {
 
+  private TestServiceGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.TestService";
 
   // Static method descriptors that strictly reflect the proto.

compiler/src/test/golden/TestServiceNano.java.txt

@@ -18,6 +18,8 @@ import java.io.IOException;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class TestServiceGrpc {
 
+  private TestServiceGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.TestService";
 
   // Static method descriptors that strictly reflect the proto.

examples/src/generated/main/grpc/io/grpc/examples/helloworld/GreeterGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class GreeterGrpc {
 
+  private GreeterGrpc() {}
+
   public static final String SERVICE_NAME = "helloworld.Greeter";
 
   // Static method descriptors that strictly reflect the proto.

examples/src/generated/main/grpc/io/grpc/examples/routeguide/RouteGuideGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class RouteGuideGrpc {
 
+  private RouteGuideGrpc() {}
+
   public static final String SERVICE_NAME = "routeguide.RouteGuide";
 
   // Static method descriptors that strictly reflect the proto.

interop-testing/src/generated/main/grpc/io/grpc/testing/integration/ReconnectServiceGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class ReconnectServiceGrpc {
 
+  private ReconnectServiceGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.ReconnectService";
 
   // Static method descriptors that strictly reflect the proto.

interop-testing/src/generated/main/grpc/io/grpc/testing/integration/TestServiceGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class TestServiceGrpc {
 
+  private TestServiceGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.TestService";
 
   // Static method descriptors that strictly reflect the proto.

interop-testing/src/generated/main/grpc/io/grpc/testing/integration/UnimplementedServiceGrpc.java

@@ -16,6 +16,8 @@ import static io.grpc.stub.ServerCalls.asyncBidiStreamingCall;
 @javax.annotation.Generated("by gRPC proto compiler")
 public class UnimplementedServiceGrpc {
 
+  private UnimplementedServiceGrpc() {}
+
   public static final String SERVICE_NAME = "grpc.testing.UnimplementedService";
 
   // Static method descriptors that strictly reflect the proto.

stub/src/main/java/io/grpc/stub/ClientCalls.java

@@ -55,6 +55,8 @@ import javax.annotation.Nullable;
  * that the runtime can vary behavior without requiring regeneration of the stub.
  */
 public class ClientCalls {
+  // Prevent instantiation
+  private ClientCalls() {}
 
   /**
    * Executes a unary call with a response {@link StreamObserver}.

stub/src/main/java/io/grpc/stub/MetadataUtils.java

@@ -35,6 +35,7 @@ import io.grpc.CallOptions;
 import io.grpc.Channel;
 import io.grpc.ClientCall;
 import io.grpc.ClientInterceptor;
+import io.grpc.ExperimentalApi;
 import io.grpc.ForwardingClientCall.SimpleForwardingClientCall;
 import io.grpc.ForwardingClientCallListener.SimpleForwardingClientCallListener;
 import io.grpc.Metadata;
@@ -47,6 +48,8 @@ import java.util.concurrent.atomic.AtomicReference;
  * Utility functions for binding and receiving headers.
  */
 public class MetadataUtils {
+  // Prevent instantiation
+  private MetadataUtils() {}
 
   /**
    * Attaches a set of request headers to a stub.
@@ -55,11 +58,11 @@ public class MetadataUtils {
    * @param extraHeaders the headers to be passed by each call on the returned stub.
    * @return an implementation of the stub with {@code extraHeaders} bound to each call.
    */
-  @SuppressWarnings({"unchecked", "rawtypes"})
+  @ExperimentalApi
-  public static <T extends AbstractStub> T attachHeaders(
+  public static <T extends AbstractStub<T>> T attachHeaders(
       T stub,
       final Metadata extraHeaders) {
-    return (T) stub.withInterceptors(newAttachHeadersInterceptor(extraHeaders));
+    return stub.withInterceptors(newAttachHeadersInterceptor(extraHeaders));
   }
 
   /**
@@ -94,12 +97,12 @@ public class MetadataUtils {
    * @param trailersCapture to record the last received trailers
    * @return an implementation of the stub with {@code extraHeaders} bound to each call.
    */
-  @SuppressWarnings({"unchecked", "rawtypes"})
+  @ExperimentalApi
-  public static <T extends AbstractStub> T captureMetadata(
+  public static <T extends AbstractStub<T>> T captureMetadata(
       T stub,
       AtomicReference<Metadata> headersCapture,
       AtomicReference<Metadata> trailersCapture) {
-    return (T) stub.withInterceptors(
+    return stub.withInterceptors(
         newCaptureMetadataInterceptor(headersCapture, trailersCapture));
   }