examples: clarify about potential mistake in unit test

Resolves grpc#6161

Author: dapengzhang0

examples/README.md

@@ -164,9 +164,11 @@ $ bazel-bin/hello-world-client
 
 Examples for unit testing gRPC clients and servers are located in [examples/src/test](src/test).
 
-In general, we DO NOT allow overriding the client stub.
-We encourage users to leverage `InProcessTransport` as demonstrated in the examples to
-write unit tests. `InProcessTransport` is light-weight and runs the server
+In general, we DO NOT allow overriding the client stub and we DO NOT support mocking final methods
+in gRPC-Java library. Users should be cautious that using tools like PowerMock or
+[mockito-inline](https://search.maven.org/search?q=g:org.mockito%20a:mockito-inline) can easily
+break this rule of thumb. We encourage users to leverage `InProcessTransport` as demonstrated in the
+examples to write unit tests. `InProcessTransport` is light-weight and runs the server
 and client in the same process without any socket/TCP connection.
 
 Mocking the client stub provides a false sense of security when writing tests. Mocking stubs and responses

examples/example-kotlin/src/test/kotlin/io/grpc/examples/helloworld/HelloWorldClientTest.kt

@@ -52,9 +52,16 @@ class HelloWorldClientTest {
     @get:Rule
     val grpcCleanup = GrpcCleanupRule()
 
-    private val serviceImpl = mock(GreeterGrpc.GreeterImplBase::class.java, delegatesTo<Any>(object : GreeterGrpc.GreeterImplBase() {
+    private val serviceImpl = mock(GreeterGrpc.GreeterImplBase::class.java, delegatesTo<Any>(
+            object : GreeterGrpc.GreeterImplBase() {
+            // By default the client will receive Status.UNIMPLEMENTED for all RPCs.
+            // You might need to implement necessary behaviors for your test here, like this:
+            //
+            // override fun sayHello(req: HelloRequest, respObserver: StreamObserver<HelloReply>) {
+            //     respObserver.onNext(HelloReply.getDefaultInstance())
+            //     respObserver.onCompleted()
+            }))
 
-    }))
     private var client: HelloWorldClient? = null
 
     @Before

examples/src/test/java/io/grpc/examples/helloworld/HelloWorldClientTest.java

@@ -56,7 +56,18 @@ public class HelloWorldClientTest {
   public final GrpcCleanupRule grpcCleanup = new GrpcCleanupRule();
 
   private final GreeterGrpc.GreeterImplBase serviceImpl =
-      mock(GreeterGrpc.GreeterImplBase.class, delegatesTo(new GreeterGrpc.GreeterImplBase() {}));
+      mock(GreeterGrpc.GreeterImplBase.class, delegatesTo(
+          new GreeterGrpc.GreeterImplBase() {
+          // By default the client will receive Status.UNIMPLEMENTED for all RPCs.
+          // You might need to implement necessary behaviors for your test here, like this:
+          //
+          // @Override
+          // public void sayHello(HelloRequest request, StreamObserver<HelloReply> respObserver) {
+          //   respObserver.onNext(HelloReply.getDefaultInstance());
+          //   respObserver.onCompleted();
+          // }
+          }));
+
   private HelloWorldClient client;
 
   @Before