Explicit Binding with #bind[T]


Please see the section on including test-jar dependencies in your project: Test Dependencies.

Embedded #bind[T]

In the cases where we’d like to easily replace a bound instance with another instance in our tests (e.g. with a mock or a simple stub implementation), we do not need to create a specific module for testing to compose into our server as an override module. Instead we can use the #bind[T] function on the embedded server.

This is especially useful if you want to replace a bound instance with a mock under control of the test.

import com.twitter.finatra.http.{EmbeddedHttpServer, HttpTest}
import com.twitter.inject.server.FeatureTest
import com.twitter.util.mock.Mockito

class ExampleFeatureTest
  extends FeatureTest
  with Mockito
  with HttpTest {

  private val mockDownstreamServiceClient = mock[DownstreamServiceClient]
  private val mockIdService = mock[IdService]

  override val server =
    new EmbeddedHttpServer(new ExampleServer)

  test("service test") {
    /* Mock GET Request performed by DownstreamServiceClient */
    mockDownstreamServiceClient.get("/tweets/123.json")(manifest[FooResponse]) returns Future(None)

For a complete example, see the TwitterCloneFeatureTest.

Note this is also available for EmbeddedApp as well:

import com.twitter.finagle.stats.InMemoryStatsReceiver
import com.twitter.inject.Test
import com.twitter.inject.app.EmbeddedApp
import com.twitter.util.mock.Mockito

class MyAppTest extends Test with Mockito {
  private val inMemoryStatsReceiver: InMemoryStatsReceiver = new InMemoryStatsReceiver
  private val mockIdService = mock[IdService]

  // build an EmbeddedApp
  def app(): EmbeddedApp = app(new MyApp)
  def app(underlying: MyApp): EmbeddedApp =
    new EmbeddedApp(underlying)

  test("assert count") {
    val undertest = app()
    undertest.main("username" -> "jack")

    val statsReceiver = undertest.injector.instance[StatsReceiver].asInstanceOf[InMemoryStatsReceiver]
    statsReceiver.counter("my_counter")() shouldEqual 1

TestInjector #bind[T]

As described in the Integration Tests section you can use the TestInjector to construct a minimal object graph for testing. The TestInjector also supports a bind[T] function to let you easily replace bound instances in the constructed object graph with another instance, like a mock or stub.


import com.twitter.inject.IntegrationTest

class ExampleIntegrationTest extends IntegrationTest {
  val mockIdService = mock[IdService]

  override val injector =
      flags =
        Map("foo.flag" -> "meaningfulValue"),
      modules =
        Seq(ExampleModule, IdServiceModule))

  test("MyTest#perform feature") {

In this example, the bound IdService would be replaced with the mockIdService. For a more complete example, see the DarkTrafficCanonicalResourceHeaderTest.

#bind[T] DSL

The primary DSL can be expressed as such (and is similar to the Guice Linked and Instance bindings):

bind[T].to[U <: T]
bind[T].to[Class[U <: T]]

bind[T].annotatedWith[Ann].to[U <: T]
bind[T].annotatedWith[Ann].to[Class[U <: T]]

bind[T].annotatedWith[Class[Ann]].to[U <: T]
bind[T].annotatedWith[Class[Ann]].to[Class[U <: T]]

bind[T].annotatedWith(Annotation).to[U <: T]
bind[T].annotatedWith(Annotation).to[Class[U <: T]]

bindClass(Class[T]).to[Class[U <: T]]

bindClass(Class[T]).annotatedWith[Class[Ann]].[Class[U <: T]]

bindClass(Class[T]).annotatedWith(Annotation).[Class[U <: T]]

Usage from Java

The #bind[T] DSL also provides several Java-friendly methods for binding:

bindClass(Class[T], T)
bindClass(Class[T], Annotation, T)
bindClass(Class[T], Class[Annotation], T)

bindClass(Class[T], Class[U <: T])
bindClass(Class[T], Annotation, Class[U <: T])
bindClass(Class[T], Class[Annotation], Class[U <: T])


import java.util.Collections;

import com.google.inject.Stage;

import org.junit.AfterClass;
import org.junit.Assert;
import org.junit.BeforeClass;
import org.junit.Test;

import com.twitter.finagle.http.Request;
import com.twitter.finagle.http.Response;
import com.twitter.finagle.http.Status;
import com.twitter.finatra.http.EmbeddedHttpServer;
import com.twitter.finatra.httpclient.RequestBuilder;
import com.twitter.inject.annotations.Flags;

public class HelloWorldServerFeatureTest extends Assert {

    private static final EmbeddedHttpServer SERVER = setup();

    private static EmbeddedHttpServer setup() {
        EmbeddedHttpServer server = new EmbeddedHttpServer(
            new HelloWorldServer(),

        server.bindClass(Integer.class, Flags.named("magic.number"), 42);
        server.bindClass(Integer.class, Flags.named("module.magic.number"), 9999);
        return server;


    /** test magicNum endpoint */
    public void testMagicNumEndpoint() {
        Request request = RequestBuilder.get("/magicNum");
        Response response = SERVER.httpRequest(request);
        assertEquals(Status.Ok(), response.status());
        assertEquals("42", response.contentString());

See the java-http-server for a full example of using the #bind[T] DSL in test to override a binding in a server.

Injecting Members of a Test


Do not inject members of a test class into the server or application under test.

We strongly discourage injecting members of a test into your server or application. This includes using @Inject or @Bind (the com.google.inject.testing.fieldbinder.Bind annotation) on a test member field then using the server or application’s c.t.inject.Injector to do injector.underlying.injectMembers(this).


The server or application under test has a different lifecycle than the test class. Mixing them in this manner is first, hard to reason about and second, can lead to possibly non-deterministic tests. You will also make it hard to test multiple servers or applications in a single test file.

Lastly, but perhaps most importantly, the Injector of a server or application under test is meant to be encapsulated to that server or application. Any mutation of its object graph should happen within the constraints of its lifecycle and thus you should not mutate its object graph after it has started. The framework thus provides ways to do this safely taking into account the server or application’s lifecyle: either provide an override module or use the #bind[T] DSL here to replace a binding when instantiating the server or application under test.