Explicit Binding with #bind[T]

Embedded Server #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.

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

class ExampleFeatureTest
  extends FeatureTest
  with Mockito
  with HttpTest {

  val mockDownstreamServiceClient = smartMock[DownstreamServiceClient]
  val mockIdService = smartMock[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.


Using @Bind (the com.google.inject.testing.fieldbinder.Bind annotation) is to be considered deprecated.

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 = smartMock[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.