Tutorial - Build the Backend Server

The backend server is a simple web application that runs on Google App Engine and responds to incoming HTTP requests from the sample Android and iOS apps.

Here, you'll do the following:

  • Review the source files for the app
  • Update the WORKSPACE file
  • Create a BUILD file
  • Run the build
  • Find the build outputs
  • Run the application on a local development server
  • Deploy to Google App Engine

Bazel provides a set of App Engine build rules written using the Skylark framework. You'll use these in the steps below to build the application.

Review the source files

The source files for the backend server are located in $WORKSPACE/backend/.

The key files and directories are:

Name Location
Source file directory src/main/java/com/google/bazel/example/app/
Web application metadata directory webapp/WEB-INF/

Update the WORKSPACE file

As with the Android app, you must add references to external dependencies to your WORKSPACE file. For the backend server, these are references to the App Engine SDK, the Java Servlet SDK and other libraries needed to build the App Engine applications.

Add the App Engine rule

When you built the Android app, you added a reference to the location on your filesystem where you downloaded and installed the Android SDK. For the backend server, however, you'll give Bazel instructions for downloading the required App Engine SDK package from a remote server. This is optional. You can also download and install the SDK manually on your filesystem and reference it from that location as described in the App Engine rule documentation.

Add the following to your WORKSPACE file:

    name = "io_bazel_rules_appengine",
    sha256 = "f4fb98f31248fca5822a9aec37dc362105e57bc28e17c5611a8b99f1d94b37a4",
    strip_prefix = "rules_appengine-0.0.6",
    url = "",
load("@io_bazel_rules_appengine//appengine:appengine.bzl", "appengine_repositories")

http_archive downloads the AppEngine rules from a GitHub archive. We could also have used git_repository to fetch the rules directly from the Git repository. Then the next two lines use the appengine_repositories function defined in these rules to download the libraries and SDK needed to build AppEngine applications.

Now, save and close the file. You can compare your WORKSPACE file to the completed example in the master branch of the GitHub repo.

Create a BUILD file

Now that you have set up the external dependencies, you can go ahead and create the BUILD file for the backend server, as you did previously for the sample Android and iOS apps.

Open your new BUILD file for editing:


Add a java_binary rule

Add the following to your BUILD file:

    name = "app",
    srcs = glob(["src/main/java/**/*.java"]),
    main_class = "does.not.exist",
    deps = [

The java_binary tells Bazel how to build a Java .jar library for your application, plus a wrapper shell script that launches the application code from the specified main class. Here, we're using this rule instead of the java_library because we need the .jar file to contain all the dependencies required to build the final App Engine .war file. For this reason, we specify a bogus class name for the main_class attribute.

Add an appengine_war rule

Add the following to your BUILD file:

load("@io_bazel_rules_appengine//appengine:appengine.bzl", "appengine_war")

    name = "backend",
    data = [":webapp"],
    data_path = "/backend/webapp",
    jars = [":app_deploy.jar"],

    name = "webapp",
    srcs = glob(["webapp/**/*"]),

The appengine_war rule builds the final App Engine war file from the library .jar file and web application metadata files in the webapp directory.

Save and close the file. Again, the completed example is in the master branch of the GitHub repo.

Run the build

Make sure that your current working directory is inside your Bazel workspace:


Now, enter the following to build the sample app:

bazel build //backend:backend

Bazel now launches and builds the sample app. During the build process, its output will appear similar to the following:

INFO: Found 1 target...
Target //backend:backend up-to-date:
INFO: Elapsed time: 56.867s, Critical Path: 2.72s

Find the build outputs

The .war file and other outputs are located in the $WORKSPACE/bazel-bin/backend directory.

In particular, the appengine_war rule generates scripts that you can use to run your backend locally or deploy it to Google App Engine:

Run the application on a local development server

Here, you'll start a local App Engine development server in your environment and run your application on it.

To run the application, enter the following:

bazel-bin/backend/backend --port=12345

Your application will be available at http://localhost:12345

Deploy to Google App Engine

You can also deploy the application to the live App Engine serving environment on Google Cloud Platform. For this scenario, you must first create a new Cloud Platform project and App Engine application using the Google Cloud Platform Console. Follow this link to perform these actions.

Build the target that allows to deploy to App Engine:

bazel build --java_toolchain=@io_bazel_rules_appengine//appengine:jdk7 //backend:backend.deploy

Then, to deploy the application, enter the following:

bazel-bin/backend/backend.deploy <project-id>

The deployment script prompts you to authorize access to Google Cloud Platform. After you have authorized access the first time, you can deploy the application using the bazel command and the following rule target:

bazel run //backend:backend.deploy <project-id>

Your application URL will be http://<project-id>

What's next

Now let's review the tutorial steps.