bazel-for-android
├── app
│   
├── my-feature
│   └── src
│       └── io
│           └── morfly
│               └── bfa
│
└── WORKSPACE

Now, we are ready to write some code.

Under a my-feature/src/io/morfly/bfa directory create a MyFeature.java file and fill it with the following source code:

MyFeature class contents

Next, in the my-feature directory create a file named BUILD. It will contain all the build configurations for the new feature module. Add there the code below:

my-feature/BUILD file contents

We are using android_library Bazel rule to create a target that builds our module. In the simplest use case, it only has to have a unique name and list of source files (srcs) specified.

That’s it. Now we are ready to build it. To do it, run the command below in your terminal:

bazelisk build //my-feature:my-feature

If all is done correctly, the build should be successfully completed. Below is the file structure of our project so far.

bazel-for-android
├── app
│   
├── my-feature
│   ├── src
│   │   └── io/morfly/bfa
│   │       └── MyFeature.java
│   │
│   └── BUILD
│
└── WORKSPACE

Multiple modules in a single BUILD file

Bazel is flexible enough to let you define multiple modules in the same BUILDfile.

Let’s say we want to define another feature that does the same as the previous one, but for the sake of example, uses resources from strings.xml file instead of a hardcoded string.

We can define a new MyFeatureWithRes class that would refer to R.string.my_feature_info resource string. To do this:

• Under the my-feature/src/io/morfly/bfa directory create a new file MyFeatureWithRes.java.
• After that, create a my-feature/res directory that would hold Android resources.
• Finally, under the my-feature/res/values directory create a strings.xml file.

You can find the contents of MyFeatureWithRes.java and strings.xml files below.

MyFeatureWithRes class that uses string resources

Now we need to configure this module with Bazel. Open my-feature/BUILD file and add there a new my-feature-with-res target as shown below:

Adding another target to the same my-feature/BUILD file

As you can see, this target has more params to configure. In addition to name and srcs it has:

• resource_files — that defines Android resources to be used in the module.
• manifest — which is mandatory when defining resource_files param.
• custom_package — is also required to define a correct Java package that is used for R class generation.

Finally, we need to actually create a manifest file that we’ve just specified. Under my-feature directory create AndroidManifest.xml file and fill it with the content below.

my-feature/AndroidManifest.xml file content

Now, we are ready to build this module. To do this, run the following command in your terminal:

bazelisk build //my-feature:my-feature-with-res

If all is done correctly, the build should be successfully completed. Below you can find an updated file structure of the project.

bazel-for-android
├── app
│   
├── my-feature
│   ├── res
│   │   └── values
│   │       └── strings.xml
│   ├── src
│   │   └── io/morfly/bfa
│   │       ├── MyFeature.java
│   │       └── MyFeatureWithRes.java
│   │
│   ├── AndroidManifest.xml
│   └── BUILD
│
└── WORKSPACE

Running the app

Now, that we created brand new feature modules, we need to include them in the application target. To do so, follow these steps:

• Open app/BUILD file.
• Add //my-feature:my-feature and //my-feature:my-feature-with-res as depsparam to the bin Bazel target, as shown below.

Updated app/BUILD file contents

Now, let’s use our newly created modules in the source code of the app. Open app/src/io/morfly/bfa/MainActivity.java file and update its source code as shown below.

Updated MainActivity.java file contents

As you can see, we instantiate both of our features and display information about them on a TextView.

Now, let’s run the app and see the result of our work. Run the command below.

bazelisk mobile-install //app:bin --start_app

Don’t worry if the build has failed, as this is expected behavior. Your build actually should fail with these 2 errors:

ERROR: /.../bazel-for-android/app/BUILD:3:15: in android_binary rule //app:bin: target '//my-feature:my-feature' is not visible from target '//app:bin'. Check the visibility declaration of the former target if you think the dependency is legitimate
ERROR: /.../bazel-for-android/app/BUILD:3:15: in android_binary rule //app:bin: target '//my-feature:my-feature-with-res' is not visible from target '//app:bin'. Check the visibility declaration of the former target if you think the dependency is legitimate

If we read carefully the error message we can notice that our newly created modules are not visible to our binary target.

This is due to a concept in Bazel called visibility. It controls whether a target can be used (depended on) by targets in other packages. Learn more about it from the official documentation.

By default, all Bazel targets have private visibility unless otherwise is specified. To fix this we need to explicitly make our module targets public. There are two ways to do it:

• Set visibility for each target specifically using visibility param.
• Set default visibility for all targets in the package (declared in the same BUILDfile). This can be done using package function by specifying default_visibility param. We will go with this approach.

To do so, open my-feature/BUILD file and add the following line right below all load statements.

package(default_visibility = ["//visibility:public"])

The complete source code of a my-feature/BUILD file is shown below.

Complete contents of my-feature/BUILD file

Now, we are finally ready to build and launch the app:

bazelisk mobile-install //app:bin --start_app

Making my-feature a Java-only library

You can find a complete source code of a my-feature/BUILD file below.

Complete contents of my-feature/BUILD file

Now, we can run the app again and verify that it still builds and launches successfully.

bazelisk mobile-install //app:bin --start_app

Conclusion

In this blog post, we’ve discovered how to create multimodule applications built with Bazel by creating new Android and Java feature modules.

You can find source code on GitHub at the link below.

In the next part, we will see how to use a Kotlin language in Bazel Android projects.

This article was originally published on proandroiddev.com on February 21, 2021