Bazel is an open-source build and test tool similar to Make, Maven, and Gradle. The Google-internal version of Bazel, Blaze, is used to build major apps such as Google Maps and YouTube. Bazel uses a human-readable, high-level build language that supports multiple languages and platforms. There is virtually no limit on the size of the codebase or the number of uses that Bazel supports.
Note that Bazel is currently in Beta - it is not yet covered by a deprecation policy, may be subject to backward-incompatible changes, and may be missing some features.
In this codelab you will learn how to use Bazel as a core tool in your Android development flow. You will learn:
You'll need Bazel and Android Studio to complete this codelab. Skip ahead to "Get the sample project" if these are already installed.
Follow the installation instructions to install Bazel and its dependencies.
Download and install Android Studio as described in Install Android Studio. Also install the Bazel plugin.
The installer does not automatically set the
ANDROID_HOME variable. Set it to the location of the Android SDK, which defaults to
For convenience, add the above statement to your
You'll need to get the sample project from GitHub. The repo has two directories:
sample directory contains the base sample app code, while the
solution directory contains the finished project with the completed Bazel
BUILD files. You can use this directory to check your work.
Enter the following at the command line:
cd $HOME git clone https://github.com/googlecodelabs/bazel-android-intro
git clone command creates directories named
$HOME/bazel-android-intro/solution. We'll be working in the
A workspace is a directory that contains the source files for one or more software projects, as well as a
WORKSPACE file and
BUILD files that contain the instructions that Bazel uses to build the software.
A workspace directory can be located anywhere on your filesystem and is denoted by the presence of the
WORKSPACE file at its root. In this codelab, your workspace directory is
$HOME/bazel-android-intro/sample/, which contains the sample project files you cloned from the GitHub repo in the previous step.
Note that Bazel itself doesn't make any requirements about how you organize source files in your workspace. The sample source files in this codelab are organized according to Android conventions.
For your convenience, set the
$WORKSPACE environment variable now to refer to your workspace directory. At the command line, enter:
export WORKSPACE=$HOME/bazel-android-intro/sample cd $WORKSPACE
Every workspace must have a text file named
WORKSPACE located in the top-level workspace directory. For some languages the
WORKSPACE file can be empty, but for Android projects Bazel requires the Android SDK build tools to be configured. These are used to build your Android application.
And add the following lines:
android_sdk_repository( name = "androidsdk", path = "/Users/codelab/Library/Android/sdk", )
This will use the Android SDK referenced by
path, and automatically detect the highest API level and the latest version of build tools installed within that location.
Alternatively, you can explicitly specify the API level and the version of build tools to use by including the
build_tools_version attributes. You can specify any subset of these attributes (but isn't required for this codelab):
android_sdk_repository( name = "androidsdk", path = "/path/to/Android/sdk", api_level = 27, build_tools_version = "26.0.1", )
Optional: This is not required by this codelab, but if you want to compile native code into your Android app, you also need to download the Android NDK and tell Bazel where to find it by adding the following rule to your
android_ndk_repository( name = "androidndk", )
api_level is the version of the Android API the SDK and the NDK target (for example, 23 for Android 6.0 and 25 for Android 7.1). If not explicitly set,
api_level will default to the highest available API level for
android_ndk_repository. It's not necessary to set the API levels to the same value for the SDK and NDK. This web page contains a map from Android releases to NDK-supported API levels.
android_sdk_repository, the path to the Android NDK is inferred from the
ANDROID_NDK_HOME environment variable by default. The path can also be explicitly specified with a
path attribute on
Verify your workspace and SDK configuration by running the executing the following command:
bazel fetch @androidsdk//...
If your workspace configuration is correct you will see no console output. If there is an error with your SDK configuration you will see an error message.
Let's take a look at the source files for the app. These are located in
The key files and directories are:
Activity source file
Resource file directory
Note that you're just looking at these files now to become familiar with the structure of the app. You don't have to edit any of the source files to complete this codelab.
BUILD file is a text file that describes the relationship between a set of build outputs -- for example, compiled software libraries or executables -- and their dependencies. These dependencies may be source files in your workspace or other build outputs.
BUILD files are written in Skylark, a Python-inspired language.
In Bazel, adding a
BUILD file to a directory in your workspace turns it into a package. Adding
BUILD files to subdirectories of the package turns them into subpackages. This forms the package hierarchy. The package hierarchy is a logical structure that overlays the directory structure in your workspace. Each package is a directory (and its subdirectories) that contains a related set of source files and a
BUILD file. The package also includes any subdirectories, excluding those that contain their own
BUILD file. The package name is the name of the directory where the
BUILD file is located.
Note that this package hierarchy is distinct from, but coexists with, the Java package hierarchy for your Android app.
In most cases, each directory in your project will have its own
BUILD file. For this project we will create two
BUILD files: one for the common library and one for the main application.
Let's start with the common library. At a command-line prompt, open a new
BUILD file for editing:
BUILD file contains several different types of instructions for Bazel. The most important type is the build rule, which tells Bazel how to build an intermediate or final software output from a set of source files or other dependencies.
Bazel provides two primary build rules,
android_binary, that you can use to build an Android app. For this codelab, you'll first use the
android_library rule to tell Bazel how to build an Android library module from the app source code and resource files. Then you'll use the
android_binary rule to tell it how to build the Android application package.
Add the following to your
android_library( name = "media", srcs = glob(["*.java"]), visibility = ["//mediarecorder:__subpackages__"], )
As you can see, the
android_library build rule contains a set of attributes that specify the information that Bazel needs to build a library module from the source files. It contains:
name: unique name (within this BUILD file) for this target
srcs: list of source files (
visibility: controls which other packages can depend on this rule
name attribute forms part of the target
label, which is a combination of the package name (
mediarecorder/java/com/example/android/common/media) and the name (
media). The full label for this target is thus
//mediarecorder/java/com/example/android/common/media:media. Labels are used to reference targets from other rules, as you'll see in the next section.
We set the
visibility attribute to allow all subpackages of our project,
//mediarecorder, to depend on this rule. There are a number of built-in visibility labels, such as
//visibility:public which allows anyone to depend on the target. In most cases you will want to specify which packages are allowed to depend on your targets, to provide proper encapsulation for modules in your project.
Save and close this file.
At a command-line prompt, open a new
BUILD file for editing:
Add the following to your
android_library( name = "main", srcs = ["MainActivity.java"], manifest = "AndroidManifest.xml", resource_files = glob(["res/**"]), deps = [ "//mediarecorder/java/com/example/android/common/media:media", ], )
Once again we create an
android_library rule, but this time we supply more information about the application by referencing the manifest and the resources. Here, the
deps attribute references the output of the
media rule you added to the
BUILD file above. This means that when Bazel builds the output of this rule, it first checks whether the output of the
media library rule has been built and is up-to-date. If not, it builds it and then uses that output to build the main library.
Save this file, but don't close. We'll be adding another rule in the next step.
android_binary rule builds the Android application package (
.apk file) for your app.
Add the following to the same
BUILD file from above:
android_binary( name = "mediarecorder", manifest = "AndroidManifest.xml", resource_files = glob(["res/**"]), deps = [":main"], )
android_binary rule references one or multiple
android_library rules, and will package them all into a single APK file. In this case we just reference the
main library defined above.
Now, save and close the file. You can compare both
BUILD files to the completed examples in the
solutions directory of the GitHub repo.
So far we have created targets using the
android_binary rules for our own source code. But what if you want to use an external library? At a high level there are two ways to depend on external libraries, and while we don't need any external libraries for this codelab, it's useful to be familiar with these rules.
Bazel provides the
aar_import rule that allows an
android_library to directly depend on local
.aar files. For example:
aar_import( name = "some-sdk", aar = "path/to/sdk-1.0.aar", ) android_library( name = "some_library", srcs = glob(["*.java"]), deps = [":some-sdk"], )
Bazel can also depend on targets from other projects known as external dependencies. Both Bazel and non-Bazel projects are supported and Bazel is able to reference external dependencies on remote source repositories such as GitHub and Maven. For details, see the external dependencies topic on the Bazel site.
You use the
bazel command-line tool to run builds, execute unit tests and perform other operations in Bazel. During installation you probably added this location to your path.
Before you build the app, make sure your current working directory is inside your Bazel workspace. Then execute a
bazel build command:
cd $WORKSPACE bazel build //mediarecorder/java/com/example/android/mediarecorder:mediarecorder
build subcommand instructs Bazel to build the target that follows. The target is specified as the name of a build rule inside a
BUILD file, with along with the package path relative to your workspace directory. Note that you can sometimes omit the package path or target name, depending on your current working directory at the command line and the name of the target.
You can run a
bazel build command on any target in your
BUILD file, not just an
android_binary target. This can be useful during development to isolate build errors and when you don't want to build your entire app to validate a change.
You will see terminal output during the build process, with the final output appearing similar to the following:
INFO: Analysed target //mediarecorder/java/com/example/android/mediarecorder:mediarecorder (29 packages loaded). INFO: Found 1 target... Target //mediarecorder/java/com/example/android/mediarecorder:mediarecorder up-to-date: bazel-bin/mediarecorder/java/com/example/android/mediarecorder/mediarecorder_deploy.jar bazel-bin/mediarecorder/java/com/example/android/mediarecorder/mediarecorder_unsigned.apk bazel-bin/mediarecorder/java/com/example/android/mediarecorder/mediarecorder.apk INFO: Elapsed time: 12.299s, Critical Path: 10.59s INFO: Build completed successfully, 85 total actions
Bazel stores the outputs of both intermediate and final build operations in a set of per-user, per-workspace output directories. These directories are symlinked from the following locations:
$WORKSPACE/bazel-bin, which stores binary executables and other runnable build outputs
$WORKSPACE/bazel-genfiles, which stores intermediary source files that are generated by Bazel rules
$WORKSPACE/bazel-out, which stores other intermediate build outputs
Most of the time you will only be interested in
bazel-bin as it contains the final build outputs.
The console output from
bazel build above includes paths to the key output artifacts. In this case,
bazel-bin/mediarecorder/java/com/example/android/mediarecorder/mediarecorder.apk is the file we are most interested in.
Once your device is properly configured for use with the Android Debug Bridge (
adb), you can install this output apk on your device (either physical or emulated) using
The "standard" development flow of edit, build, push can be slow. Bazel's
mobile-install functionality introduces an incremental build-and-deploy flow to accelerate and simplify the process of iterating on your application. In a single command, you can build and sync just the changes you made instead of rebuilding and reinstalling the entire app.
To use this functionality, replace the
bazel build commands with
bazel mobile-install on your
android_binary target. For example:
bazel mobile-install //mediarecorder/java/com/example/android/mediarecorder:mediarecorder --start_app
This command performs an incremental build, taking advantage of a number of optimizations to the Android build process. Bazel
mobile-install will build and push a diffset of resources and
.dex files rather than building a single
You can build your project directly from Android Studio by using the Bazel plugin, which you installed in the first step of this codelab.
Note that Bazel is not currently compatible with Android Studio 3.1. Expect to see this support soon, but in the meantime you can use Android Studio 3.0 to develop with Bazel.
Complete the following steps to create a new Bazel project from a BUILD file:
WORKSPACEpath by browsing to the directory containing the
WORKSPACEfile we created earlier.
BUILDfile path relative to the
directories: mediarecorder/java/com/example/android/mediarecorder targets: //mediarecorder/java/com/example/android/mediarecorder/...:all additional_languages: # Uncomment any additional languages you want supported # c # python android_sdk_platform: android-27
This step only needs to be completed once. In the future, you can re-import this project definition and manually sync by clicking Sync Project with BUILD Files.
Complete the following steps to build your app using Bazel:
BUILDfile and double click to open it.
android_binarytarget and click Run.
Step 3 creates a
Run Configuration for your target. This can be customised via the Run -> Edit Configurations menu for advanced settings.
Congratulations! You've completed the Building Android Apps with Bazel codelab.
android_librarytarget for each module in your application
android_binarytarget for your application