Using the Android Native Development Kit with Bazel

If you're new to Bazel, please start with the Building Android with Bazel tutorial.

Overview

Bazel can run in many different build configurations, including several that use the Android Native Development Kit (NDK) toolchain. This means that normal cc_library and cc_binary rules can be compiled for Android directly within Bazel. Bazel accomplishes this by using the android_ndk_repository repository rule.

Prerequisites

Please ensure that you have installed the Android SDK and NDK.

To set up the SDK and NDK, add the following snippet to your WORKSPACE:

android_sdk_repository(
    name = "androidsdk", # Required. Name *must* be "androidsdk".
    path = "/path/to/sdk", # Optional. Can be omitted if `ANDROID_HOME` environment variable is set.
)

android_ndk_repository(
    name = "androidndk", # Required. Name *must* be "androidndk".
    path = "/path/to/ndk", # Optional. Can be omitted if `ANDROID_NDK_HOME` environment variable is set.
)

For more information about the android_ndk_repository rule, see the Build Encyclopedia entry.

If you're using a recent version of the Android NDK (r22 and beyond), use the Starlark implementation of android_ndk_repository. Follow the instructions in its README.

Quick start

To build C++ for Android, simply add cc_library dependencies to your android_binary or android_library rules.

For example, given the following BUILD file for an Android app:

# In <project>/app/src/main/BUILD.bazel

cc_library(
    name = "jni_lib",
    srcs = ["cpp/native-lib.cpp"],
)

android_library(
    name = "lib",
    srcs = ["java/com/example/android/bazel/MainActivity.java"],
    resource_files = glob(["res/**/*"]),
    custom_package = "com.example.android.bazel",
    manifest = "LibraryManifest.xml",
    deps = [":jni_lib"],
)

android_binary(
    name = "app",
    deps = [":lib"],
    manifest = "AndroidManifest.xml",
)

This BUILD file results in the following target graph:

Figure 1. Build graph of Android project with cc_library dependencies.

To build the app, simply run:

bazel build //app/src/main:app

The bazel build command compiles the Java files, Android resource files, and cc_library rules, and packages everything into an APK:

$ zipinfo -1 bazel-bin/app/src/main/app.apk
nativedeps
lib/armeabi-v7a/libapp.so
classes.dex
AndroidManifest.xml
...
res/...
...
META-INF/CERT.SF
META-INF/CERT.RSA
META-INF/MANIFEST.MF

Bazel compiles all of the cc_libraries into a single shared object (.so) file, targeted for the armeabi-v7a ABI by default. To change this or build for multiple ABIs at the same time, see the section on configuring the target ABI.

Example setup

This example is available in the Bazel examples repository.

In the BUILD.bazel file, three targets are defined with the android_binary, android_library, and cc_library rules.

The android_binary top-level target builds the APK.

The cc_library target contains a single C++ source file with a JNI function implementation:

#include <jni.h>
#include <string>

extern "C"
JNIEXPORT jstring

JNICALL
Java_com_example_android_bazel_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

The android_library target specifies the Java sources, resource files, and the dependency on a cc_library target. For this example, MainActivity.java loads the shared object file libapp.so, and defines the method signature for the JNI function:

public class MainActivity extends AppCompatActivity {

    static {
        System.loadLibrary("app");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
       // ...
    }

    public native String stringFromJNI();

}

Configuring the target ABI

To configure the target ABI, use the --android_platforms flag as follows:

bazel build //:app --android_platforms=comma-separated list of platforms

Just like the --platforms flag, the values passed to --android_platforms are the labels of platform targets, using standard constraint values to describe your device.

For example, for an Android device with a 64-bit ARM processor, you'd define your platform like this:

platform(
    name = "android_arm64",
    constraint_values = [
        "@platforms//os:android",
        "@platforms//cpu:arm64",
    ],
)

Every Android platform should use the @platforms//os:android OS constraint. To migrate the CPU constraint, check this chart:

And, of course, for a multi-architecture APK, you pass multiple labels, for example: --android_platforms=//:arm64,//:x86_64 (assuming you defined those in your top-level BUILD.bazel file).

Bazel is unable to select a default Android platform, so one must be defined and specified with --android_platforms.

Depending on the NDK revision and Android API level, the following ABIs are available:

See the NDK docs for more information on these ABIs.

Multi-ABI Fat APKs are not recommended for release builds since they increase the size of the APK, but can be useful for development and QA builds.

Selecting a C++ standard

Use the following flags to build according to a C++ standard:

For example:

bazel build //:app --cxxopt=-std=c++11

Read more about passing compiler and linker flags with --cxxopt, --copt, and --linkopt in the User Manual.

Compiler and linker flags can also be specified as attributes in cc_library using copts and linkopts. For example:

cc_library(
    name = "jni_lib",
    srcs = ["cpp/native-lib.cpp"],
    copts = ["-std=c++11"],
    linkopts = ["-ldl"], # link against libdl
)

Building a cc_library for Android without using android_binary

To build a standalone cc_binary or cc_library for Android without using an android_binary, use the --platforms flag.

For example, assuming you have defined Android platforms in my/platforms/BUILD:

bazel build //my/cc/jni:target \
      --platforms=//my/platforms:x86_64

With this approach, the entire build tree is affected.

These flags can be put into a bazelrc config (one for each ABI), in project/.bazelrc:

common:android_x86 --platforms=//my/platforms:x86

common:android_armeabi-v7a --platforms=//my/platforms:armeabi-v7a

# In general
common:android_<abi> --platforms=//my/platforms:<abi>

Then, to build a cc_library for x86 for example, run:

bazel build //my/cc/jni:target --config=android_x86

In general, use this method for low-level targets (like cc_library) or when you know exactly what you're building; rely on the automatic configuration transitions from android_binary for high-level targets where you're expecting to build a lot of targets you don't control.