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 on the android_ndk_repository rule, see the Build Encyclopedia entry.

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 STL

To configure the C++ STL, use the flag --android_crosstool_top.

bazel build //:app --android_crosstool_top=target label

The available STLs in @androidndk are:

For r16 and below, the default STL is gnustl. For r17 and above, it is libc++. For convenience, the target @androidndk//:default_crosstool is aliased to the respective default STLs.

Please note that from r18 onwards, STLport and gnustl will be removed, making libc++ the only STL in the NDK.

See the NDK documentation for more information on these STLs.

Configuring the target ABI

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

bazel build //:app --fat_apk_cpu=comma-separated list of ABIs

By default, Bazel builds native Android code for armeabi-v7a. To build for x86 (such as for emulators), pass --fat_apk_cpu=x86. To create a fat APK for multiple architectures, you can specify multiple CPUs: --fat_apk_cpu=armeabi-v7a,x86.

If more than one ABI is specified, Bazel will build an APK containing a shared object for each ABI.

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
)

Integration with platforms and toolchains

Bazel's configuration model is moving towards platforms and toolchains. If your build uses the --platforms flag to select for the architecture or operating system to build for, you will need to pass the --extra_toolchains flag to Bazel in order to use the NDK.

For example, to integrate with the android_arm64_cgo toolchain provided by the Go rules, pass --extra_toolchains=@androidndk//:all in addition to the --platforms flag.

bazel build //my/cc:lib \
  --platforms=@io_bazel_rules_go//go/toolchain:android_arm64_cgo \
  --extra_toolchains=@androidndk//:all

You can also register them directly in the WORKSPACE file:

android_ndk_repository(name = "androidndk")
register_toolchains("@androidndk//:all")

Registering these toolchains tells Bazel to look for them in the NDK BUILD file (for NDK 20) when resolving architecture and operating system constraints:

toolchain(
  name = "x86-clang8.0.7-libcpp_toolchain",
  toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
  target_compatible_with = [
      "@platforms//os:android",
      "@platforms//cpu:x86_32",
  ],
  toolchain = "@androidndk//:x86-clang8.0.7-libcpp",
)

toolchain(
  name = "x86_64-clang8.0.7-libcpp_toolchain",
  toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
  target_compatible_with = [
      "@platforms//os:android",
      "@platforms//cpu:x86_64",
  ],
  toolchain = "@androidndk//:x86_64-clang8.0.7-libcpp",
)

toolchain(
  name = "arm-linux-androideabi-clang8.0.7-v7a-libcpp_toolchain",
  toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
  target_compatible_with = [
      "@platforms//os:android",
      "@platforms//cpu:arm",
  ],
  toolchain = "@androidndk//:arm-linux-androideabi-clang8.0.7-v7a-libcpp",
)

toolchain(
  name = "aarch64-linux-android-clang8.0.7-libcpp_toolchain",
  toolchain_type = "@bazel_tools//tools/cpp:toolchain_type",
  target_compatible_with = [
      "@platforms//os:android",
      "@platforms//cpu:aarch64",
  ],
  toolchain = "@androidndk//:aarch64-linux-android-clang8.0.7-libcpp",
)

How it works: introducing Android configuration transitions

The android_binary rule can explicitly ask Bazel to build its dependencies in an Android-compatible configuration so that the Bazel build just works without any special flags, except for --fat_apk_cpu and --android_crosstool_top for ABI and STL configuration.

Behind the scenes, this automatic configuration uses Android configuration transitions.

A compatible rule, like android_binary, automatically changes the configuration of its dependencies to an Android configuration, so only Android-specific subtrees of the build are affected. Other parts of the build graph are processed using the top-level target configuration. It may even process a single target in both configurations, if there are paths through the build graph to support that.

Once Bazel is in an Android-compatible configuration, either specified at the top level or due to a higher-level transition point, additional transition points encountered do not further modify the configuration.

The only built-in location that triggers the transition to the Android configuration is android_binary's deps attribute.

For example, if you try to build an android_library target with a cc_library dependency without any flags, you may encounter an error about a missing JNI header:

ERROR: project/app/src/main/BUILD.bazel:16:1: C++ compilation of rule '//app/src/main:jni_lib' failed (Exit 1)
app/src/main/cpp/native-lib.cpp:1:10: fatal error: 'jni.h' file not found
#include <jni.h>
         ^~~~~~~
1 error generated.
Target //app/src/main:lib failed to build
Use --verbose_failures to see the command lines of failed build steps.

Ideally, these automatic transitions should make Bazel do the right thing in the majority of cases. However, if the target on the Bazel command-line is already below any of these transition rules, such as C++ developers testing a specific cc_library, then a custom --crosstool_top must be used.

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 --crosstool_top, --cpu and --host_crosstool_top flags.

For example:

bazel build //my/cc/jni:target \
      --crosstool_top=@androidndk//:default_crosstool \
      --cpu=<abi> \
      --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

In this example, the top-level cc_library and cc_binary targets are built using the NDK toolchain. However, this causes Bazel's own host tools to be built with the NDK toolchain (and thus for Android), because the host toolchain is copied from the target toolchain. To work around this, specify the value of --host_crosstool_top to be @bazel_tools//tools/cpp:toolchain to explicitly set the host's C++ toolchain.

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 --crosstool_top=@androidndk//:default_crosstool
common:android_x86 --cpu=x86
common:android_x86 --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

common:android_armeabi-v7a --crosstool_top=@androidndk//:default_crosstool
common:android_armeabi-v7a --cpu=armeabi-v7a
common:android_armeabi-v7a --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

# In general
common:android_<abi> --crosstool_top=@androidndk//:default_crosstool
common:android_<abi> --cpu=<abi>
common:android_<abi> --host_crosstool_top=@bazel_tools//tools/cpp:toolchain

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.