Bazel: Android target for a _transitive closure_ library aar

[mrdomino]

One of my work projects is an Android library with a substantial JNI footprint, designed for inclusion in third-party Android apps. I'd love to be able to specify it as a single bazel target (say for an aar, which seems to be what the Android people want us to generate these days) that depends on my android_library and cc_library.

My current planned stopgap is to build an android_binary and then take the .jar and .so out of the target for redistribution. Improvements and suggestions welcome.

[aj-michael]

android_library has an implicit AAR output called name.aar.

As an example,

$ cat java/com/foo/BUILD
cc_library(
    name = "cc_lib",
    srcs = ["foo.cc"],
)
android_library(
    name = "android_lib",
    srcs = ["Foo.java"],
    deps = [":cc_lib"],
)
$ bazel build //java/com/foo:android_lib.aar
INFO: Found 1 target...
Target //java/com/foo:android_lib.aar up-to-date:
  bazel-bin/java/com/foo/android_lib.aar
INFO: Elapsed time: 4.138s, Critical Path: 3.59s
$

[aj-michael]

Augh, I should not have closed this. Per the documentation of android_library:

name.aar: An android 'aar' bundle containing the java archive and resources of this target. It does not contain the transitive closure.

https://bazel.build/versions/master/docs/be/android.html#android_library

So the .aar output of android_library will not contain the .so from the cc_library.

I'm going to use this as the tracking bug for support transitive-closure-aars from android_library.

[andrewharp]

This would also be useful for TensorFlow so that we can build a full Android distributable in a single file (using only Bazel) during our nightly build process.

edit: We'd like to be able to package all relevant architectures in the same AAR, much like --fat_apk_cpu packages multiple target architectures in APKs.

[spinorx]

+1. Any update of expected support for this?

[ahumesky]

We're working on getting our testing for android apps in better shape, this is something we'll likely work on after that.

[steeve]

hey guys, i'm having another issue: the cc_library on which my android_library depends is not built when building via name.aar.
And when I build via name, it seems it is not built for the android ndk (unable to include jni.h, for instance)

[balazsbanyai]

+1

[spinorx]

Use sample in https://github.com/spinorx/grpc_andr_ios to unblock urself till this is fixed.

[balazsbanyai]

@spinorx thank you very much. I can build the app using the provided project, but the android_library target (eg. bazel build --config android //android:GreeterClient) won't build in this example either.

[spinorx]

Yeah, android_library does not propogate needed config to cc_library yet. See detailed discussion in: https://github.com/bazelbuild/bazel/issues/3924

[aj-michael]

@steeve, see my response in https://github.com/bazelbuild/bazel/issues/4407#issuecomment-356658776 if you want to build the cc_library for Android directly. Note that that will not get you a .so. To build a .so outside of android_binary, you need a cc_binary.

[steeve]

@aj-michael thanks! however, I'm trying to build an aar (not an apk) file that depends on the cc_library.
I would assume the toolchain configuration to be transitive to the cc_library with android_library the way android_binary does.

Also, there is the case of fat apks, which is better handled by building for the Android env via android_{binary,library} rules imho.

[aj-michael]

Yes, I was responding to your comment:

And when I build via name, it seems it is not built for the android ndk (unable to include jni.h, for instance)

android_library .aar output does not currently support bundling native libraries. This is a known deficiency that I believe @dkelmer has plans to work on.

If you're so inclined, you could workaround this deficiency by creating an additional android_binary and a genrule for the android_library, something like this:

genrule(
    name = "aar_with_jni",
    srcs = [
        ":abin_unsigned.apk",
        ":alib.aar",
    ],
    outs = ["alib_with_jni.aar"],
    cmd = """
cp $(location :alib.aar) $(location :alib_with_jni.aar)
chmod +w $(location :alib_with_jni.aar)
origdir=$$PWD
cd $$(mktemp -d)
unzip $$origdir/$(location :abin_unsigned.apk) "lib/*"
cp -r lib jni
zip -r $$origdir/$(location :alib_with_jni.aar) jni/*/*.so
""",
)

android_binary(
    name = "abin",
    manifest = "AndroidManifest.xml",
    custom_package = "does.not.matter",
    deps = [":alib"],
)

android_library(
    name = "alib",
    srcs = ["A.java"],
    deps = [":clib"],
)

cc_library(
    name = "clib",
    srcs = ["c.cc"],
)

[steeve]

nice, I hadn't though about this! thanks @aj-michael

[aj-michael]

This is a common enough request (JNI in .aar) that I decided to throw together an example repository for how to workaround it:

https://github.com/aj-michael/aar_with_jni

[andrewharp]

FWIW you can just use zip -ur to add files to an existing archive (we do this to create the TensorFlow AAR for release), no need to unzip first.

[jcayzac]

@aj-michael adding to this discussion, in my understanding the aar produced by bazel are also "incomplete" in the sense there is no (obvious) way to specify a _consumer_ proguard file yet.

Should I file a separate issue for this, or better keep it here together with the related jni problem?

[aj-michael]

@jcayzac , please file a separate issue. If I understand what you mean, that is probably a much easier problem to solve. The JNI issue is a little tricky because at the moment android_library doesn't do any linking.

[cpsauer]

Ran across this while looking for a workaround for our team. Can confirm that there are still people who'd use this :) (looks like Tensorflow/MediaPipe are also using AJ's good workaround.)

Any updates on official support?

[steeve]

Once starlark rules_android lands (they are in alpha iirc), it'll be much easier to add it