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README.md

Rust Android Gradle Plugin

Cross compile Rust Cargo projects for Android targets.

Usage

To begin you must first install the rust toolchains for your target platforms.

rustup target add armv7-linux-androideabi   # for arm
rustup target add i686-linux-android        # for x86
rustup target add aarch64-linux-android     # for arm64
rustup target add x86_64-linux-android      # for x86_64
rustup target add x86_64-unknown-linux-gnu  # for linux-x86-64
rustup target add x86_64-apple-darwin       # for macOS (darwin)
rustup target add x86_64-pc-windows-gnu     # for win32-x86-64-gnu
rustup target add x86_64-pc-windows-msvc    # for win32-x86-64-msvc
...

Add the plugin to your root build.gradle, like:

buildscript {
    repositories {
        maven {
            url "https://plugins.gradle.org/m2/"
        }
    }
    dependencies {
        classpath 'gradle.plugin.org.mozilla.rust-android-gradle:plugin:0.8.0'
    }
}

Next add the cargo configuration to android project. Point to your cargo project using module and add targets. Currently supported targets are arm, arm64, x86, x86_64, and linux-x86-64, darwin, win32-x86-64-gnu, and win32-x86-64-msvc.

cargo {
    module = "../rust"
    targets = ["arm", "x86"]
}

Run the cargoBuild task to cross compile

./gradlew cargoBuild

Configuration

The cargo Gradle configuration accepts many options.

Linking Java code to native libraries

Generated libraries will be added to the Android jniLibs source-sets, when correctly referenced in the cargo configuration through the libname and/or targetIncludes options. The latter defaults to ["lib${libname}.so", "lib${libname}.dylib", "{$libname}.dll"], so the following configuration will include all libbackend libraries generated in the Rust project in ../rust:

cargo {
    module = "../rust"
    libname = "backend"
}

Now, Java code can reference the native library using, e.g.,

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

Native apiLevel

The Android NDK also fixes an API level, which can be specified using the apiLevel option. This option defaults to the minimum SDK API level. As of API level 21, 64-bit builds are possible; and conversely, the arm64 and x86_64 targets require apiLevel >= 21.

Cargo release profile

The profile option selects between the --debug and --release profiles in cargo. Defaults to debug!

Extension reference

module

The path to the Rust library to build with Cargo; required. module is interpreted as a relative path to the Gradle projectDir.

cargo {
    module = '../rust'
}

libname

The library name produced by Cargo; required.

libname is used to determine which native libraries to include in the produced AARs and/or APKs. See also targetIncludes.

libname is also used to determine the ELF SONAME to declare in the Android libraries produced by Cargo. Different versions of the Android system linker depend on the ELF SONAME.

In Cargo.toml:

[lib]
name = "test"

In build.gradle:

cargo {
    libname = 'test'
}

targets

A list of Android targets to build with Cargo; required.

Valid targets are arm, arm64, x86, x86_64 (Android), and 'linux-x86-64', 'darwin', 'win32-x86-64-gnu', and 'win32-x86-64-msvc' (Desktop).

The desktop targets are useful for testing native code in Android unit tests that run on the host, not on the target device. Better support for this feature is planned.

cargo {
    targets = ['arm', 'x86', 'linux-x86-64']
}

verbose

When set, execute cargo build with or without the --verbose flag. When unset, respect the Gradle log level: execute cargo build with or without the --verbose flag according to whether the log level is at least INFO. In practice, this makes ./gradlew ... --info (and ./gradlew ... --debug) execute cargo build --verbose ....

Defaults to null.

cargo {
    verbose = true
}

profile

The Cargo release profile to build.

Defaults to "debug".

cargo {
    profile = 'release'
}

features

Set the Cargo features.

Defaults to passing no flags to cargo.

To pass --all-features, use

cargo {
    features {
        all()
    }
}

To pass an optional list of --features, use

cargo {
    features {
        defaultAnd("x")
        defaultAnd("x", "y")
    }
}

To pass --no-default-features, and an optional list of replacement --features, use

cargo {
    features {
        noDefaultFeatures()
        noDefaultFeatures("x")
        noDefaultFeatures "x", "y"
    }
}

targetDirectory

The target directory into which Cargo writes built outputs.

Defaults to ${module}/target. targetDirectory is interpreted as a relative path to the Gradle projectDir.

cargo {
    targetDirectory = 'release'
}

targetIncludes

Which Cargo outputs to consider JNI libraries.

Defaults to ["lib${libname}.so", "lib${libname}.dylib", "{$libname}.dll"].

cargo {
    targetIncludes = ['libnotlibname.so']
}

apiLevel

The Android NDK API level to target. NDK API levels are not the same as SDK API versions; they are updated less frequently. For example, SDK API versions 18, 19, and 20 all target NDK API level 18.

Defaults to the minimum SDK version of the Android project's default configuration.

cargo {
    apiLevel = 21
}

Specifying NDK toolchains

The plugin looks for (and will generate) per-target architecture standalone NDK toolchains as generated by make_standalone_toolchain.py.

The toolchains are rooted in a single Android NDK toolchain directory. In order of preference, the toolchain root directory is determined by:

  1. rust.androidNdkToolchainDir in the per-(multi-)project ${rootDir}/local.properties
  2. the environment variable ANDROID_NDK_TOOLCHAIN_DIR
  3. ${System.getProperty(java.io.tmpdir)}/rust-android-ndk-toolchains

Note that the Java system property java.io.tmpdir is not necessarily /tmp, including on macOS hosts.

Each target architecture toolchain is named like $arch-$apiLevel: for example, arm-16 or arm64-21.

Specifying local targets

When developing a project that consumes rust-android-gradle locally, it's often convenient to temporarily change the set of Rust target architectures. In order of preference, the plugin determines the per-project targets by:

  1. rust.targets.${project.Name} for each project in ${rootDir}/local.properties
  2. rust.targets in ${rootDir}/local.properties
  3. the cargo { targets ... } block in the per-project build.gradle

The targets are split on ','. For example:

rust.targets.library=linux-x86-64
rust.targets=arm,linux-x86-64,darwin

Specifying paths to sub-commands (Python and Cargo)

The plugin invokes Python and Cargo. In order of preference, the plugin determines what command to invoke for Python by:

  1. rust.pythonCommand in ${rootDir}/local.properties
  2. the environment variable RUST_ANDROID_GRADLE_PYTHON_COMMAND
  3. the default, python

In order of preference, the plugin determines what command to invoke for Cargo by:

  1. rust.cargoCommand in ${rootDir}/local.properties
  2. the environment variable RUST_ANDROID_GRADLE_CARGO_COMMAND
  3. the default, cargo

Paths must be host operating system specific. For example, on Windows:

rust.pythonCommand=c:\Python27\bin\python

On Linux,

env RUST_ANDROID_GRADLE_CARGO_COMMAND=$HOME/.cargo/bin/cargo ./gradlew ...

Passing arguments to cargo

The plugin passes project properties named like RUST_ANDROID_GRADLE_target_..._KEY=VALUE through to the Cargo invocation for the given Rust target as KEY=VALUE. Target should be upper-case with "-" replaced by "_". (See the links from this Cargo issue.) So, for example,

project.RUST_ANDROID_GRADLE_I686_LINUX_ANDROID_FOO=BAR

and

./gradlew -PRUST_ANDROID_GRADLE_ARMV7_LINUX_ANDROIDEABI_FOO=BAR ...

and

env ORG_GRADLE_PROJECT_RUST_ANDROID_GRADLE_ARMV7_LINUX_ANDROIDEABI_FOO=BAR ./gradlew ...

all set FOO=BAR in the cargo execution environment (for the "armv7-linux-androideabi` Rust target, corresponding to the "x86" target in the plugin).

Development

At top-level, the publish Gradle task updates the Maven repository under samples:

$ ./gradlew publish
...
$ ls -al samples/maven-repo/org/mozilla/rust-android-gradle/org.mozilla.rust-android-gradle.gradle.plugin/0.4.0/org.mozilla.rust-android-gradle.gradle.plugin-0.4.0.pom
-rw-r--r--  1 nalexander  staff  670 18 Sep 10:09
samples/maven-repo/org/mozilla/rust-android-gradle/org.mozilla.rust-android-gradle.gradle.plugin/0.4.0/org.mozilla.rust-android-gradle.gradle.plugin-0.4.0.pom

Sample projects

To run the sample projects:

$ ./gradlew -p samples/library :assembleDebug
...
$ ls -al samples/library/build//outputs/aar/library-debug.aar
-rw-r--r--  1 nalexander  staff  8926315 18 Sep 10:22 samples/library/build//outputs/aar/library-debug.aar

Real projects

To test in a real project, use the local Maven repository in your build.gradle, like:

buildscript {
    repositories {
        maven {
            url "file:///Users/nalexander/Mozilla/rust-android-gradle/samples/maven-repo"
        }
    }

    dependencies {
        classpath 'org.mozilla.rust-android-gradle:plugin:0.3.0'
    }
}
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