Intro

update 09.03.2015: this solution sucks, try a new one

If you use scala for android development, you most likely need proguard to work around limitations of dex format. But shrinking takes quite an amount of build time, for instance for my project I got these numbers:

with shrinking

compile	10s
proguard	18s
dex	13s
other	9s
total	50s

without shrinking

compile	9s
dex	15s
other	10s
total	34s

So removing proguard saves a third of a build time. It is absolutely not nececcary to perform it each build. Having apk built with proguard you may extract scala library from it and use instead of the origin one without proguard. Enjoy until you face NoSuchMethodError or NoClassDefFoundError (which means you used scala feature you didn’t use before), repeat that all over again. That might be not handy, if we couldn’t automate the process, but I managed to set up maven to get the work done and will show you right that.

Sample project

There is an example project where you can see all the details

git clone https://github.com/yaroslav-ulanovych/pre-proguard-scala-library-for-android-with-maven-example
cd pre-proguard-scala-library-for-android-with-maven-example

Build instructions

You should be able to build android applications with maven. In short you need to install android sdk, then clone Maven Android SDK Deployer and perform mvn install in folders maven-android-sdk-deployer/platforms/android-15 and maven-android-sdk-deployer/extras/compatibility-v4. Also dex2jar should be installed and available in path.

# first install the parent pom
# -N says not to install submodules
mvn install -N

# install other modules

cd androidapp-deps
mvn install

# build android app with proguard
cd ../androidapp
mvn install -P proguard

# check that it works
mvn android:deploy android:run -P proguard

# once we have proguarded apk
# we can build our scala library
cd ../scala-library
mvn install

# and build android app without proguard
cd ../androidapp
# notice the absence of -P proguard
mvn package

# check that it works
mvn android:deploy android:run

Maven configuration

Project consists of a parent project and three modules: scala-library, androidapp and androidapp-deps. There is nothing interesting about parent project, it’s ordinary, just common settings for submodules. Real magic happens in androidapp, androidapp-deps and scala-library modules (and if maven was a bit smarter, we could get rid of andoidapp-deps module too).

scala-library module

Module produces a jar artifact which contains shrunk scala library. That is just five console commands but in maven it’s a hundred of lines of xml. There may exist a nicer way, but I’m not a maven expert. Nevertheless, it does the job.

Tool that I use to convert dex to jar names differently in Windows and Linux, fixing that.

...
<profiles>
    <profile>
        <id>Windows</id>
        <activation><os><family>Windows</family></os></activation>
        <properties><dex2jarExecutable>d2j-dex2jar.bat</dex2jarExecutable></properties>
    </profile>
    <profile>
        <id>Linux</id>
        <activation><os><family>Linux</family></os></activation>
        <properties><dex2jarExecutable>d2j-dex2jar.sh</dex2jarExecutable></properties>
    </profile>
</profiles>
...

Don’t pay attention at phases plugins are bound to, I had problems running the same plugin twice in one phase in correct order, so just put them in different phases.

First, copy proguarded apk from local repository to build directory

...
<artifactId>maven-dependency-plugin</artifactId>
...
        <id>copy apk</id>
...
                <artifactItem>
                    <groupId>com.example.preproguard</groupId>
                    <artifactId>androidapp</artifactId>
                    <version>${version}</version>
                    <classifier>proguard</classifier>
                    <type>apk</type>
                    <overWrite>true</overWrite>
                    <outputDirectory>${project.build.directory}</outputDirectory>
                    <destFileName>androidapp.apk</destFileName>
                </artifactItem>
...

unzip apk

...
<artifactId>maven-antrun-plugin</artifactId>
...
                <unzip src="${project.build.directory}/androidapp.apk" dest="${project.build.directory}/apk" />
...

convert classes.dex to jar via dex2jar

...
<artifactId>exec-maven-plugin</artifactId>
...
        <id>dex2jar</id>
...
                <argument>${project.build.directory}/apk/classes.dex</argument>
                <argument>-o</argument>
                <argument>${project.build.directory}/classes.jar</argument>
...

unzip classes

...
<artifactId>maven-antrun-plugin</artifactId>
...
        <id>unzip classes.jar</id>
...
                <unzip src="${project.build.directory}/classes.jar" dest="${project.build.outputDirectory}"/>
...

remove everything but scala library classes

...
<artifactId>exec-maven-plugin</artifactId>
...
        <id>remove not scala-library classes</id>
...
            <executable>rm</executable>
            <arguments>
                <argument>-rf</argument>
                <argument>${project.build.outputDirectory}/com</argument>
                <argument>${project.build.outputDirectory}/android</argument>
...

maven jar plugin will do the rest.

androidapp module

By default it should depend on shrunk scala library and via proguard profile on the origin one. But the problem with that is that in maven we can’t just exclude dependencies in profile, but we can do that with transitive ones and that is what androidapp-deps module is for. It holds dependencies of the androidapp module so that we can exlude shrunk scala library in proguard profile

...
<dependency>
    <groupId>com.example.preproguard</groupId>
    <artifactId>androidapp-deps</artifactId>
    <version>1.0-SNAPSHOT</version>
    <type>pom</type>
    <exclusions>
        <exclusion>
            <groupId>org.scala-lang</groupId>
            <artifactId>scala-library</artifactId>
        </exclusion>
...

Exclude shrunk scala library and include original one in proguard profile

...
<profiles>
    <profile>
        <id>proguard</id>
        <dependencies>
            <dependency>
                <groupId>org.scala-lang</groupId>
                <artifactId>scala-library</artifactId>
                <version>${scalaVersionFull}</version>
            </dependency>
            <dependency>
                <groupId>com.example.preproguard</groupId>
                <artifactId>androidapp-deps</artifactId>
                <version>1.0-SNAPSHOT</version>
                <type>pom</type>
                <exclusions>
                    <exclusion>
                        <groupId>com.example.preproguard</groupId>
                        <artifactId>scala-library</artifactId>
                    </exclusion>
                </exclusions>
...

Enable proguard

...
<proguard><skip>false</skip></proguard>
...

Give proguarded apk a classifier to distinct it from a full one

...
<classifier>proguard</classifier>
...

That’s almost it, just one strange moment more

...
<dependency>
    <groupId>org.scala-lang</groupId>
    <artifactId>scala-library</artifactId>
    <version>${scalaVersionFull}</version>
    <scope>provided</scope>
</dependency>
...

I had troubles compiling the project without original scala library in dependencies, cause scala maven plugin was complaining about missing scala library. I managed to satisfy it, supplying scala version explicitly, but not intellij idea. So I just added scala library with provided scope.

Conclusions

This is not a perfect solution, cause it complicates project structure, and is not fully automated, but it works, it saves my time, and hopefully will save yours. If there is a plugin or a build tool that does job better, please let me know.