About executing native code on Android

The primary language for Android being Java, running a native C++ application on this platform actually means that the Java application will be linked to native code using a Java functionality called Java Native Interface. The thing is quite easy if you only need to call C++ functions from Java but it becomes tricky when you want to call Java methods from C++. Thanks to the SDL it is possible to build the application with almost no work in the Java side.

Before detailing how I built the Android application for my game, I would like to describe the basics behind native code usage.

In order to make your native code available in the Java application, you have to build a shared library and import it in the Java side using System.loadLibrary(). On the Java side, the C++ functions must be declared using the native Java keyword. On the C++ side, the function must be named following a specific syntax recalling the full name of the Java class.

Let's take an example. Suppose we have built the shared library libbest-game-ever.so from our C++ code. In the Java side, we write something like this:

package example.native;

public class BestGameEver {

    static {
        // Here I tell Java to link with the shared library.
        System.loadLibrary("best-game-ever");
    }

    // Implemented in the library. You can tell with the "native" keyword.
    private static native doNativeStuff();

    public void doStuff() {
        // There the native code will be executed.
        doNativeStuff();
    }
}

On the C++ side we name the function with the Java prefix followed with an underscore and the full class name where dots are replaced by underscores:

void Java_example_native_BestGameEver_doNativeStuff(JNIEnv* env, jclass jcls)
{
  // yep, I do stuff in C++
}

Calling a Java method from C++ is a bit more tricky. We need to retrieve the class of the object from the JNI environment, then the method to call, then we can call it. If the method has arguments, we have to convert them too.

Let's say we add the public void method BestGameEver.doJavaStuff() in our Java class. Calling the method from C++ would look like this:

void bge::some_class::call_the_java_method
( JNIEnv* env, jobject java_object ) const
{
  // java_object is an instance of BestGameEver. Here we retrieve its Java
  // class
  jclass java_class( env->GetObjectClass(java_object) );

  // From the class we can retrieve the method given its name and signature.
  const jmethodID method_id
    ( env->GetMethodID( java_class, "doJavaStuff", "()V" ) );

  // And we can call the method.
  env->CallVoidMethod( java_object, method_id );
}

Thanks to the SDL, during the initial port of Andy's Super Great Park I did not have to call any function from one language to the other. Only for the second release did I need to call Java functions from C++ for stuff like opening a link in the phone's browser. In case you need to call Java code from C++ too, the SDL provides some utility functions to help us, like:

  • SDL_AndroidGetJNIEnv() returns the JNI environement in which the application runs,
  • SDL_AndroidGetActivity() returns the main Java class of the application.

Preparing the Android project

The SDL2 comes with a template directory structure for a typical native application, named android-project and located at the root of the library's sources.

The first thing to do is to look at its content and to copy this folder in our game directory. Inside the directory we can find, among other things:

  • src/org/libsdl/app/SDLActivity.java: the main Java class of the application. Our application will inherit from it.
  • AndroidManifest.xml The description of our Android application.

Since I use static libraries for my game, I had to edit SDLActivity.java in order to comment out all the System.loadLibrary() lines. You may also want to edit this file if you need to load more SDL libraries, like SDL2_mixer, SDL2_ttf, etc.

Writing the activity class

Then I have created the main application class for my game, named com.stuffomatic.asgp.ASGP and inheriting SDLActivity. As you can see in the commit, the class is quite empty:

package com.stuffomatic.asgp;

import org.libsdl.app.SDLActivity;

public class ASGP extends SDLActivity
{
    static {
        System.loadLibrary( "gnustl_shared" );
        System.loadLibrary( "andy-super-great-park" );
    }
}

The second call to System.loadLibrary() effectively loads the main shared object of the game. Also, since I could not find a way to link statically with the gnustl library, I had to load it there too.

Configuring the application

The application is configured in the AndroidManifest.xml file. The file provided with the SDL sets everything fine and I only had to add few attributes to the <activity> tag:

    <activity android:name="ASGP"
              android:label="@string/app_name"
              android:screenOrientation="landscape"
              android:launchMode="singleInstance"
              android:keepScreenOn="true"
              android:configChanges="orientation|screenSize"
              >

Here is the explanation of the values:

  • name: the name of the class that implements the activity,
  • label: the name of the application, as displayed to the user,
  • screenOrientation: the orientation of the screen. I force the landscape mode to avoid the rotation of the screen in portrait mode, since the game cannot handle it.
  • launchMode: tells how the activity is launched. Since I do not want the game to be relaunched when the user switches back from another application, I chose singleInstance.
  • keepScreenOn: tells to keep the screen on when the application is in the foreground. Without this flag the screen would turn off while the player watches the scripted sequences.
  • configChanges: the configuration changes handled by the application. I had to set orientation in order for the SDL to be able to restore the game correctly when switching back from another application or from the lock screen. According to the documentation, screenSize must be set when orientation is set.

Creating the main shared object file

Because of the way our engine works, Andy's Super Great Park was already built as a shared library. Thus, the work required in order to use it in the Android application was quite low. After the work previously done to port the game (compiling the required libraries, updating to SDL2 and from OpenGL direct mode to GLES), I have been able to compile the various libraries that make the game. Now I only need to link them together in a shared library which will be added in the APK file.

This shared library is built from two source files. The first one is provided with the SDL2, in the source archive at src/main/android/SDL_android_main.c. Since I use C++, I had to surround its content with an extern "C" {} block. The second file contains our main() function:

#include <SDL/SDL_main.h>

int main( int argc, char* argv[] )
{
  game_loop();

  return 0;
} // main()

Easy, isn't it?

I could not manage to configure the compilation of the final shared library with CMake, mainly because the project uses a lot of libraries which must be linked together with different flags. The most straightforward way I've found to compile the library has thus been to write the g++ command myself and to put it in a script. The command looks like this:

arm-linux-androideabi-g++ -fPIC -shared \
    -Wl,-soname,libandy-super-great-park.so \
    -Wl,--export-dynamic -rdynamic \
    ${INCLUDE_DIRECTORIES}
    ${CPP_FILES} \
    -o 'path/to/java-project/libs/armeabi-v7a/libandy-super-great-park.so' \
    ${LINK_DIRECTORIES} \

    -Wl,--no-as-needed -Wl,--whole-archive \
    -lrp \
    -lbear_generic_items \

    -Wl,--as-needed -Wl,--no-whole-archive \
    ${BEAR_ENGINE_LIBRARIES} \

    -Wl,-Bstatic \
    ${OTHER_LIBRARIES} \ # Claw, jpeg, png, Boost, SDL2, intl
    -lsupc++ \
    -lstdioext \

    -Wl,-Bdynamic \
    -lgnustl_shared \
    -lz \
    -lGLESv2 \
    -lGLESv1_CM \
    -llog \
    -ldl \
    -landroid

The first lines are quite typical: I tell the compiler which sources to compile, where to find the headers and the libraries and how to name the result.

The second part concerns the libraries of items from the game and from the engine. The classes declared in these libraries are almost not referenced anywhere in the code. Actually, most of them are instantiated only when loading the levels, using factories. If I link them using the default linker flags (which include --as-needed and --no-whole-archive), the linker will drop their content, since it won't find any references in the code. In order to have their code available in the final binary, I must force these libraries to be included as is using the --no-as-needed and --whole-archive arguments. It is a way to tell the linker not to try to be smart.

The third part links the libraries of the Bear Engine, which are compiled as static libraries. For them, I can restore the default behavior by setting --as-needed and --no-whole-archive again.

The fourth section links the various libraries used by the engine. Since these libraries may be available as static or shared, I force the use of the static version by setting @-Bstatic@@.

Finally, the last section lists the libraries which I could not link statically. For them, I force the shared version by setting -Bdynamic.

You can find the complete compilation script in GitHub.

About accessing the game's resources

The game's resources are usually read from the hard disk, from the directory where the game is installed. For Android applications, there is no such thing as installing the application's files; the resources are stored in the application package and that's all. From there we have only two options which both require to extract the resources from the package: either do we use the extracted resource on the fly or we save it on the external storage as needed and load them from here in future runs. The latter may result in faster loading time on multiple runs, at the cost of a larger consumption of the user's storage.

For Andy's Super Great Park I chose to use the resources extracted on the fly. It is the solution which requires the less work and the performances appeared to be good enough.

Once again, the SDL provides a function to help us, namely SDL_RWFromFile(). If the resources of the application are stored in the assets directory of the package (i.e. in the assets directory in the Java project's directory), then passing the relative path to this function will open it as if it was a regular file.

Since the Bear Engine already had a resource_pool object to abstract the access to the resources, which returns a std::istream from the relative path of a resource; using the function of the SDL was quite straightforward. I had to adjust the existing resource_pool to accept different implementations, then I wrote an implementation which uses SDL_RWFromFile().

Building the APK and running it on the phone

Let's recap what we have in our Android project directory now:

android-project/
|- assets/                              # The resources of the game.
|- libs/armeabi-v7a/                    # The native part of the game.
|  |- libandy-super-great-park.so
|  |- libgnustl_shared.so
|- res/                                 # Various resources of the
|                                       # application, including its icon.
|- src/
|  |- org/libsdl/app/SDLActivity.java   # The Java Android application as
|  |                                    # provided by the SDL2
|  |- com/stuffomatic/asgp/ASGP.java    # The Java Android application of our
|                                       # game
|- AndroidManifest.xml

In order to build and test the APK, all we have to do is to enter the android-project directory and to type the following command:

$ ant debug

At the end of the build the Android package can be found in ./bin, namely ./bin/ASGP-debug.apk for my game.

This command builds the Android package in debug mode, which means that it is signed with the default debugging key. The package can be installed on a device using the adb tool but it cannot be submitted to the Play Store.

In order to install the package on the device, we must enable the developer mode in Android then plug the device on a USB port. The device should appear in the list printed when typing adb devices in the shell, along with its identifier:

$ adb devices
List of devices attached
0123456789      device

Write this identifier somewhere as it will be required for future access to the device with the adb tool. In the following, this identifier will be denoted by $DEVICE_ID

With this information, we can finally install the package on the device:

$ adb -s $DEVICE_ID install -r bin/ASGP-debug.apk

The first run is very exciting. Let's open the phone's applications menu and tap the game's icon. Then…

First execution of Andy's Super Great Park on my phone

Well… not so bad for a first run, but totally not playable. Actually I made a little mistake in the computation of the vertex colors when converting the OpenGL code and the screen is not in the right orientation (it was prior to setting the configuration in AndroidManifest.xml). Once these problem were fixed, here is what I had:

First working execution of Andy's Super Great Park on my phone

Great!

Building the final APK

As stated before, the debug package of the application cannot be submitted to the Play Store. The process of building the final package requires some extra steps which I explain below.

The first thing we can easily do is to build the package in release mode, using the ant release command in the Android project's directory. It will build an intermediate package in the bin directory, named ASGP-release-unsigned.apk for my game.

As the name suggests, the package is not signed. We will need a private key in order to sign it, which we can generate using the keytool command. For example:

$ keytool -genkey -keystore ~/.keytool/stuffomatic.keystore \
  -alias gplays -keyalg RSA -keysize 2048 -validity 10000

Once we have our key, it is easy to sign the package using the jarsigner tool:

$ jarsigner -sigalg SHA1withRSA -digestalg SHA1 \
  -keystore ~/.keytool/stuffomatic.keystore \
  ./bin/ASGP-release-unsigned.apk gplays

The package must then be aligned (I don't know why, but the documentation says it must):

$ zipalign 4 ./bin/ASGP-release-unsigned.apk ./bin/ASGP.apk

And that's it. We can now submit the final APK to the Play Store.

If you try to install this package on your phone using the adb command, ensure that you have uninstalled the previously installed debug package. Otherwise the installation will fail with an incomprehensible message.

What's next

Well, I didn't think this part would need so many words! At the end of it, I am able to run Andy's Super Great Park on my phone and to release it on the Play Store. It would be a lie to say the game is actually ready to be released as is but it is still quite a nice achievement.

In this state, the game ran at most at 15 frames per second, which is very far from the 60 frames expected. I had to rework some parts of the engine in order to obtain a decent frame rate, one of them being to put the rendering work in a separate thread. Also, the translations did not work well and I had to spend some time to fix some errors in libintl-lite and to find a way to retrieve the phone's locale in native code. Finally, I had to handle the finger events and to adjust the controls in order to be able to play. These may be interesting parts to talk about in future notes too.