Lasersköld

2015-06-21

Add native code to Android Studio project.

I felt a bit lost when switching from eclipse to the Android Studio environment. Also I found it difficult to interpret the documentations and tutorials I found online that tried to explain how to setup android studio projects. After struggling for a while I decided to learn to compile a project with the terminal first to get a grip around what was happening in the background, and then the pieces started to fall in place.

First of I added the gradle and the ndk-build to the PATH variable. The path variable is a environment variable (both in Linux and Windows) that tells the environment which programs can be executed. Here is a bit discussion that I found about it. You fix it by adding some lines in your ~/.bashrc file. This makes it possible to be able to run grandle without specifying the path on every call. I did the same for ndk-build that is located in the native sdk folder.

Create a project

If you do not already have a project select File → New → New Project... and create a project that suits for you. Alternatively you can import an eclipse project by selecting File → New → Import project... Some things will differ, as I will mention in the text.

Add a jni folder to your project.

In your project explorer (probably on your left) right-click on "app" then go to New → Folder → New JNI Folder ... Select it and create a folder. Note that if you are in Android-mode (the text "android" is visible above the explorer) it might work differently, but then just press the drop-down menu (with the text android) and select "project".

Include library files into your project

Android studio will automatically call ndk-build in the jni-folder, so what you will have to do is to simply include your library into your code. The fastest way I found is to simply add this to your app/build.gradle

build.gradle

android {
  ... //old code
    sourceSets.main {
        jni.srcDirs = [] //This prevents the auto generation of Android.mk
        jniLibs.srcDir 'src/main/libs' //this is not necessary unless you have precompiled libraries in your project
  }
}

You could do a bit more fancy stuff to match to different types of processors, but that will do for now.

If you have imported from an eclipse project you probably have a part in your gradle script that says "ndk {... }". I use to remove this statement because otherwise grandle will complain about duplicate libraries. Another alternative is to remove the jniLibs.srcDir statement above, it seems to have a similar effect.

I have also noticed that when importing from an eclipse project, Android Studio puts the old files in app/jniLib/... This caused an error message for me that confused me at first, but the solution was to simply remove the old copied so-files (the ones in app/jniLib) because new ones will be generated anyway.

Create some source files

Because the ndk-build

Lazy way to discover function names

You need to come up with a name that reflects the native functions. However if you are a bit lazy, like me you could just run the program, without implementing the right functions and then look for an error message like this in logcat.

java.lang.UnsatisfiedLinkError: No implementation found for void se.laserskold.mattias.android1.MainActivity.test() (tried Java_se_laserskold_mattias_android1_MainActivity_test and Java_se_laserskold_mattias_android1_MainActivity_test__)

then you know what to call your function.

Here are some more good resources http://ph0b.com/android-studio-gradle-and-ndk-integration/