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Let's Build Chuck Norris! - Part 7: Android and JNA

Note: This is part 7 of the Let’s Build Chuck Norris! series.

Last time we managed to cross-compile and run C++ code for Android.

It’s now time to write some Java code, but we need to take a detour on the desktop first.

Java bindings #

Let’s create a new Java library project with gradle:

$ cd chucknorris
$ mkdir java && cd java
$ gradle init --type java-library

Gradle created a bunch of files, but for now we just care about the sources and the tests.

Our goal is to write a test that demonstrates we can indeed get some ChuckNorris facts.

Let’s start by removing cruft from the generated build.gradle:

dependencies {
-    api 'org.apache.commons:commons-math3:3.6.1'
-    implementation ''

Then let’s fix the source files that gradle created so that we have proper package:

$ tree java
├── build.gradle
├── gradle
└── src
    ├── main
    │   └── java
    │       └── com
    │           └── chucknorris
    │               └──
    └── test
        └── java
            └── com
                └── chucknorris

Now we can write a failing test:

/* In */
public class ChuckNorrisTest {

    public void testGetFact() {
        ChuckNorris ck = new ChuckNorris();
        String fact = ck.getFact();
        assertThat(fact, containsString("Chuck Norris"));
/* In */

public class ChuckNorris {
    String getFact() {
        return "";

Let’s run the tests:

$ ./gradlew test
> Task :test FAILED

com.chucknorris.ChuckNorrisTest > testGetFact FAILED
    java.lang.AssertionError at

1 test completed, 1 failed

# open build/reports/tests/test/index.html

Expected: a string containing "Chuck Norris"
     but: was ""

OK, this fails for the good reason.

Now we can try and load our shared library using JNA.

First, we add the dependency in the build.gradle file:

dependencies {
+   api ''
    testImplementation 'junit:junit:4.12'

Then we’re ready to use JNA:

  • We use Native.loadLibrary() to load the shared library
  • We create a CLibrary interface that implements the C functions we want to call as methods. (just chuck_norris_init for now).
  • We call chuck_norris_init in the constructor of our ChuckNorris class, storing the result into a JNA Pointer:
public class ChuckNorris {
  private Pointer ckPointer;

  private static CLibrary loadChuckNorrisLibrary() {
    return (CLibrary) Native.loadLibrary("chucknorris", CLibrary.class);

  public interface CLibrary extends Library {
    CLibrary INSTANCE = loadChuckNorrisLibrary();

    void chuck_norris_init();

  public ChuckNorris() {
    ckPointer = CLibrary.INSTANCE.chuck_norris_init();

  public String getFact() {
    return "";

And we run the tests:

$ ./gradlew test
java.lang.UnsatisfiedLinkError: Unable to load library 'chucknorris':
  Native library (linux-x86-64/
  not found in resource path (...)
  at com.sun.jna.NativeLibrary.loadLibrary(
  at com.sun.jna.NativeLibrary.getInstance(

This is expected. We never told JNA where the file is.

As a reminder, the file currently lives in the build/default folder. Here’s how we built it:

$ cd build/default
$ conan install ../..
$ cmake -GNinja -DBUILD_SHARED_LIBS=ON ../..
$ ninja

There are several ways to tell JNA about the location of the shared library file. Here we’ll set a system property in the test block of the Gradle script:

def thisFile  = new File(project.file('build.gradle').absolutePath)
def projectPath = thisFile.getParentFile()
def topPath = projectPath.getParentFile()
def cppPath = new File(topPath, "cpp")
def cppBuildPath = new File(cppPath, "build/default/lib")

test {
    systemProperty 'jna.library.path', cppBuildPath

Now if we re-run the tests we get back our first failure:

$ ./gradlew test

Expected: a string containing "Chuck Norris"
     but: was ""

But we did manage to instantiate the ChuckNorris class, so this is progress :)

Let’s implement getFact(), and while we’re at it, add a .close() method:

  public interface CLibrary extends Library {
    // ...
    void chuck_norris_init();
    String chuck_norris_get_fact(Pointer pointer);
    void chuck_norris_deinit(Pointer pointer);

  public ChuckNorris() {
    ckPointer = CLibrary.INSTANCE.chuck_norris_init();

  public String getFact() {
    return CLibrary.INSTANCE.chuck_norris_get_fact(ckPointer);

  public void close() {

Re-run the tests:

$ ./gradlew test


Creating a new Android project #

Now we know:

  • How to compile the C++ code for Android.
  • How to load some C++ in Java, but only for the desktop.

It’s time to glue things together.

To do so, the best thing is to use Android Studio to create the gradle project, starting with a with a basic activity so we don’t have to deal with all the Android boilerplate.

Adapting the GUI #

Let’s pretend the ChuckNorris class already exists for now. 1

We start by adding a text_view ID for the text view in the content_main layout.

Then we adapt the file to update the text view when clicking on the floating button action:

public class MainActivity extends AppCompatActivity {
  private ChuckNorris chuckNorris;

  protected void onCreate(Bundle savedInstanceState) {
    chuckNorris = new ChuckNorris();

  protected void onDestroy() {

  // ...

  final TextView textView = (TextView) findViewById(;
  FloatingActionButton fab = (FloatingActionButton) findViewById(;
  fab.setOnClickListener(new View.OnClickListener() {
    public void onClick(View view) {
      String fact = chuckNorris.getFact();

Adding ChuckNorris sources #

One of Java’s slogan is “Write Once, Run Everywhere” 2.

So let’s:

  • Add JNA in the dependencies
  • Add the file we wrote earlier

And everything should work, right?

Fun with #

To check if our code works, let’s create an emulator, and click on play.

We’re faced with:

ChuckNorris has stopped

Open App Again

What? Chuck Norris can’t be stopped, this is unacceptable!

Chuck Norris punching the screen

Time to look at the logs:

06-18 14:27:18.553 6890-6890/info.dmerej.chucknorris E/AndroidRuntime:
Process: info.dmerej.chucknorris, PID: 6890
Native library (com/sun/jna/android-x86-64/
not found in resource path (.)
  at com.sun.jna.Native.loadNativeDispatchLibraryFromClasspath
  at com.sun.jna.Native.loadNativeDispatchLibrary
  at info.dmerej.chucknorris.ChuckNorris.loadChuckNorrisLibrary

That’s a fun one. Turns out the name of dependency changes when compiling for Android, you need a @aar prefix 3:

dependencies {
  // ...
  implementation ''

Let’s try again!

We get the same error message, but this time it’s the library that is not found:

06-18 14:27:18.553 6890-6890/info.dmerej.chucknorris E/AndroidRuntime:
Process: info.dmerej.chucknorris, PID: 6890
java.lang.UnsatisfiedLinkError: Unable to load library 'chucknorris':
  Native library (android-x86-64/ not found

Fortunately, there’s a more or less standard solution.

If you put a .so file in a folder named src/main/jniLibs/<arch>, it will be included inside the Java application, and the Java code will be able to load it without any configuration.

The shared option #

For simplicity purposes, we built the ChuckNorris library as a static library, just to show that the C++ binary still needed to run. But JNA needs a shared library to run.

Remember in part 4 we had to call CMake with -DBUILD_SHARED_LIBS=ON to get a shared library.

We’ll do the same thing, but going through Conan this time.

First, let’s add the ChuckNorris:shared option in the android profile:


Then adapt the recipe:

class ChucknorrisConan(ConanFile):
    name = "ChuckNorris"
    options = {"shared": [True, False]}
    default_options = "shared=False"

    def build(self):
        cmake = CMake(self)
        cmake_definitions = {}
        if self.options.shared:
            cmake_definitions["BUILD_SHARED_LIBS"] = "ON"

    def package(self):
        self.copy("lib/", dst="lib", keep_path=False)
        self.copy("lib/", dst="lib", keep_path=False)

Then let’s re-create the Conan package:

$ conan create . dmerej/test --profile android --setting arch=x86_64
Exporting package recipe
package(): Copied 2 '.so' files:,
Package '<hash>' created

Finally let’s create symlinks to all .so files from the package.

$ cd android/app
$ cd src/main
$ mkdir -p jniLibs/x86_64
$ cd jniLibs/x86_64
$ ln -s ~/.conan/data/ChuckNorris/0.1/dmerej/test/<hash>/ .
$ ln -s ~/.conan/data/ChuckNorris/0.1/dmerej/test/<hash>/ .

Let’s try again:

Chuck Norris app running

Victory \o/

That’s all for today. See you next time!

  1. This is also known as wishful thinking programming ↩︎

  2. As always, the Wikipedia page contains lots of interesting stuff about this topic. ↩︎

  3. You can find a note about this in JNA’s FAQ, but as far as I know, not anywhere else in the documentation. ↩︎

Thanks for reading this far :)

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