Android: Java and C++ synergy

This is a short checklist how Java and C++ code can synergize and how it is processed. All necessary steps are based on DWG Viewer.

1. Java: Create a nativ method

2. C++: Create a matching implementation

3. Store the C++ implementation inside a shared library

4. Compile the shared library with CMake

5. Load the shared library with System.LoadLibrary()

Note: When a Java native method is called before the implementation is loaded it leads to a crash!

1. Java: Create a nativ method

JNI.java

public static native void SetBackgroundColor( int color );

2. C++: Create a matching implementation

ARESSimplified_JNI.cpp

JNIEXPORT void JNICALL Java_com_graebert_aressimplified_JNI_SetBackgroundColor( JNIEnv * env, jclass clazz, jint color ){...}

3. Store the C++ implementation inside a shared library

See CFx/AndroidSimplified_JNI/app/CMakeLists.txt
See CFx/CMakeLists.txt

4. Compile the shared library with CMake

Enable CMake compilation inside build.gradle file. Android Studio will take care about the necessary files and compiles everything with the NDK linked to the project.

build.gradle

...externalNativeBuild {
   cmake {
       arguments "-DANDROID_STL=c++_shared", "-DVIEWER=1"
       cppFlags "-std=c++11 -frtti -fexceptions"
   }
}
...
externalNativeBuild {
   cmake {
       path file('../../CMakeLists.txt')
   }
}
...

5. Load the shared library with System.LoadLibrary()

To complete the Java and C++ synergy the shared library must be loaded into the host application once before any native method is called.

JNI.java

public static void Load()
{
   String lib = "";
   try
   {
       lib = "JNI";             System.loadLibrary(lib);
   }
   catch (UnsatisfiedLinkError use)
   {
       Log.e("JNI", "WARNING: Could not load JNI library " + lib, use );
   }
...
}