A quick tutorial to NDK development using new version of Android Studio 1.4

1. New a simple android application project in the AS and build the project. This will generate the class target objects for the java class. The generated objs will be located in:

   app/build/intermediates/classes/debug.

2. Creat a jni directory in app/src/main/, so you will get:

   src
   ├── androidTest
   │   └── java
   │   │   └── com
   │   │   │   └── example
   │   │   │   │   └── project_name
   │   │   │   │   │   └── module_name
   ├── main
   │   ├── java
   │   │   └── com
   │   │   │   └── example
   │   │   │   │   └── project_name
   │   │   │   │   │   └── module_name
   │   ├── jni
   │   └── res
   │   │   ├── drawable
   │   │   ├── layout
   │   │   ├── menu
   …

3. Make an declaration of the native method in the java code (main activity for example):

      native String StringFromJni();  
   

4. Generate the JNI header file for the native method declared in step 3.
   • launch a terminal, go to the class obj directory: ProjectName/app/build/intermediates/classes/debug

   • use javah command to generate the c/c++ header files

     $JDKPath$\bin\javah -jni -d $ModuleFileDir$/src/main/jni $FileClass$

     Note:
     JDKPath - refers to your JDK installation directory.
     -jni - means generate JNI-style header file (default).
     -d - output directory, point to your jni directory just crated in step 2.
     FileClass - com.example.georgeyan.nimes.Avignon. (the package name)

5. Now you can find the generated header file in your jni ditrctory (app/src/main/jni). The file name is com_package_name_projectname_module.h, containing:

   JNIEXPORT jstring JNICALL Java_com_example_georgeyan_nimes_Avignon_StringFromJni(JNIEnv *, jobject);

6. Now create a c/c++ source file to implement it.

   JNIEXPORT jstring JNICALL Java_com_example_georgeyan_nimes_Avignon_StringFromJni(JNIEnv * env, jobject ){

                 return (env)->NewStringUTF("HelloFromJni"); }  
   

   Two ways to compile the .so module:
   [1] gradle: (if you want more about gradle, refer to this:gradle.)
   add the following in the file app/build.gradle, in the defaultConfig bracket:

                 ndk {  
                      moduleName "hello-jni"  
                 }  
           you may need add `android.useDeprecatedNdk=true` in gradle.property
   

   [2] Android.mk: build manually, not pratice using Android Studio.

7. The last thing is using the .so module in the java class.

        static {  
            System.loadLibrary("hello-jni");  
        }  
   

   Example:
   In the Oncreate function:

       TextView text = (TextView)findViewById(R.id.hw);  
          
       text.setText(StringFromJni());  
   

8. Demo on github:
   Nimes.

19 Oct 2015