How do I add a file to an Android project and then load it using the NDK

Question

I am using the latest version of Android Studio (2.2.3) and I have loaded up the HelloGL2 sample project.

I now want to add a file (any type of file) to my app, and then be able to open it and read it in the c++ code using something like c's fopen etc (any direct file access api is fine)

How do I do this?

Answer 1

There are two options, it will depend on your target. If your file is a basic text configuration file, you can use both cases, but if your file is a 3D object such as (.obj, .max, .dae) you should use AssetManager class.

First option: (store your files in res raw (You can use fopen())).

• Create a folder called raw inside res directory (res->raw).
• Write your files in the apk private directory.

In Java:

    public void writeFileToPrivateStorage(int fromFile, String toFile) 
    {

       InputStream is =   mContext.getResources().openRawResource(fromFile);
       int bytes_read;
       byte[] buffer = new byte[4096];  
         try 
         {
            FileOutputStream fos = mContext.openFileOutput(toFile, Context.MODE_PRIVATE);

            while ((bytes_read = is.read(buffer)) != -1)            
                fos.write(buffer, 0, bytes_read); // write

            fos.close();
            is.close();

        } 
        catch (FileNotFoundException e) 
        {            
            e.printStackTrace();
        } 
         catch (IOException e) 
         {            
            e.printStackTrace();
        }                 
    }  

Then, call to your function:

        writeFileToPrivateStorage(R.raw.your_file,"your_output_file.txt");

• Get your private path

    path=mContext.getApplicationContext().getFilesDir().toString();

• Define your JNI funcion in Java:

    public static native void setconfiguration(String yourpath);

• Implement it in C/C++:

     JNIEXPORT void JNICALL Java_com_android_gl2jni_GL2JNILib_setconfiguration(JNIEnv * env, jobject obj, jstring path)
        {
            //convert your string into std::string. 
            const char *nativeString = env->GetStringUTFChars(config_path, 0);
            //make here your fopen.
            fopen(nativeString,"r");
        }

Second option (use assetManager, usually for opengl resources).

The parameter, in this case, is not the path of the directory is the asset manager.

• Store your files in the asset directory.
• Define your native function in C/C++

     public static native void yourfunction(AssetManager assetManager);

• Call in java to this function:

     loadYourFile(m_context.getAssets());

• Create your jni function in C/C++

    JNIEXPORT void   Java_com_android_gl2jni_GL2JNILib_(JNIEnv * env, jobject obj,jobject java_asset_manager)
        {
         AAssetManager* mgr = AAssetManager_fromJava(env,java_asset_manager);
            AAsset* asset = AAssetManager_open(mgr, (const char *) js, AASSET_MODE_UNKNOWN);
            if (NULL == asset) {
                __android_log_print(ANDROID_LOG_ERROR, NF_LOG_TAG, "_ASSET_NOT_FOUND_");
                return JNI_FALSE;
            }
            long size = AAsset_getLength(asset);
            char* buffer = (char*) malloc (sizeof(char)*size);
            AAsset_read (asset,buffer,size);
            __android_log_print(ANDROID_LOG_ERROR, NF_LOG_TAG, buffer);
            AAsset_close(asset);
        }

Note: Do not forget to add the permissions in your AndroidManifest.xml.

Note II: Do not forget to add:

#include <android/asset_manager.h>
#include <android/asset_manager_jni.h>

I hope this answer helps you.