Adding native C++ code to your application

Let us expand our minimalistic Java template, which was discussed in the previous recipe, so we can create a placeholder for our native C++ code.

Getting ready

We need to copy all the files from our App1 project to save time while creating the initial project files. This recipe will focus on the changes to be made to the App1 project in order to add the C++ code to it.

How to do it...

Carry out the following steps to create a placeholder for our C++ code:

1. Add the jni/Wrappers.cpp file with the following code:

   #include <stdlib.h>
   #include <jni.h>
   #include <android/log.h>
   #define LOGI(...) ((void)__android_log_print(ANDROID_LOG_INFO, "App2", __VA_ARGS__))
   
   extern "C"
   {
     JNIEXPORT void JNICALL
   Java_com_packtpub_ndkcookbook_app2_App2Activity_onCreateNative( JNIEnv* env, jobject obj )
       {
         LOGI( "Hello World!" );
       }
   }

2. We need to change our Activity class from the previous recipe to make use of the native code we just added in the preceding section, through the following code:

   package com.packtpub.ndkcookbook.app2;
   
   import android.app.Activity;
   import android.os.Bundle;
   
   public class App2Activity extends Activity
   {
       static
       {

   Here we load the native library named libApp2.so. Note the omitted lib prefix and .so extension:

         System.loadLibrary( "App2" );
       }
       @Override protected void onCreate( Bundle icicle )
       {
         super.onCreate( icicle );
         onCreateNative();
       }
       public static native void onCreateNative();
   };

3. Tell the NDK build system how to treat the .cpp file. Create the jni/Android.mk file. The Android.mk file is used by the Android NDK build system to find out how to treat the source code of your project:

   TARGET_PLATFORM := android-7
   LOCAL_PATH := $(call my-dir)
   include $(CLEAR_VARS)
   LOCAL_ARM_MODE := arm
   LOCAL_MODULE     := App2
   LOCAL_SRC_FILES += Wrappers.cpp
   LOCAL_ARM_MODE := arm
   COMMON_CFLAGS := -Werror -DANDROID -DDISABLE_IMPORTGL \
   -isystem $(SYSROOT)/usr/include/
   ifeq ($(TARGET_ARCH),x86)
   	LOCAL_CFLAGS   := $(COMMON_CFLAGS)
     else
   	LOCAL_CFLAGS   := -mfpu=vfp -mfloat-abi=softfp \
     -fno-short-enums $(COMMON_CFLAGS)
   endif
   LOCAL_LDLIBS     := -llog -lGLESv2 -Wl,-s
   LOCAL_CPPFLAGS += -std=gnu++0x
   include $(BUILD_SHARED_LIBRARY)

   Note the ifeq ($(TARGET_ARCH),x86) section. Here we specify architecture-specific compiler flags for floating point support on ARMv7. This will give you hardware floating-point support on the ARM architecture and a warnings-free log on the x86 Android target architecture..

4. Paste the following code into the jni/Application.mk file:

   APP_OPTIM := release
   APP_PLATFORM := android-7
   APP_STL := gnustl_static
   APP_CPPFLAGS += -frtti 
   APP_CPPFLAGS += -fexceptions
   APP_CPPFLAGS += -DANDROID
   APP_ABI := armeabi-v7a
   APP_MODULES := App2
   NDK_TOOLCHAIN_VERSION := clang

How it works...

1. First of all, we need to compile the native code. From the root of your App2 project, run the following command:

   >ndk-build
   

2. You should see the following output:

   Compile++ arm: App2 <= Wrappers.cpp
   SharedLibrary: libApp2.so
   Install      : libApp2.so => libs/armeabi-v7a/libApp2.so
   

3. Now proceed to the .apk creation as in the previous recipe by running the following command:

   >ant debug
   

4. Your libApp2.so native shared library will be packed into the App2-debug.apk package. Install and run it. It will output a Hello World! string into the device log.

There's more...

You can use the adb command to view the device log. A nice clean formatted log with timestamps can be created using the following command:

>adb logcat -v time > 1.txt

The actual output from your device will look similar to the following command:

05-22 13:00:13.861 I/App2    ( 2310): Hello World!