Import existing c++ library (.a or .so file) ndk android

Importing Existing C++ Libraries (.a or .so) into Android NDK

This article will guide you through the process of importing existing C++ libraries, in the form of static (.a) or dynamic (.so) files, into your Android NDK project.

Prerequisites

• Android Studio
• Android NDK installed
• Existing C++ library (.a or .so) file

Steps

1. Preparing your C++ Library

Ensure your library is compiled for the appropriate architecture (armeabi-v7a, arm64-v8a, etc.) targeted by your Android project.

2. Creating a Native Library Project

• In Android Studio, create a new project.
• Choose “Native C++” as the project template.

3. Adding Library Files

• Create a new folder in your project’s jni directory named libs.
• Place your .a or .so library file in the corresponding architecture subfolder within libs. For example, if your library is for armeabi-v7a, the path would be: jni/libs/armeabi-v7a/your_library.a

Creating the JNI Wrapper

You’ll need a Java class to interact with your C++ library. This is called the JNI wrapper.

1. Defining Native Methods

Declare native methods in your Java class using the native keyword. These methods will be implemented in your C++ code.

public class NativeLibrary {
   public native void myNativeMethod();
}

2. Implementing Native Methods in C++

Create a C++ header file (e.g., native_library.h) and implementation file (e.g., native_library.cpp) to implement the native methods. This will link your library with the wrapper code.

// native_library.h
#ifndef NATIVE_LIBRARY_H
#define NATIVE_LIBRARY_H

#include 

extern "C" {
JNIEXPORT void JNICALL Java_com_example_myproject_NativeLibrary_myNativeMethod(JNIEnv* env, jobject thiz);
}

#endif //NATIVE_LIBRARY_H

// native_library.cpp
#include "native_library.h"
#include "your_library.h" // Include your library header

JNIEXPORT void JNICALL Java_com_example_myproject_NativeLibrary_myNativeMethod(JNIEnv* env, jobject thiz) {
    // Call functions from your C++ library
    yourLibraryFunction();
}

Compiling and Linking

1. Building the Native Library

Use the NDK’s build tools (ndk-build) to compile the C++ wrapper and link it to your existing library. Add the following to your Android.mk file:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := native_library
LOCAL_SRC_FILES := native_library.cpp

LOCAL_LDLIBS := -L$(LOCAL_PATH)/../libs/armeabi-v7a -lyour_library

include $(BUILD_SHARED_LIBRARY)

Replace your_library with the actual name of your library. The LOCAL_LDLIBS line defines the path to your library and links it.

2. Loading the Library

In your Java activity, load the native library using System.loadLibrary().

public class MainActivity extends AppCompatActivity {
  static {
     System.loadLibrary("native_library");
  }

  @Override
  protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);

     NativeLibrary.myNativeMethod();
  }
}

Troubleshooting

• Library Not Found: Ensure the library path in LOCAL_LDLIBS is correct.
• Architecture Mismatch: The library must be compiled for the same architecture as your target device.
• Linking Errors: Check for errors in the Android.mk file or in the C++ code. Refer to NDK documentation for specific errors.

Summary

By following these steps, you can successfully import existing C++ libraries into your Android NDK project. This allows you to leverage pre-existing code and functionality within your Android apps.