Android native library linking against another native library from aar

Introduction

In Android development, native libraries written in languages like C/C++ can be packaged into Android Archive (AAR) files. These AARs can then be easily reused across different projects. However, linking a native library within an AAR against another native library present in the same AAR or a different AAR can pose challenges.

Linking Native Libraries within the Same AAR

Challenges

• Static Linking: Statically linking native libraries within the same AAR can lead to larger APK sizes as all the code is bundled together.
• Dependency Management: Managing dependencies between the native libraries can become complex, especially when dealing with multiple AARs.

Solution: Using the Android NDK’s Static Linking

The Android NDK provides tools to statically link native libraries. You can create a static library that includes all the necessary code from the native libraries within the AAR. This static library can then be linked against your application’s native code.

Example

// Example CMakeLists.txt file for static linking
cmake_minimum_required(VERSION 3.10)
add_library(native_library STATIC native_library.cpp)
target_link_libraries(native_library native_library_dep)
install(TARGETS native_library DESTINATION lib/armeabi-v7a)

Solution: Using Dynamic Linking

Dynamic linking allows your application to load and link against native libraries at runtime. This approach can result in smaller APK sizes and allows for easier dependency management.

Example

// Example CMakeLists.txt file for dynamic linking
cmake_minimum_required(VERSION 3.10)
add_library(native_library SHARED native_library.cpp)
target_link_libraries(native_library native_library_dep)
install(TARGETS native_library DESTINATION lib/armeabi-v7a)

Solution: Using a Shared Library

Create a shared library that acts as a bridge between the two native libraries. This shared library would handle the linking process, ensuring the two libraries interact seamlessly.

Example

// Example CMakeLists.txt file for using a shared library
cmake_minimum_required(VERSION 3.10)
add_library(shared_library SHARED shared_library.cpp)
target_link_libraries(shared_library native_library_1 native_library_2)
install(TARGETS shared_library DESTINATION lib/armeabi-v7a)

Linking Native Libraries from Different AARs

Challenges

• Dependency Management: Managing dependencies between native libraries in separate AARs can become complex.
• Build System Integration: Integrating the build systems of the different AARs to achieve proper linking can be challenging.

Solution: Building Dependencies Explicitly

Explicitly define the dependencies between the native libraries in your application’s build system. This ensures that the necessary libraries are linked together during the compilation process.

Example (CMake)

// Example CMakeLists.txt file for linking external native libraries
cmake_minimum_required(VERSION 3.10)
add_library(native_library STATIC native_library.cpp)
target_link_libraries(native_library external_native_library)
include_directories(external_native_library/jni/include)

Solution: Using a Centralized Library

Introduce a central library that acts as a bridge between the native libraries from different AARs. This centralized library can handle the communication and linking between the native libraries, simplifying the linking process.

Comparison of Linking Approaches

Conclusion

Linking native libraries from AARs can be a complex task. Understanding the different approaches and choosing the most suitable one based on the project requirements is crucial. Static linking can be a viable option for smaller projects with well-defined dependencies. However, for larger projects with complex dependencies, dynamic linking or using a centralized library might be a better choice.