Android NDK build cmake include .so library included in .aar library

Question

I have an android_library.aar file that containing library.so and some other resources and java files.

I imported android_library.aar to my project. My project uses c++ code with NDK. My problem is I can not compile C++ code that dependent with library.so with CMake. How could I extract library.so from aar before CMake compile? I want to include library.so to my C++ code directly without java. I know how to include .so in a project but I don't know how to do it from inside of aar.

Very nice indeed! I believe that gradle does extract the .so files if the .aar is a dependency. — Alex Cohn
– Alex Cohn, Commented Sep 3, 2018 at 14:50
I thought so. But it does not extract before cmake build. I don't know why. I searched for it and did not find any answer easy than this method. — Almas
– Almas, Commented Sep 6, 2018 at 0:05
… but in your solution the aar is not declared as dependency of your project, s it? — Alex Cohn
– Alex Cohn, Commented Sep 6, 2018 at 4:54
It's also must declare library as dependency. Because of Java part of library. I don't know much about dependency decleration also affecting to JNI part or not. — Almas
– Almas, Commented Sep 6, 2018 at 15:09

Almas · Accepted Answer · 2020-05-15 15:45:20Z

I found a solution on my own after 1-week research. I found some code from Google GVR code. But it hasn't worked for my case. I changed build.dependsOn to preBuild.dependsOn then it worked. It might help some people in the future.

Here is the example code:

In the module's build.gradle file:

sourceSets { main { jniLibs.srcDirs = ["jniLibs"] } } task extractSo(type: Copy) { println 'Extracting *.so file(s)....' from zipTree("${project.rootDir}/ModuleName/ModuleName.aar") into "${project.rootDir}/${project.name}/src/main/jniLibs/" include "jni/**/*.so" eachFile { def segments = it.getRelativePath().getSegments() as List println segments it.setPath(segments.tail().join("/")) return it } includeEmptyDirs = false } preBuild.dependsOn extractSo

Michael Troger · Accepted Answer · 2024-02-29 00:36:29Z

Nowadays this is possible without much boiler plate. Prerequisite is that your AAR file includes a prefab folder. So the library maintainer needs to properly set things up.

If this is the case then it's as easy as adding following to app/build.gradle:

android { // ... buildFeatures { prefab true // extracts native libraries from AAR for usage in C++ } }

Then in CMakeLists.txt you can find and register the lib:

# Locate the native library: find_package(ModuleName REQUIRED CONFIG) # Replace ModuleName with the actual name of your module. # It can be found in `prefab/prefab.json` file of your AAR. target_link_libraries( app ModuleName::nameOfSoFileWithoutFileEnding )

Example with the OpenCV AAR library on Maven:

# ... find_package(OpenCV REQUIRED CONFIG) target_link_libraries( app OpenCV::opencv_java4 )

This will register opencv_java4.so that is part of the AAR file. The C++ header files would be added as well.

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