How to add JNI (C/C++ native code) to existing Android Studio project

Since Android Studio 3.1 its possible easy way:

1. Create cpp folder inside app\src\main.

2. Create <YOUR_FILE_NAME>.cpp file in app\src\main\cpp path (e.g. native-lib.cpp)

3. Add CMakeLists.txt file to app folder.

In that file name of the library, .cpp file path and some other settings should be defined, e.g. (from new, empty Android Studio Project with C++ support):

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.4.1)

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native-lib.cpp )
                          ^^^^^^^^^^^^^^
                          YOUR_CPP_FILE_NAME

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
                       native-lib

                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

4. Add to build.gradle (Module app) externalNativeBuild tag with reference to CMakeLists.txt into android section:

android {
    compileSdkVersion 26
    defaultConfig {
        ...
    }
    buildTypes {
        ...
    }
    externalNativeBuild {               <--- these lines should be added
        cmake {                         <--- these lines should be added
            path "CMakeLists.txt"       <--- these lines should be added
        }                               <--- these lines should be added
    }                                   <--- these lines should be added
}

5. Add to build.gradle (Module app) externalNativeBuild tag with cmake tag into defaultConfig section:

...
defaultConfig {
    applicationId "<YOUR_APP_ID>"
    minSdkVersion 26
    targetSdkVersion 26
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"


    externalNativeBuild {   <--- these lines should be added
        cmake {             <--- these lines should be added
            cppFlags ""     <--- these lines should be added
        }                   <--- these lines should be added
    }                       <--- these lines should be added
}
...

(example of “basic” build.gradle file also available in new, empty Android Studio project with C++ support)

6. Resync Project with Gradle files

By clicking Sync Project in the toolbar. NB! In Android Studio 3.3, the icon is .

Also, take a look at Official Tutorial.

PS. If files not shown in cpp folder:

try File/Invalidate Caches & Restart as Thinh Vu mentioned in his comment.