CMake could not find JNI

I created a new Android Studio Project with C++ support and did not add anything to it. The project builds, compiles and runs with CMake. I then added the following two lines of code to CMakeLists.txt at the bottom of the file and I get a CMake build error.

find_package(Java COMPONENTS Development)       # Line 47
find_package(JNI REQUIRED)                      # Line 48

And this is the error I get when building the project

-- Found Java: /usr/lib/jvm/java-10-oracle/bin/java (found version "10.0.1") found components:  Development 
-- Configuring incomplete, errors occurred!
CMake Error at /home/xxxxx/Android/Sdk/cmake/3.6.4111459/share/cmake-3.6/Modules/FindPackageHandleStandardArgs.cmake:148 (message):
  Could NOT find JNI (missing: JAVA_AWT_LIBRARY JAVA_JVM_LIBRARY
See also "/home/xxxxx/AndroidStudioProjects/MyApplication/app/.externalNativeBuild/cmake/release/x86_64/CMakeFiles/CMakeOutput.log".
  JAVA_INCLUDE_PATH2 JAVA_AWT_INCLUDE_PATH)
Call Stack (most recent call first):
  /home/xxxxxx/Android/Sdk/cmake/3.6.4111459/share/cmake-3.6/Modules/FindPackageHandleStandardArgs.cmake:388 (_FPHSA_FAILURE_MESSAGE)
  /home/xxxxxx/Android/Sdk/cmake/3.6.4111459/share/cmake-3.6/Modules/FindJNI.cmake:314 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:48 (find_package)
External native generate JSON release: JSON generation completed with problems

CMake runs when being called form the command line, but fails inside Android Studio for some reason and I am not sure why.

EDIT 1

Here is the CMakeOutput.log file

EDIT 2

This is all that is in CMakeLists.txt file. It compiles in cmd with the command cmake.

cmake_minimum_required(VERSION 3.4.1)

find_package(Java COMPONENTS Development)
find_package(JNI REQUIRED)

This is the cmd output

    E:UsersxxxxxAndroidStudioProjectsMyApplicationappbuild>cmake ..
-- Configuring done
-- Generating done
-- Build files have been written to: E:/Users/xxxxx/AndroidStudioProjects/MyApplication/app/build

When I try to add it compile it in Android Studio using gradle I get the following error on Windows 10 pro 64 bit

Microsoft Windows [Version 10.0.17134.112]
(c) 2018 Microsoft Corporation. All rights reserved.

E:UsersxxxxxAndroidStudioProjectsMyApplication>gradlew build --stacktrace

> Task :app:generateJsonModelDebug 
External native generate JSON debug: starting JSON generation
External native generate JSON debug: using platform version 24 for ABI ARMEABI_V7A and min SDK version 24
External native generate JSON debug: noticing that build file 'E:UsersxxxxxAndroidStudioProjectsMyApplicationappCMakeLists.txt' is out of date with respect to E:UsersxxxxxAndroidStudioProjectsMyApplicationapp.extern
alNativeBuildcmakedebugarmeabi-v7aandroid_gradle_build.json
External native generate JSON debug: rebuilding JSON E:UsersxxxxxAndroidStudioProjectsMyApplicationapp.externalNativeBuildcmakedebugarmeabi-v7aandroid_gradle_build.json due to:
External native generate JSON debug: - a dependent build file changed
External native generate JSON debug: keeping json folder 'E:UsersxxxxxAndroidStudioProjectsMyApplicationapp.externalNativeBuildcmakedebugarmeabi-v7a' but regenerating project
External native generate JSON debug: executing cmake Executable : E:AndroidSDKcmake3.6.4111459bincmake.exe
arguments :
-HE:UsersxxxxxAndroidStudioProjectsMyApplicationapp
-BE:UsersxxxxxAndroidStudioProjectsMyApplicationapp.externalNativeBuildcmakedebugarmeabi-v7a
-DANDROID_ABI=armeabi-v7a
-DANDROID_PLATFORM=android-24
-DCMAKE_LIBRARY_OUTPUT_DIRECTORY=E:UsersxxxxxAndroidStudioProjectsMyApplicationappbuildintermediatescmakedebugobjarmeabi-v7a
-DCMAKE_BUILD_TYPE=Debug
-DANDROID_NDK=E:AndroidSDKndk-bundle
-DCMAKE_CXX_FLAGS=
-DCMAKE_TOOLCHAIN_FILE=E:AndroidSDKndk-bundlebuildcmakeandroid.toolchain.cmake
-DCMAKE_MAKE_PROGRAM=E:AndroidSDKcmake3.6.4111459binninja.exe
-GAndroid Gradle - Ninja
jvmArgs :


CMake Error at E:/Android/SDK/cmake/3.6.4111459/share/cmake-3.6/Modules/FindPackageHandleStandardArgs.cmake:148 (message):
  Could NOT find JNI (missing: JAVA_AWT_LIBRARY JAVA_JVM_LIBRARY
  JAVA_INCLUDE_PATH2 JAVA_AWT_INCLUDE_PATH)
Call Stack (most recent call first):
  E:/Android/SDK/cmake/3.6.4111459/share/cmake-3.6/Modules/FindPackageHandleStandardArgs.cmake:388 (_FPHSA_FAILURE_MESSAGE)
  E:/Android/SDK/cmake/3.6.4111459/share/cmake-3.6/Modules/FindJNI.cmake:314 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  CMakeLists.txt:4 (find_package)
-- Configuring incomplete, errors occurred!
See also "E:/Users/xxxxx/AndroidStudioProjects/MyApplication/app/.externalNativeBuild/cmake/debug/armeabi-v7a/CMakeFiles/CMakeOutput.log".

Answer

As the CMake version bundled with Android tries to tell you, it can’t find the JNI package because some parts were missing:

missing: JAVA_AWT_LIBRARY JAVA_JVM_LIBRARY JAVA_INCLUDE_PATH2 JAVA_AWT_INCLUDE_PATH

Checking the documentation for FindJNI (https://cmake.org/cmake/help/latest/module/FindJNI.html) these variables are set to the locations of libraries and headers not shipped with the Android version of JNI (to little surprise, Android does not included the AWT library for instance). When running find_package(JNI REQUIRED), the FindJNI code checks if these variables are set and if not, issues an error.

A workaround is to set these variables yourself, before calling find_package:

# We are only interested in finding jni.h: we do not care about extended JVM
# functionality or the AWT library.
set(JAVA_AWT_LIBRARY NotNeeded)
set(JAVA_JVM_LIBRARY NotNeeded)
set(JAVA_INCLUDE_PATH2 NotNeeded)
set(JAVA_AWT_INCLUDE_PATH NotNeeded)
find_package(JNI REQUIRED)

Be aware though, that your code will only be able to use jni.h and its functionality: if it tries to access any other part of the JNI package it will fail (probably at compile time) because essentially, you have tricked CMake into thinking that the entire package was found, when in reality only a part of it exists in the Android setup.