AndroidStudio JNI开发，通过Cmake编译so文件

官方文档：https://developer.android.com/ndk/guides/cmake

1、创建项目，在首界面勾选上Include C++ support，然后点击next，直至创建完成

2、项目会默认生成jni事例代码，观察项目结构，在app/src/main下有个cpp文件夹，我们要写或者要添加的c/c++文件就写在该文件夹下。

3、打开MainActivity.java，在里面可以看到这段代码

 // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("native-lib");
    }
    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();

其中native-lib就是我们要生成的so文件的名称，在后续配置文件中还会用到，stringFromJNI就是调用c/c++的方法，我们打开app/src/main/cpp/native-lib.cpp文件，就会在其中发现有个Java_com_lianluo_jnitest_MainActivity_stringFromJNI方法，就是与之对应的。现在我们自己写个native方法，public native String helloWorld()；此时我们发现helloWorld方法名是红色的，这是把鼠标指针移上去，然后点击alt+enter键，选择create function Java_com_lianluo_jnitest_MainActivity_helloWorld，这时会在native-lib.cpp文件中自动生成对应的c/c++方法。

4、打开app下的CMakeLists.txt文件，这个是进行配置cmake的，详细的配置请看官网，这里简单的写一下要注意的地方

# 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.
            //这里要写上我们要生成的so文件名称，要与MainActivity.java里的名称对应
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             //这里要写上我们放在项目里的c/c++文件
             src/main/cpp/native-lib.cpp )

# 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.
                        //这里要写上我们要生成的so文件名称，要与上面的名称对应
                       native-lib

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

4、打开app下build.gradle，将Cmake配置到gradle中
要在

android {
    compileSdkVersion 27
    defaultConfig {

        ...

        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }

    ...

    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
}

5、编译so文件，当写好代码后，我们clean项目，然后在工具栏build里面选择Make Project，这是在app的build/intermediates/cmake/debug(release)/obj文件夹下生成对应的so文件

注意：使用cmake编译so文件时，绝对不能有中文路径，否则会报这个错误

Build command failed.
Error while executing process /Users/krubo/Library/Android/sdk/cmake/3.6.4111459/bin/cmake with arguments {--build /Users/krubo/Documents/工作/项目代码/AndroidStudioProjects/JniTest/app/.externalNativeBuild/cmake/debug/x86_64 --target native-lib}
ninja: warning: premature end of file; recovering
[1/2] Building CXX object CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o
ninja: build stopped: Error writing to deps log: No such file or directory.