CMake方式编译生成so库

在CMake构建工具没有发布之前，都是ndk-build方式来编译生成so库，但是ndk-build比较繁琐且容易出错，我们用CMake方式，比较简单，这样我们就可以专注于我们的开发，细心的哥们可能早就注意到了Android Studio创建项目的时候，可以勾选支持这种方式：

看图：

这样是最省事的，我先看下项目都生成了什么特别的东西：
CMakeLists.txt 这个是最显眼的文件，没错，系统帮助我们完成了所有的东西，最重要的就是这个文件，文件内容如下：

# 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.
#最低要求版本号
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. 设置so库的名字，这里叫native-lib
             native-lib

             # Sets the library as a shared library.动态库，会被动态链接，在运行时被加载
             SHARED

             # Provides a relative path to your source file(s).要编译的源代码文件的路径，如果你的不是cpp下的，这里记得同步改下
             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.日志支持库并将其路径存储在 log-lib 中
              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.为了确保您的原生库可以在 log 库中调用函数
                       native-lib

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

接着看模块下的build.gradle文件（记住不是工程下）：

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.example.administrator.myapplication"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }

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

这是默认生成的。到这里基本上就没了，所以很简单。不过这是新建项目的时候就选择支持CMake，但是这不符合我们正常开发，好多都是中途要引入Jni\Ndk开发的，怎么办？我觉得我们可以新建一个项目，然后把CMakeLists.txt文件copy到我们现有项目中，然后修改配置，重新编辑即可，是不是很机智QAQ。

大家坐好，看我表演~
首先，新建c文件夹（其实无所谓，因为我待会在文件夹下写c语言，所以取名为c）
然后，新建C文件，C的后缀是.c而C++是.cpp
native-lib.c 代码：

#include <jni.h>

jstring Java_com_example_administrator_kotlinapp_ui_JniActivity_sayHello(JNIEnv* env,jobject jobj){
          char* text="I am from C";
          return (*env)->NewStringUTF(env,text);
 }

看不懂的朋友要去补补C语言咯T^T

接着我们把CMakeLists.txt文件copy过来，如下：

注意：源文件的路径一定要正确

再看下build.gradle:

android {
    compileSdkVersion 26
    defaultConfig {
        applicationId "com.example.administrator.kotlinapp"
        minSdkVersion 15
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }

        ndk {
            // Specifies the ABI configurations of your native
            // libraries Gradle should build and package with your APK.
            abiFilters 'armeabi', 'x86'
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
}

这样我们就可以用Java调用C语言了：

public class JniActivity extends AppCompatActivity {

     {
        System.loadLibrary("native-lib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_jni);
        ((TextView)findViewById(R.id.textView)).setText(sayHello());
    }

    public native String sayHello();
}

这时候rebuild下，就可以看到生成的so库了：