Android studio 使用Cmake完成C/C++ 的使用以及生成so文件

今天，简单讲讲android中关于Cmake进行NDK编程的相关知识。

Android studio 2.2版本以后对C/C++的支持可以说很方便了，当然官方推荐使用Cmake完成对C/C++的支持

2.2版本以上的同学新建一个项目就知道了，步骤如下：

File -> New -> New Project,如下图：

然后勾选Include C++ support,一直next ，最后Finish以后，项目就出现了，和一般的项目略有不同，其实只要多了几个文件，而已：

1：目录下多了个CmakeLists.txt文件

2：src目录下多了一个cpp目录，里面有个.cpp文件，C++文件都是以.cpp结尾的。

3：就是build.gradle内容中添加了几行配置

一一解读一下，先来看看CmakeLists.txt 文件里面的内容是什么：

# 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 )

# 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} )

这里有设置Cmake版本啊，引用的cpp文件的路径啊等等，如果自己引用的话 其实值需要修改引用的cpp路径即可，

其他的暂时不需要动

看下native_lib.cpp文件内容：

#include <jni.h>
#include <string>

extern "C"
JNIEXPORT jstring JNICALL
Java_com_process_main_myapplication_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

很简单 其实就是输出Hello from C++这段文字：

再看下build.gradle的配置改变：

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "com.process.main.myapplication"
        minSdkVersion 22
        targetSdkVersion 25
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:25.1.1'
    compile 'com.android.support.constraint:constraint-layout:1.0.0-beta4'
    testCompile 'junit:junit:4.12'
}

主要多了两个地方的改变：

1：defaultConfig中添加：

externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }

2：在android{}中添加：

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

其实也就是引用CmakeLists.txt文件。

现在重点讲讲CmakeLists.txt里面的代码的含义。

以上的CmakeLists.txt对于与图片的目录结构，可以看出主要是add_liberary里写入需要编译的C文件的路径，其余的基本不用修改。

还有就是要是在add_liberary中添加.cpp文件的话记得路径，如果你的CMakeLists和你的.cpp在一个问价夹下就不用管路径。

Android studio 使用Cmake完成C/C++ 的使用以及生成so文件就讲完了。

就这么简单。