Android Studio已创建项目下配置CMake

使用Android Studio在新的项目添加CMake支持，操作十分简单，只需要勾选“Include C/C++ support”，Android Studio就会自动为新项目配置好CMake环境。
但对于已创建的项目，Android Studio似乎并没有提供自动配置的功能。
要在已创建的项目中配置CMake，需要进行以下步骤:
首先，在项目的build.gradle文件下，修改配置如下：

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.2.1'
        

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

建议使用Gradle 3.2.1版本，使用其他版本编译，可能会出现配置错误的问题。由于Gradle 3.2.1版本对应的zip为gradle-4.6-all.zip，因此还要修改gradle-wrapper.properties文件:

#Sun Jun 16 13:00:26 CST 2019
distributionBase=GRADLE_USER_HOME
distributionPath=wrapper/dists
zipStoreBase=GRADLE_USER_HOME
zipStorePath=wrapper/dists
distributionUrl=https\://services.gradle.org/distributions/gradle-4.6-all.zip

在app文件夹下build.gradle文件中，分别在android和defaultConfig配置中添加externalNativeBuild:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "cn.axen.demo"
        minSdkVersion 19
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        // defaultConfig中添加externalNativeBuild
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    // android中添加externalNativeBuild
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

在app文件夹下添加CMakeLists.txt，在CMakeLists.txt添加如下内容：

cmake_minimum_required(VERSION 3.4.1)


add_library( # Sets the name of the library.
             jni_lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             )

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 )

target_link_libraries( # Specifies the target library.
                       jni_lib

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

点击项目右键，创建JNI文件夹:

创建jni文件夹.png
新建C/C++ Source File，这里命名为jni_hello.cpp:
新建C/C++ Source File.png
新建C/C++ Source File.png
jni_hello.cpp代码如下：

#include<jni.h>

#include <string>

extern "C"JNIEXPORT

jstring JNICALL Java_cn_axen_demo_MainActivity_stringFromJNI(JNIEnv *env, jclass thiz)
{
    std::string hello = "Hello,I from C++";
    return env->NewStringUTF(hello.c_str());
}

将jni_hello.cpp添加到CMakeLists.txt中：

cmake_minimum_required(VERSION 3.4.1)


add_library( # Sets the name of the library.
             jni_lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/jni/jni_hello.cpp)

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 )

target_link_libraries( # Specifies the target library.
                       jni_lib

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

大功告成，rebuild项目之后，就可以使用jni_hello里面的函数啦:

package cn.axen.demo;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView= findViewById(R.id.tv);
        textView.setText(stringFromJNI);

    }

    static{
        System.loadLibrary( "jni_lib");
    }

    private static native String stringFromJNI();