基于CMake Android NDK学习

网上很多NDK开发入门都是基于Ecplise，ndk-build方式的开发教程，本文主要介绍在现有项目中用CMake脚本方式添加C/C++代码。

Android NDK 安装教程参考官网

官网开发教程

NDK开发教程

一.创建源文件

• IDE左侧，切换项目为Project试图
• 导航到src，右键点击main目录，选择New > Directory
• 在目录中给文件夹取名（官网取名cpp，随便取）并点击OK
• 右键点击上一步创建目录，然后选择New > C/C++ Source File
• 为源文件输入一个名称，如native-lib（爱咋取咋取），然后点击OK

二.创建CMake脚本

右键点击app，选择New > File,创建名为CMakeLists.txt作为文件名并点击OK（名字不能看心情💢发挥了）

接下来我们添加CMake以下命令，开始构建脚本（其他相关操作配置，，根据相关需求参考官方文档）

# Sets the minimum version of CMake required to build your native library.
# This ensures that a certain set of CMake features is available to
# your build.
#定义Cmake 版本号，每个版本新增功能，参考CMake官方文档
cmake_minimum_required(VERSION 3.4.1)

# Specifies a library name, specifies whether the library is STATIC or
# SHARED, and provides relative paths to the source code. You can
# define multiple libraries by adding multiple add.library() commands,
# and CMake builds them for you. When you build your app, Gradle
# automatically packages shared libraries with your APK.
#定义library名字（生成的so库名：lib库名称.so）和定义library相对路径
add_library( # Specifies 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 )
#引用库
find_library( # Defines the name of the path variable that stores the
              # location of the NDK library.
              log-lib

              # Specifies the name of the NDK library that
              # CMake needs to locate.
              log )

# Links your native library against one or more other native libraries.
# 关联相关lib
target_link_libraries( # Specifies the target library.
                       native-lib

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

三.使用Android GUI关联CMake

• 左侧IDE打开Project选择app模块
• 右键app文件夹，从菜单中选择Link C++ Project with Gradle
• 在弹框中，Build System选择CMake方式，Path选择你的CMakeLists.txt文件，点击OK等待gradle构建完成

四.开始在java中定义native方法

• 定义native方法

public class NativeDemo {
    //静态native方法
    public static native String helloFromJNI();
}
//非静态native方法
public class Hello {
    public int property;
    public int function(int foo, Date date, int[] arr) {
        System.out.println("function");
        Log.d("Yxjie","jni输入数字："+foo);
        return foo;
    }
    public boolean func(int a ,double b,char c){
        return true;
    }
    public native void test();
}

• 开始写native-lib.cpp文件
  开始编写之前我们需要了解一下签名格式：
  

<font color=red>注：char类型参数要在前面加个L，如L'a',java
中字符在Unicode是双字节的，而C++中的字符是单字节，需要加宽</font>

#include "jni.h"

extern "C"
JNIEXPORT jstring  JNICALL
Java_com_example_ndktest_NativeDemo_helloFromJNI(JNIEnv *env, jclass jclz) {
    //静态native方法 需要两个参数JNIEnv，以及jclass
    return env->NewStringUTF("Hello From JNI");
}

extern "C"
JNIEXPORT void Java_com_example_ndktest_Hello_test(JNIEnv *env, jobject obj) {
    //非静态方法需要JNIEnv以及jobject（java中Hello，class对象引用）
    jclass hello_clazz = env->GetObjectClass(obj);//获取java端 hello.class
    jfieldID fieldId_prop = env->GetFieldID(hello_clazz, "property", "I");//获取属性
    jint prop_int = env->GetIntField(hello_clazz, fieldId_prop);//获取java prop默认值

    jmethodID methodId_function = env->GetMethodID(hello_clazz, "function",
                                                   "(ILjava/util/Date;[I)I");//获取对应方法

    //批量传入参数
    jmethodID methodId_func = env->GetMethodID(hello_clazz, "func", "(IDC)Z");
    jvalue *args = new jvalue[3];
    args[0].i = 10L;
    args[1].d = 3.14;
    args[3].c = L'j';//Java 中字符双字节，C++字符为单子接需要编程器宽字节
    env->CallBooleanMethod(hello_clazz, methodId_func, args);
    delete[] args;
    //直接传值
    env->SetIntField(hello_clazz, fieldId_prop, 100L);//设置java类中props的值
    env->CallIntMethod(obj, methodId_function, 11L, NULL, NULL);//调用方法

}

• 加载load lib
  我是直接放在我的入口Activity里面，直接静态代码块加载lib

public class MainActivity extends AppCompatActivity {

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView txt=findViewById(R.id.txt);
        txt.setText(NativeDemo.helloFromJNI());

        Hello hello=new Hello();
        hello.test();
    }
}

<font color=red>注：如有错误欢迎留言指正</font>