Android NDK 开发流程

一、下载并配置NDK

下载地址：https://developer.android.google.cn/ndk/downloads/
下载成功，直接解压配置自己的NDK路径，注意中间不要有空格，否则后面开发中会出问题。

二、创建 Android 项目，进行NDK配置

1、配置好NDK路径之后记得在工程目录的local.properties文件中进行关联，注意要配置自己NDK的路径

2、如果建立的是普通的 Android 项目，可按照文章内容进行配置，如果直接建立C/C++项目，下面的配置可以直接忽略的，因为系统自动配置好了。

三、在 Android 项目中声明 native 方法

public class Jnitest {
    // 声明所需的 native 方法
    static {
        System.loadLibrary("Jnitest");
    }
    public native String getString();
    public native void setString(String str);
}

四、编译 native 方法生成 .h 头文件

1、点击 Build –> Make Project，生成 class 文件

2、找到class文件，在app->build->intermediates->classes->debug目录下：

3、通过javah命令生成.h头文件，点击 Android Studio 底下菜单中的 Terminal ,一次输入：

cd app\build\intermediates\classes\debug

// 这里注意改成自己的包名
javah -jni com.mtf.ndktest.Jnitest 

入下图所示，您的 .h 头文件已经生成成功，默认位置在 debug 目录下

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_mtf_ndktest_Jnitest */
#ifndef _Included_com_mtf_ndktest_Jnitest
#define _Included_com_mtf_ndktest_Jnitest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_mtf_ndktest_Jnitest
 * Method:    getString
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_mtf_ndktest_Jnitest_getString
  (JNIEnv *, jobject);
/*
 * Class:     com_mtf_ndktest_Jnitest
 * Method:    setString
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_com_mtf_ndktest_Jnitest_setString
  (JNIEnv *, jobject, jstring);
#ifdef __cplusplus
}
#endif
#endif

五、创建 C/C++ 文件实现 .h 文件

创建jni目录，将刚才生成的.h文件复制过来，创建.c/.cpp文件，实现.h文件中的方法，方法名称可以直接从.h文件中复制过来。

六、编译生成 .so 文件

1、用cmake方式编译so库，创建CMakeLists.txt文件。

# 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.
             Jnitest
             # Sets the library as a shared library.
             SHARED
             # Provides a relative path to your source file(s).
             src/main/jni/Jnitest.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.
                       Jnitest
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

2、配置app module目录下的build.gradle文件

android {
    ......
    defaultConfig {
        ......
        externalNativeBuild {
            cmake {
                cppFlags ""
                abiFilters "armeabi", "armeabi-v7a", "x86" //输出指定cpu体系结构下的so库。
            }
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
    ......
}

3、编译并看一下结果

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        TextView tv = (TextView) findViewById(R.id.tv_test);
        tv.setText(Jnitest.getString());
    }
}

（完）