概述

Android Studio使用ndk的简单步骤。

NDK环境搭建

下载NDK

下载链接：https://developer.android.com/ndk/downloads/index.html

PS：需要翻墙，建议下载r9+的版本。

国内下载地址：

http://www.wanandroid.com/tools/ide

解压 NDK包【建议在未打开Android Studio的情况下】

注：解压路径不要出现空格和中文。

建议：将文件解压到SDK目录里，并命名为ndk-bundle。好处：启动Android Studio时，Android Studio会自动检查它并直接添加到ndk.dir中，那么在使用时，就不用配置Android Studio与NDK的关联【解压的时候需要直接解压到sdk安装目录/ndk-bundle目录中才可以实现自动关联，否则需要手动关联】

因为我个人觉得不是每一个项目都需要用到ndk，所以就采用了手动关联的方式。

自动关联：sdk安装目录/ndk-bundle

手动关联：其他目录

下载安装Cmake

我是通过新建一个项目，根据Android Studio的提示信息进行安装的。其实也可以跳过新建项目的步骤，直接安装SDK Manager中安装。具体操作步骤见下文。

新建项目

勾选Include C++ support

Next

选择C++标准，一般选择默认即可

手动关联NDK

对于解压ndk未解压到自动关联的目录（sdk安装目录/ndk-bundle）的情况，新建项目后会出现下面的提示信息，解决方案就是手动关联ndk。

对于解压ndk到自动关联的目录（sdk安装目录/ndk-bundle）的情况，可以跳过。因为不会出现下面的提示信息。如果万一出现了的话，那么就手动关联ndk即可。

File——Project Structure…

选择ndk路径

查看项目根目录的local.properties文件，会发现多了一行代码：

添加对旧版本的NDK支持

在工程中gradle.properties中添加以下代码：android.useDeprecatedNdk=true

下载安装Cmake

第一次运行会报错，原因是未安装Cmake。

打开SDK Manager

方式一

方式二：File——Settings…

安装cmake

运行

新建的项目含有一个cpp文件，可以看下效果：

将指定的.h和.cpp文件编译成so文件

首先修改生成的so文件的名称

打开CMakeLists.txt

最开始的：

# 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} )

修改后的：

# 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.
# 编译出一个动态库 ndklib（名字随意命名），源文件只有 src/main/cpp/native-lib.cpp（如果含有多个的话，需要添加多行类似的代码）
add_library( # Sets the name of the library.
ndklib # 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.
# 找到预编译库 log_lib 并link到我们的动态库 ndklib（跟上面的保持一致）中
target_link_libraries( # Specifies the target library.
ndklib # Links the target library to the log library
# included in the NDK.
${log-lib} )

这样命名的话生成的so文件如下（前缀自动有个lib）：

新建一个类文件LibNDKDemo.java（名字随意），并且引用我们新建的so库

添加以下代码：

    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("ndklib");
    }    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public static native String stringFromJNI();

一般会有红色报错，不过不用着急。

鼠标定位到stringFromJNI方法那里，然后点击Alt+Enter，选择第一个AS会自动帮我们生成实现：

自动跳转到native-lib.cpp文件

将returnValue修改成真实的数据，并删除旧数据（上方黑色边框标记的代码），修改后的代码如下：

#include <jni.h>
#include <string>extern "C"
JNIEXPORT jstring JNICALL
Java_com_why_project_ndkdemo_LibNDKDemo_stringFromJNI(JNIEnv *env, jobject instance) {    // TODO
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

Build——Rebuild Project

此时，生成了debug模式下的so文件。

调用

将MainActivity.java下面的代码删除

修改后的代码：

package com.why.project.ndkdemo;import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;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.sample_text);
        tv.setText(LibNDKDemo.stringFromJNI());
    }
}

运行不报错，说明成功。

生成release版本的so文件

PS:（Debug版本和release版本，做个C++的都知道。debug版本是调试的使用的，里面包含很多的调试信息，文件体积也是比较大；release版本发布的时候使用的，会自动的去除掉里面的调试信息，文件体积比较小）。通过Gradle projects生成release版本：

生成的release版本so文件的位置：

将so文件结合module生成arr文件

其实上面的so文件就可以集成到项目中使用了，不过如果想要进一步封装so文件，比如指定cpu类型的so文件，或者还有其他代码需要配合调用等。

新建module（命名随意，包名需要跟so文件中的java文件包名一致）

module的包名必须跟so文件中的java文件（比如上面步骤中的LibNDKDemo.java）包名一致。

将需要用到的cpu类型的so文件（release版本）和java类文件拷贝到module中

一般不用将所有cpu类型的so文件拷贝到module中，根据实际项目情况。我这里将常用的cpu类型的so文件复制到module中。

将module生成arr文件

选择边上的Gradle——选择{module}目录下的 Tasks->build->assembleRelease方法

生成的arr文件位置：

将arr文件集成到其他项目中

注意：集成到的项目的编译、目标、最低SDK版本号应该大于等于生成arr文件的module中设置的版本号。

具体步骤，参考《【Android Studio安装部署系列】十七、Android studio引用第三方库、jar、so、arr文件》

调用

运行效果：

遇到的问题

如果cpp目录下含有C文件，并且别的cpp文件引用这个C文件了，那么CMakeLists.txt文件需要添加以下配置

# 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)add_library(aes-lib STATIC src/main/cpp/aes.c)# 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.
# 编译出一个动态库 urlpath，源文件有 src/main/cpp/native-lib.cpp等
add_library( # Sets the name of the library.
urlpath # Sets the library as a shared library.
SHARED # Provides a relative path to your source file(s).
src/main/cpp/base64.cpp
src/main/cpp/url_auth.cpp
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.
#找到预编译库 log_lib 并link到我们的动态库 urlauth中
target_link_libraries( # Specifies the target library.
urkpath
aes-lib
# Links the target library to the log library
# included in the NDK.
${log-lib} )