概要

在日常开发中，android NDK的作用无外乎有两种：一种是通过调用底层C/C++的算法，提高app的运行效率；另一种则是通过C/C++的特性，或者和驱动交互等，实现一些功能性的需求。接下来将用详细的步骤，给大家演示一下android ndk调用三方so库的过程。

1.NDK环境准备

1.1.通过android studio在SDK Tools中安装LLDB、NDK、CMake，如下图：

1.2.安装好了NDK之后，将ndk-bundle目录设置到系统环境变量Path中去，如下图：

2.生成一个三方so库

2.1.创建一个jni目录，在该目录下创建四个文件(注意目录的名字一定要是jni)：MyLib.h,MyLib.cpp,Android.mk,Application.mk

a.MyLib.h

#ifndef TESTNDK_MYLIB_H

#define TESTNDK_MYLIB_H

class MyLib {

public:

MyLib();

~MyLib();

int add(int a,int b);

};

#endif //TESTNDK_MYLIB_H

b.MyLib.cpp

#include "MyLib.h"

MyLib::MyLib()

{

}

MyLib::~MyLib()

{

}

int MyLib::add(int a,int b)

{

return a+b;

}

c.Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE := mylib

LOCAL_SRC_FILES := MyLib.cpp

NDK_APP_DST_DIR := ./jniLibs/$(TARGET_ARCH_ABI)

include $(BUILD_SHARED_LIBRARY)

d.Application.mk

APP_STL:=c++_shared

APP_PLATFORM := android-16

APP_CPPFLAGS:=-frtti -fexceptions

APP_ABI := all

2.2.进入到CMD，执行ndk-build命令：

命令执行成功之后，就会在当前目录下生成jniLibs的目录，里面就是我们需要的so库文件了。

3.Android项目中引入三方so库

3.1.创建android项目，配置NDK环境

3.2.将jniLibs文件夹拷贝到项目中

3.3.配置CMakeLists.txt文件，将三方so配置进去

完整的CMakeLists.txt如下

# 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).

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 )

add_library(

mylib

SHARED

IMPORTED)

set_target_properties(

mylib

PROPERTIES IMPORTED_LOCATION

${PROJECT_SOURCE_DIR}/../jniLibs/${ANDROID_ABI}/libmylib.so

)

include_directories(src/main/cpp/include)

# 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.

mylib

${log-lib} )

3.4.在模块级build.gradle中配置c++_shared的库，因为mylib.so依赖于它

arguments "-DANDROID_STL=c++_shared"

4.调用so库中的方法

4.1.将MyLib.h文件拷贝到cpp/include目录下(这个目录需要配置到CMakeLists.txt文件中，上面的操作已经配置过了)

4.2.在默认生成的native-lib.cpp中调用我们so库中的方法

#include

#include

#include "./include/MyLib.h"

extern "C" JNIEXPORT jstring JNICALL

Java_com_example_testndk_MainActivity_stringFromJNI(JNIEnv* env,jobject) {

MyLib myLib;

int result = myLib.add(10,10);

return env->NewStringUTF(std::to_string(result).c_str());

}

5.效果