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 <jni.h>
#include <string>
#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