cmake jni简说

CMake配置Jni开发

1 安装工具

2 新建项目，注意include C++ support选项

项目创建好后查看目录结构

新增三块.externalNativeBuild；cpp（c、c++代码）；CMakeList.txt

CMakeList.txt

# 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.

# 配置so库信息
add_library( # Sets the name of the library.
             # 生成的so库名称，名称是libnative-lib.so
             native-lib

# SHARED：动态库，会被动态链接，在运行时被加载
             # Sets the library as a shared library.
             SHARED

# 要编译的c/c++文件列表（可以多个），文件路径想对于cmake文件路径
             # 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.

# 从系统查找依赖库
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.

# 配置库的链接（依赖关系）
target_link_libraries( # Specifies the target library.
                       # 目标库
                       native-lib

# 依赖于
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

app/build.gradle 文件变动，配置CMake

defaultConfig {
　　　...
    externalNativeBuild {
        cmake {
            cppFlags ""
        }
    }
}

externalNativeBuild {
    cmake {
        path "CMakeLists.txt"
    }
}

代码部分
java类
首先引入

// Used to load the 'native-lib' library on application startup.
static {
    System.loadLibrary("native-lib");
}

创建本地方法

public native String stringFromJNI();

快键键atl+回车，弹出框中选Gread function ***,自从在C++文件中创建对应的类

extern "C"
JNIEXPORT jstring
JNICALL
Java_com_test_myjniapplication_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

运行程序，java端获取C++端返回数据"Hello from C++"

C++端添加回调
java端添加函数

public void callbackFromJni(){}

public void callbackFromJni2J(int num, int num1){}

c++端函数内添加

extern "C"
JNIEXPORT jstring
JNICALL
Java_com_test_myjniapplication_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {

　　// 直接用GetObjectClass找到Class, 也就是Sdk.class.
　　jclass jSdkClass = env->GetObjectClass(instance);

　　// jmethodID javaCallback = env->GetMethodID(jSdkClass, "callbackFromJni", "()V");
　　// env->CallVoidMethod(instance, javaCallback);

　　//找到需要调用的方法ID(类似反射)
　　jmethodID javaCallback = env->GetMethodID(jSdkClass, "callbackFromJni2J", "(II)V");

　　//进行回调
　　env->CallVoidMethod(instance, javaCallback, 10, 1);

　　std::string hello = "Hello from C++";
　　return env->NewStringUTF(hello.c_str());
}