初心者_JNI开发备忘

不BB，开怼。

环境： Android studio 3.2   Cmake  

app目录下 添加一个CMakeLists.txt文件 

文件内容 ： 

# 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( # so文件调用名称. 
        mycall
        # Sets the library as a shared library.
        SHARED
        # C文件路劲.
        src/main/cpp/mycall.c)

# 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(
        mycall
        ${log-lib})

 

build.gradle 添加

defaultConfig {
        applicationId "com.example.administrator.ndkbuilddemo"
        minSdkVersion 19
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
                //生成多个版本的so文件
                abiFilters 'arm64-v8a', 'armeabi-v7a', 'x86', 'x86_64'
            }
        }
    }

 

android {
    ......

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

    ......
}

 

java 调用 C 

 native  方法  然后在cpp/c 文件中添加对应的方法  然后编译即可。

或者对着方法名  

生成一哈。 

就会在C/CPP 文件内 生成对应的方法 

然后就开始搞事情就可以了。。

 

C调用JAVA方法

//  反射java类的字节码  jclass      (*FindClass)(JNIEnv*, const char* = 包名+类名);
jclass clz=(*env)->FindClass(env,"com/example/administrator/ndkdemo3/JNI");
//调取方法   jmethodID   (*GetMethodID)(JNIEnv*, jclass=类, const char* = 方法名 , const char*  =签名 );
//关于签名 需要生成class 后 到class文件夹下  执行  javap -s 文件名 编译得到方法签名列表 

jmethodID mtd=(*env)->GetMethodID(env,clz,"showText","(II)V");


//得到类对象jobject     (*AllocObject)(JNIEnv*, jclass = 上面的字节码 );
jobject jb=(*env)->AllocObject(env,clz);

// 执行方法  void    (*CallVoidMethod)(JNIEnv*, jobject=类对象, jmethodID=方法 , ... = 参数xN);
(*env)->CallVoidMethod(env,jb,mtd,77,66);

 

签名需要说一下：

方法签名需要到编译后的class文件下  执行 javap -s 文件.class 

 

 

全部完事后 执行 得到C调java的返回结果

 

 jni.h的文件  有很多示例可以参考。

比如说：酱紫的。

 

剩下自由发挥。。