Android Studio JNI 使用模板：c/cpp源文件的集成编译，快速上手

一、前言

• JNI 技术，使得Java可以调用C/CPP编写的代码库，也是老技术了，对于不想花时间研究的同学，可以照抄本文的编译模板。
• JNI代码的AS编译，有两种途径，其一是NDK配置编译，其二是cmake的配置编译，本文采用第二种，也是AS直接支持创建的方式。

二、实现步骤

2.1 创建 cpp目录

• tv-settings\app\src\main\cpp

2.2 创建jni文件

1. CMakeList.txt 文件，负责处理编译
2. 你的 c/cpp 源码文件，本文： JniMotor.c

图示如下：

2.3 编辑CMakeList.txt

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html.
# For more examples on how to use CMake, see https://github.com/android/ndk-samples.

# Sets the minimum CMake version required for this project.
cmake_minimum_required(VERSION 3.22.1)

# Declares the project name. The project name can be accessed via ${ PROJECT_NAME},
# Since this is the top level CMakeLists.txt, the project name is also accessible
# with ${CMAKE_PROJECT_NAME} (both CMake variables are in-sync within the top level
# build script scope).
# 备注：如直接采用AS创建的JNI项目，会将project name作为so库的名字
project("TvSettings")

# 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.
#
# In this top level CMakeLists.txt, ${CMAKE_PROJECT_NAME} is used to define
# the target library name; in the sub-module's CMakeLists.txt, ${PROJECT_NAME}
# is preferred for the same purpose.
#
# In order to load a library into your app from Java/Kotlin, you must call
# System.loadLibrary() and pass the name of the library defined here;
# for GameActivity/NativeActivity derived applications, the same library name must be
# used in the AndroidManifest.xml file.
# 备注：此处罗列你的 c / cpp 文件
add_library(jnimotor SHARED
        # List C/C++ source files with relative paths to this CMakeLists.txt.
        JniMotor.c)

# Specifies libraries CMake should link to your target library. You
# can link libraries from various origins, such as libraries defined in this
# build script, prebuilt third-party libraries, or Android system libraries.
# 备注：此处第一个参数jnimotor，就是生产的库的名字：libjnimotor.so
target_link_libraries(jnimotor
        # List libraries link to the target library
        android
        log)
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2.4 编辑 JniMotor.c

• 以下为C源文件模板
• 如采用CPP的同学，记得添加extern “C” 标记，因为CPP编译产生的符号与C不同，需要产生C的符号供JAVA调用。

//略……
#include <jni.h>
#include <android/log.h>
#define TAG "JniMonitor"

#define LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG, TAG, __VA_ARGS__)

//略……


static int motorioctl(int des, int step)
{
//略……
}

JNIEXPORT jint JNICALL Java_com_android_tv_settings_device_displaysound_aw_1displaysound_JniMotor_ioctl
  (JNIEnv *env, jclass clas, jint des, jint step){
    int i = des;
    int j = step;
    return motorioctl(i, j);
}

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2.5 编辑build.gradle

• 在 app/build.gradle 的android{} 增加如下配置，指明使用Native build

android {
 …… 略 ……
    externalNativeBuild {
        cmake {
            path file('src/main/cpp/CMakeLists.txt')
            version '3.22.1'
        }
    }
 …… 略 ……
}
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三、执行编译

• 如下图所示， 直接产生了个eabi所需的SO文件，大功告成

编译结果：图示一

编译结果：图示二