Android studio配置opencv的JNI接口，实现C++代码编程

一、新建c++项目，C++版本选C++14，在工程导入Java的包，命名为opencv

二、配置opencv的JNI接口，拷贝opencv所需的库文件和头文件

1）把opencv Android sdk下的OpenCV-android-sdk/sdk/native/jni/include文件夹拷贝到你的项目中src/main/cpp下面。

2）把opencv Android sdk下的OpenCV-android-sdk/sdk/native/libs文件夹拷贝到你的项目中src/main/下面。并修改名字为JniLibs。

三、对CMake文件进行修改，打开CMakeLists.txt文件，添加如下代码，修改的地方只有最上面和最下面两部分，其中需要注意的是，设置include文件夹的地址：include_directories($CMAK{E}_{S}OURC{E}_{D}IR/include)，这里直接写/include，不要写成/src/main/include这种格式，否则无法调用。se{t}_{t}arge{t}_{p}roperties(libopenc{v}_{j}ava4PROPERTIESIMPORTE{D}_{L}OCATION（你自己JniLibs的地址）/${ANDROID_ABI}/libopencv_java4.so）
注意Java4.so的版本，我的版本为4。代码如下：

# 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 3.10.2)

# Declares and names the project.
# 设置include文件夹的地址
include_directories(${CMAKE_SOURCE_DIR}/include)
add_library(libopencv_java4 SHARED IMPORTED)
set_target_properties(libopencv_java4 PROPERTIES IMPORTED_LOCATION
        D:/LenovoSoftstore/Install/Android/Demo/JniDemo/app/src/main/JniLibs/${ANDROID_ABI}/libopencv_java4.so)


project("jnidemo")

# 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 )

# 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

                       libopencv_java4
                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

如果实验代码中含有图片，则可在target_link_libraries中加入jnigraphics，如下：

四、配置app的build.gradle文件，打开app的build.gradle文件，进行如下添加。
1，添加的的第一部分，在cmake中加上arguments “-DANDROID_STL=c++_shared”，如下：
cmake {
cppFlags ‘-std=c++14’
arguments “-DANDROID_STL=c++_shared”
}
2，添加的的第二部分：
ndk {
}

2，添加第三部分：
sourceSets {
}
splits {
}
3，添加的第四部分：
project.ext.versionCodes
android.applicationVariants.all {
}

整个代码：

plugins {
    id 'com.android.application'
}

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.2"

    defaultConfig {
        applicationId "com.example.jnidemo"
        minSdkVersion 19
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags '-std=c++14'
                arguments "-DANDROID_STL=c++_shared"
            }
        }
        ndk{
            abiFilters "armeabi-v7a", "arm64-v8a"
        }


    }

    sourceSets {
        main {

            jniLibs.srcDirs = ['libs']
        }
    }

//    packagingOptions {
//        exclude 'lib/arm64-v8a/libopencv_java4.so'
//    }

    splits {
        abi {
            enable true
            reset()
            include 'x86', 'x86_64', 'armeabi-v7a', 'arm64-v8a' //select ABIs to build APKs for
            universalApk true //generate an additional APK that contains all the ABIs
        }
    }
    project.ext.versionCodes = ['armeabi': 1, 'armeabi-v7a': 2, 'arm64-v8a': 3, 'mips': 5, 'mips64': 6, 'x86': 8, 'x86_64': 9]

    android.applicationVariants.all { variant ->
        variant.outputs.each { output ->
            output.versionCodeOverride =
                    project.ext.versionCodes.get(output.getFilter(com.android.build.OutputFile.ABI), 0) * 1000000 + android.defaultConfig.versionCode
        }
    }





    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    externalNativeBuild {
        cmake {
            path file('src/main/cpp/CMakeLists.txt')
            version '3.10.2'
        }
    }
    buildFeatures {
        viewBinding true
    }
}

dependencies {

    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'com.google.android.material:material:1.2.1'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.1'
    implementation project(path: ':opencv')
    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'
}

注意：jniLibs.srcDirs = [‘libs’] 千万不要写为：jniLibs.srcDirs = [src/main/JniLibs’] 否则程序会报错

四、在build.gradle(:opencv)中把application换成library，然后把defaultConfig注释掉，其他的不做修改，如下：五、这个时候请先点击Build-Make project构建一下程序。然后打开native-lib.cpp文件，这是我们要写C++代码的文件，写过C++的应该都知道.cpp文件。先试着引入一下包，输入#include ，如果变成绿色而不是红色，那么恭喜你，配置成功了，可以开始在Android平台下用C++来开发opencv代码了。