NDK Cmake 爬坑

官网介绍

https://developer.android.com/studio/projects/add-native-code.html#link-gradle

首先来说一下我今天遇到的问题
1 在自动生成的项目中，添加了一个native方法后，调用此方法报错说找不到。
2 新建cpp文件提示一堆错误。

1在MainActivity中新建一个native方法并调用
按照官方介绍新建项目，选择支持c++

文件预览：

MainActivity

public class MainActivity extends AppCompatActivity {    // Used to load the 'native-lib' library on application startup.    static {        System.loadLibrary("native-lib");    }    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        // Example of a call to a native method        TextView tv = (TextView) findViewById(R.id.sample_text);        tv.setText(stringFromJNI());    }    /**     * A native method that is implemented by the 'native-lib' native library,     * which is packaged with this application.     */    public native String stringFromJNI();}

native-lib.cpp

#include <jni.h>#include <string>extern "C"jstringJava_com_yeliang_ndktest_MainActivity_stringFromJNI(        JNIEnv *env,        jobject /* this */) {    std::string hello = "Hello from C++";    return env->NewStringUTF(hello.c_str());}

CMakeLists.txt

# Sets the minimum version of CMake required to build the native# library. You should either keep the default value or only pass a# value of 3.4.0 or lower.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 it 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).             # Associated headers in the same location as their source             # file are automatically included.             src/main/cpp/native-lib.cpp )# Searches for a specified prebuilt library and stores the path as a# variable. Because system libraries are included 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 the# 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.                       ${log-lib} )

运行结果

在MainActivity中新增加一个方法

public native String getStringFromC();

调用此方法

 tv.setText(getStringFromC());

native-lib.cpp 在此文件中增加一个方法

#include <jni.h>#include <string>extern "C"jstringJava_com_yeliang_ndktest_MainActivity_stringFromJNI(        JNIEnv *env,        jobject /* this */) {    std::string hello = "Hello from C++";    return env->NewStringUTF(hello.c_str());}jstringJava_com_yeliang_ndktest_MainActivity_getStringFromC(        JNIEnv *env,        jobject /* this */) {    std::string hello = "这句话来自C";    return env->NewStringUTF(hello.c_str());}

运行后将会报错

    Process: com.yeliang.ndktest, PID: 25628                                                   java.lang.UnsatisfiedLinkError: No implementation found for java.lang.String com.yeliang.ndktest.MainActivity.getStringFromC() (tried Java_com_yeliang_ndktest_MainActivity_getStringFromC and Java_com_yeliang_ndktest_MainActivity_getStringFromC__)                                                       at com.yeliang.ndktest.MainActivity.getStringFromC(Native Method)                                                       at com.yeliang.ndktest.MainActivity.onCreate(MainActivity.java:21)

后来在这个方法上加了一个

extern "C"

我也不知道这个是干嘛的 然后在运行

再此运行好了。

2 新建cpp文件 并调用
新建java类

接下来自己新建ndk-test.cpp文件

添加这个文件到Cmake

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).             # Associated headers in the same location as their source             # file are automatically included.             src/main/cpp/native-lib.cpp             src/main/cpp/ndk-test.cpp)

最后一行是添加进来的

src/main/cpp/ndk-test.cpp

重新运行后：

3 Cmake文件中各标签的含义

C文件和C文件编译生成的so

添加倒入的so 并指定倒入so的位置

指定库的路径

包含头文件的路径
添加这个标签后就不用写出头文件的路径全称，而只需要写包含路径的子目录即可。

本地库和倒入库头文件的映射关系

添加关联库

下面是Cmake完整版

# Sets the minimum version of CMake required to build the native# library. You should either keep the default value or only pass a# value of 3.4.0 or lower.cmake_minimum_required(VERSION 3.4.1)add_library(             native-lib             SHARED             src/main/cpp/video_player.c)find_library(              log-lib              log )set(distribution_DIR ${CMAKE_SOURCE_DIR}/../../../../libs)add_library(libavcodec-56             SHARED             IMPORTED )set_target_properties( libavcodec-56                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libavcodec-56.so )add_library( libavdevice-56             SHARED             IMPORTED )set_target_properties( libavdevice-56                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libavdevice-56.so )add_library( libavfilter-5             SHARED             IMPORTED )set_target_properties( libavfilter-5                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libavfilter-5.so)add_library( libavformat-56             SHARED             IMPORTED)set_target_properties( libavformat-56                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libavformat-56.so )add_library( libavutil-54             SHARED             IMPORTED)set_target_properties( libavutil-54                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libavutil-54.so )add_library( libpostproc-53             SHARED             IMPORTED)set_target_properties( libpostproc-53                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libpostproc-53.so )add_library( libswresample-1             SHARED             IMPORTED)set_target_properties( libswresample-1                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libswresample-1.so )add_library( libswscale-3             SHARED             IMPORTED)set_target_properties( libswscale-3                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libswscale-3.so)add_library(libyuv             SHARED             IMPORTED)set_target_properties( libyuv                       PROPERTIES IMPORTED_LOCATION                       ../../../../libs/armeabi-v7a/libyuv.so)set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")include_directories(libs/include)include_directories(libs/include/libyuv)#target_include_directories(native-lib PRIVATE libs/include)target_link_libraries( native-lib libavcodec-56 libavdevice-56 libavfilter-5 libavformat-56                        libavutil-54 libpostproc-53 libswresample-1 libswscale-3 libyuv android                       ${log-lib} )

4 gradle相关配置

gradle完整版

apply plugin: 'com.android.application'android {    compileSdkVersion 25    buildToolsVersion "26.0.0"    defaultConfig {        applicationId "com.ffmpeg07_thread_decode"        minSdkVersion 15        targetSdkVersion 25        versionCode 1        versionName "1.0"        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"        externalNativeBuild {            cmake {                cppFlags ""                arguments '-DANDROID_PLATFORM=android-21',                        '-DANDROID_TOOLCHAIN=clang', '-DANDROID_STL=gnustl_static'            }        }        ndk {            abiFilters  "armeabi-v7a"        }    }    buildTypes {        release {            minifyEnabled false            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'        }    }    sourceSets.main {        jniLibs.srcDirs = ['libs']        jni.srcDirs = []    }    externalNativeBuild {        cmake {            path "CMakeLists.txt"        }    }}