NDK配置及基本语法

NDK是Google为Android实现JNI调用而提供的工具
原生库的构建方法包括旧版的ndk-build，及新版的CMake

参考Google文档ndk-build

ndk-build构建方法

构建脚本：

• Android.mk

语法与AOSP的Android.mk文件类似，其内定义若干个模块

LOCAL_PATH := $(call my-dir) #文件所在目录
include $(CLEAR_VARS) #清除各类LOCAL_XXX变量（不包括LOCAL_PATH）
LOCAL_MODULE := hello-jni #模块名（生成libhello-jni.so）
LOCAL_SRC_FILES := hello-jni.c #源文件
include $(BUILD_SHARED_LIBRARY) #连接以上内容。BUILD_SHARED_LIBRARY指向GNU Makefile脚本，确定构建内容及方法

• application.mk

描述应用需要的原生模块

CMake构建方法

参考Google文档add c & c++ code

构建脚本：

• 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.

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).
             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.

#find_library从NDK现有库中查找到android/log库，并赋值给log-lib变量；
#再通过target_link_libraries把log-lib链接到native-lib库；
#基于此，native-lib中的.cpp文件可#include <android/log.h>引入对应库，并使用__android_log_print方法记录log

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

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

Gradle关联到原生库

• 在模块build.gradle中关联原生库的脚本文件：

android {
  ...
  defaultConfig {...}
  buildTypes {...}

  // Encapsulates your external native build configurations.
  externalNativeBuild {

    // Encapsulates your CMake build configurations.
    cmake {

      // Provides a relative path to your CMake build script.
      path "CMakeLists.txt"
    }
  }
}

如果需关联ndk-build项目，则使用ndkBuild { }块替换cmake { }，关联Android.mk文件。

• Gradle可选配置：

在模块build.gradle的defaultConfig{ }（可在productFlavors中使用不同配置）中配置CMake或ndk-build的可选参数和标志。

android {
  ...
  defaultConfig {
    ...
    // This block is different from the one you use to link Gradle
    // to your CMake or ndk-build script.
    externalNativeBuild {

      // For ndk-build, instead use ndkBuild {}
      cmake {

        // Passes optional arguments to CMake.
        arguments "-DANDROID_ARM_NEON=TRUE", "-DANDROID_TOOLCHAIN=clang"

        // Sets optional flags for the C compiler.
        cFlags "-D_EXAMPLE_C_FLAG1", "-D_EXAMPLE_C_FLAG2"

        // Sets a flag to enable format macro constants for the C++ compiler.
        cppFlags "-D__STDC_FORMAT_MACROS"
      }
    }
  }

  buildTypes {...}

  productFlavors {
    ...
    demo {
      ...
      externalNativeBuild {
        cmake {
          ...
          // Specifies which native libraries to build and package for this
          // product flavor. If you don't configure this property, Gradle
          // builds and packages all shared object libraries that you define
          // in your CMake or ndk-build project.
          targets "native-lib-demo"
        }
      }
    }

    paid {
      ...
      externalNativeBuild {
        cmake {
          ...
          targets "native-lib-paid"
        }
      }
    }
  }

  // Use this block to link Gradle to your CMake or ndk-build script.
  externalNativeBuild {
    cmake {...}
    // or ndkBuild {...}
  }
}