JNI編程基礎（二）——Android Studio NDK開發

由於筆者目前的工作是Android開發，所以在JNI開發系列博客中穿插一篇AndroidStudio NDK開發介紹。

隨着Android Studio 2.2的推出，Android Studio的NDK開發支持Cmake和ndk-build兩種方式，簡化了Android Studio上NDK開發流程，提升了開發效率。接下來就介紹下兩種開發方式。

1. CMake

CMake是一個跨平臺的編譯（安裝）工具，支持通過用較爲簡單的語言來描述編譯（安裝）過程，最後輸出適配不同平臺的makefile貨project文件。Android Studio通過引入CMake來簡化JNI函數的編譯。接下來通過一個例子來展示基於CMake的Android Studio NDK開發：

1. 創建一個Android工程，創建時勾選支持c++開發

創建完成後會看到在工程中會有一個c++目錄，與Java目錄同級

2. 在c++目錄下創建兩個文件：jni_lic.cpp/h。

3.在c++目錄下創建CMakeLists.txt文件，文件內容如下：

# 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.
AUX_SOURCE_DIRECTORY(./src/main/cpp DIR_SRCS)
add_library( # Sets the name of the library.
            jni-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.
             ${DIR_SRCS}
             )
include_directories(src/main/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.
                       jni-lib

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

4. 在gradle中加入相關配置

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.3"
    defaultConfig {
        applicationId "com.asia.jni"
        minSdkVersion 15
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        cmake {
            path "src/cpp/CMakeLists.txt"
        }
    }
}

5.配置NDK路徑

file->project structure->SDK Location:

如果沒有安裝NDK，可以通過SDK Manager安裝。

6. JNI函數開發

這部分可以參考上一篇博客：JNI編程基礎（一）

NDK-BUILD

ndk-build方式與cmake方式類似，只需要將cmake文件改寫爲Android.mk和Appliction.mk文件。在CMakeLists.txt加載的位置將CMakeLists.txt替換爲Android.mk即可

1.Android.mk

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
APP_ABI := all

LOCAL_MODULE    := jni-lib
LOCAL_CPPFLAGS     := -O0 -D_UNICODE -DUNICODE -DUSE_DUMP -Wno-error=format-security
LOCAL_CPP_EXTENSION := .cpp
LOCAL_LDLIBS    := -lm -llog -lz
LOCAL_SHORT_COMMANDS := true
INC_DIRS = -I$(LOCAL_PATH)/src/cpp
LOCAL_CPPFLAGS += $(INC_DIRS)

LOCAL_SRC_FILES    := \
    jni_lib.cpp    \    \


LOCAL_SHARED_LIBRARIES += libandroid_runtime


include $(BUILD_SHARED_LIBRARY)

2.Application.mk

APP_ABI := all
NDK_TOOLCHAIN_VERSION := clang
APP_SHORT_COMMANDS      := true
APP_STL := stlport_static
APP_CPPFLAGS := -std=gnu++11 -D__STDC_LIMIT_MACROS

3.替換gradle的配置中cmake

//    externalNativeBuild{
//        cmake{
//            path file("src/main//CMakeLists.txt")
//        }
//    }
    externalNativeBuild{
        ndkBuild{
            path file("src/main/cpp/Android.mk")
        }
    }