Android(安卓)NDK之一：什么是NDK？

WhatistheNDK?

TheAndroidNDKisatoolsetthatletsyouembedcomponentsthatmakeuseofnativecodeinyourAndroidapplications.

AndroidNDK是一个工具集，它让我们能够在Android应用程序中使用由本地代码（nativecode）编写的代码模块。

AndroidapplicationsrunintheDalvikvirtualmachine.TheNDKallowsyoutoimplementpartsofyourapplicationsusingnative-codelanguagessuchasCandC++.Thiscanprovidebenefitstocertainclassesofapplications,intheformofreuseofexistingcodeandinsomecasesincreasedspeed.

Android应用程序在Dalvik虚拟机中运行，NDK让我们可以将应用程序中的某些部分用本地代码（nativecode）语言（例如C和C++）来实现，这样会对部分应用程序有益处，例如可以代码重用，提高运行速度等。

TheNDKprovides:

NDK工具集提供的内容包括：

·AsetoftoolsandbuildfilesusedtogeneratenativecodelibrariesfromCandC++sources

用于将C和C++源代码生成本地代码库（nativecodelibraries）的一套工具和编译文件（buildfiles）

·Awaytoembedthecorrespondingnativelibrariesintoanapplicationpackagefile(.apk)thatcanbedeployedonAndroiddevices

将相应的本地库（nativelibraries）嵌入进应用程序包文件（apk）的方法

·AsetofnativesystemheadersandlibrariesthatwillbesupportedinallfutureversionsoftheAndroidplatform,startingfromAndroid1.5.ApplicationsthatusenativeactivitiesmustberunonAndroid2.3orlater.

一套本地系统头文件和库，这些头文件和库从Android1.5版本开始，及其后续所有版本的Android平台中都会支持。使用nativeactivities的应用程序则必须运行在Android2.3及其以后的版本上。

·Documentation,samples,andtutorials

各类文档，实例程序及各类指导材料。

ThelatestreleaseoftheNDKsupportsthefollowinginstructionsets:

最新release的NDK支持如下的指令集：

·ARMv5TE,includingThumb-1instructions(seedocs/CPU-ARCH-ABIS.htmlformoreinformation)

·ARMv7-A,includingThumb-2andVFPv3-D16instructions,withoptionalsupportforNEON/VFPv3-D32instructions(seedocs/CPU-ARM-NEON.htmlformoreinformation)

·x86instructions(seedocs/CPU-X86.htmlformoreinformation)

·MIPSinstructions(seedocs/CPU-MIPS.htmlformoreinformation)

ARMv5TEmachinecodewillrunonallARM-basedAndroiddevices.ARMv7-AwillrunonlyondevicessuchastheVerizonDroidorGoogleNexusOnethathaveacompatibleCPU.ThemaindifferencebetweenthetwoinstructionsetsisthatARMv7-AsupportshardwareFPU,Thumb-2,andNEONinstructions.Youcantargeteitherorbothoftheinstructionsets—ARMv5TEisthedefault,butswitchingtoARMv7-Aisaseasyasaddingasinglelinetotheapplication'sApplication.mkfile,withoutneedingtochangeanythingelseinthefile.Youcanalsobuildforbotharchitecturesatthesametimeandhaveeverythingstoredinthefinal.apk.CompleteinformationisprovidedintheCPU-ARCH-ABIS.HTMLintheNDKpackage.

ARMv5TE机器码可以在所有基于ARM的Android设备上运行。ARMv7-A机器码只可以在VerizonDroid或者GoogleNexusOne等这类拥有兼容CPU（compatibleCPU）的设备上运行。这两种指令集之间的主要差别是ARMv7-A支持硬件FPU，Thumb-2和NEON指令。ARMv5TE是默认的指令集，要切换到ARMv7-A也很容易，只需要在应用程序的Application.mk文件中添加一行语句，文件中的其它内容则都不需要修改。我们也可以同时使用这两种指令集，并且都存放进最终的apk文件中。完整的信息请参考NDK包中的CPU-ARCH-ABIS.HTML文件。

TheNDKprovidesstableheadersforlibc(theClibrary),libm(theMathlibrary),OpenGLES(3Dgraphicslibrary),theJNIinterface,andotherlibraries,aslistedintheDevelopmentToolssection.

NDK为libc，libm，OpenGLES，JNI接口和其它的一些库提供了稳定的头文件。（全部的库请参考DevelopmentTools章节）

WhentoDevelopinNativeCode

何时需要使用本地代码开发

TheNDKwillnotbenefitmostapplications.Asadeveloper,youneedtobalanceitsbenefitsagainstitsdrawbacks;notably,usingnativecodedoesnotresultinanautomaticperformanceincrease,butalwaysincreasesapplicationcomplexity.Ingeneral,youshouldonlyusenativecodeifitisessentialtoyourapplication,notjustbecauseyouprefertoprograminC/C++.

大部分的应用程序都不会从NDK收益。作为一个开发者，我们需要平衡它能带给我们的好处和它所带来的问题，尤其是，使用本地代码不一定会带来性能上的提升，但是一定会增加程序的复杂度。通常情况下，只有在对应用程序十分必要的情况才应该使用本地代码，而不是仅仅因为喜欢用C或者C++编程。

TypicalgoodcandidatesfortheNDKareself-contained,CPU-intensiveoperationsthatdon'tallocatemuchmemory,suchassignalprocessing,physicssimulation,andsoon.Simplyre-codingamethodtoruninCusuallydoesnotresultinalargeperformanceincrease.Whenexaminingwhetherornotyoushoulddevelopinnativecode,thinkaboutyourrequirementsandseeiftheAndroidframeworkAPIsprovidethefunctionalitythatyouneed.TheNDKcan,however,canbeaneffectivewaytoreusealargecorpusofexistingC/C++code.

典型的需要使用NDK的是那些self-contained，CPU-intensive的操作，不需要申请大量的内存，例如信号处理，物理模拟（physicssimulation）等。简单的使用C语言重新编码一个函数通常不会带来性能上的提升。在分析是否应该使用本地代码进行开发时，需要考虑我们的要求以及是否AndroidframeworkAPIs已经提供了我们需要的功能。然而，要复用大量C/C++代码，NDK是一种高效的方式。

TheAndroidframeworkprovidestwowaystousenativecode:

Androidframework提供了两种方式使用本地代码：

使用Androidframework编写应用程序，然后使用JNI访问AndroidNDK提供的API。这种方式可以利用Androidframework的方便，而且需要的时候依然可以编写本地代码。

编写nativeactivity，这样可以在本地代码中实现生命周期回调函数。AndroidSDK提供了NativeActivity类。

WriteyourapplicationusingtheAndroidframeworkanduseJNItoaccesstheAPIsprovidedbytheAndroidNDK.ThistechniqueallowsyoutotakeadvantageoftheconvenienceoftheAndroidframework,butstillallowsyoutowritenativecodewhennecessary.Ifyouusethisapproach,yourapplicationmusttargetspecific,minimumAndroidplatformlevels,seeAndroidplatformcompatibilityformoreinformation.

Writeanativeactivity,whichallowsyoutoimplementthelifecyclecallbacksinnativecode.TheAndroidSDKprovidestheNativeActivityclass,whichisaconvenienceclassthatnotifiesyournativecodeofanyactivitylifecyclecallbacks(onCreate(),onPause(),onResume(),etc).Youcanimplementthecallbacksinyournativecodetohandletheseeventswhentheyoccur.ApplicationsthatusenativeactivitiesmustberunonAndroid2.3(APILevel9)orlater.

YoucannotaccessfeaturessuchasServicesandContentProvidersnatively,soifyouwanttousethemoranyotherframeworkAPI,youcanstillwriteJNIcodetodoso.

不能够在本地代码中使用Service和ContentProvider等特性，如果要使用这些特性或者其它任何framework的API，可以编写JNI完成。

ContentsoftheNDK

NDK中包含的内容：开发工具，文档，实例程序

TheNDKcontainstheAPIs,documentation,andsampleapplicationsthathelpyouwriteyournativecode.

Developmenttools

TheNDKincludesasetofcross-toolchains(compilers,linkers,etc..)thatcangeneratenativeARMbinariesonLinux,OSX,andWindows(withCygwin)platforms.

NDK包含一套交叉工具链，能够生成运行在Linux，OSX和Windows平台上的本地ARM二进制文件（nativeARMbinaries）。

ItprovidesasetofsystemheadersforstablenativeAPIsthatareguaranteedtobesupportedinalllaterreleasesoftheplatform:

一套系统头文件，头文件中包含稳定的本地APIs，这些本地APIs在以后的release中都会支持：

·libc(Clibrary)headers

·libm(mathlibrary)headers

·JNIinterfaceheaders

·libz(Zlibcompression)headers

·liblog(Androidlogging)header

·OpenGLES1.1andOpenGLES2.0(3Dgraphicslibraries)headers

·libjnigraphics(Pixelbufferaccess)header(forAndroid2.2andabove).

·AMinimalsetofheadersforC++support

·OpenSLESnativeaudiolibraries

·AndroidnativeapplicationAPIS

TheNDKalsoprovidesabuildsystemthatletsyouworkefficientlywithyoursources,withouthavingtohandlethetoolchain/platform/CPU/ABIdetails.YoucreateveryshortbuildfilestodescribewhichsourcestocompileandwhichAndroidapplicationwillusethem—thebuildsystemcompilesthesourcesandplacesthesharedlibrariesdirectlyinyourapplicationproject.

NDK提供的编译系统让我们可以高效的编译源码，不需要去处理工具链、平台等细节。我们只需要创建很短的编译文件，描述哪些源文件要编译以及哪些应用程序会使用它们，编译系统就会编译源代码并且将编译生成的共享库（sharedlibraries）直接放到我们的应用程序中。

Important:Withtheexceptionofthelibrarieslistedabove,nativesystemlibrariesintheAndroidplatformarenotstableandmaychangeinfutureplatformversions.YourapplicationsshouldonlymakeuseofthestablenativesystemlibrariesprovidedinthisNDK.

除去上面所列的库外，其它的本地系统库是不稳定的，在以后的版本中可能会改变，因此我们的应用程序应该避免使用不稳定的库，而只使用NDK中提供的稳定的本地系统库。

Documentation

TheNDKpackageincludesasetofdocumentationthatdescribesthecapabilitiesoftheNDKandhowtouseittocreatesharedlibrariesforyourAndroidapplications.Inthisrelease,thedocumentationisprovidedonlyinthedownloadableNDKpackage.Youcanfindthedocumentationinthe<ndk>/docs/directory.Includedarethesefiles(partiallisting):

NDK包包含了一套文档，这些文档描述了NDK的功能，以及如何使用它为Android应用程序创建共享库（sharedlibraries）。在这次release中，这些文档只在下载的NDK包中提供，可以在docs目录下查看，包括如下的文件：

·INSTALL.HTML—describeshowtoinstalltheNDKandconfigureitforyourhostsystem

·OVERVIEW.HTML—providesanoverviewoftheNDKcapabilitiesandusage

·ANDROID-MK.HTML—describestheuseoftheAndroid.mkfile,whichdefinesthenativesourcesyouwanttocompile

描述Android.mk文件的使用，Android.mk文件定义要编译的本地源文件

·APPLICATION-MK.HTML—describestheuseoftheApplication.mkfile,whichdescribesthenativesourcesrequiredbyyourAndroidapplication

·CPLUSPLUS-SUPPORT.HTML—describestheC++supportprovidedintheAndroidNDK

描述AndroidNDK对C++的支持

·CPU-ARCH-ABIS.HTML—adescriptionofsupportedCPUarchitecturesandhowtotargetthem.

描述了支持的CPU架构

·CPU-FEATURES.HTML—adescriptionofthecpufeaturesstaticlibrarythatletsyourapplicationcodedetectthetargetdevice'sCPUfamilyandtheoptionalfeaturesatruntime.

描述了cpufeature静态库，它可以让我们的应用程序代码在运行时探测目标设备的CPUfamily和其它的特性

·CHANGES.HTML—acompletelistofchangestotheNDKacrossallreleases.

·DEVELOPMENT.HTML—describeshowtomodifytheNDKandgeneratereleasepackagesforit

·HOWTO.HTML—informationaboutcommontasksassociatedwithNDKdevelopment

·IMPORT-MODULE.HTML—describeshowtoshareandreusemodules

·LICENSES.HTML—informationaboutthevariousopensourcelicensesthatgoverntheAndroidNDK

·NATIVE-ACTIVITY.HTML—describeshowtoimplementnativeactivities

描述如何实现nativeactivities

·NDK-BUILD.HTML—describestheusageofthendk-buildscript

·NDK-GDB.HTML—describeshowtousethenativecodedebugger

·PREBUILTS.HTML—informationabouthowsharedandstaticprebuiltlibrarieswork

共享库和静态库如何工作方面的信息

·STANDALONE-TOOLCHAIN.HTML—describeshowtouseAndroidNDKtoolchainasastandalonecompiler(stillinbeta).

·SYSTEM-ISSUES.HTML—knownissuesintheAndroidsystemimagesthatyoushouldbeawareof,ifyouaredevelopingusingtheNDK.

·STABLE-APIS.HTML—acompletelistofthestableAPIsexposedbyheadersintheNDK.

NDK中的头文件导出的稳定API的完整列表

Additionally,thepackageincludesdetailedinformationaboutthe"bionic"ClibraryprovidedwiththeAndroidplatformthatyoushouldbeawareof,ifyouaredevelopingusingtheNDK.Youcanfindthedocumentationinthe<ndk>/docs/system/libc/directory:

·OVERVIEW.HTML—providesanoverviewofthe"bionic"Clibraryandthefeaturesitoffers.

提供了“bionic”的概述，以及它所提供的特性

Sampleapplications

TheNDKincludessampleapplicationsthatillustratehowtousenativecodeinyourAndroidapplications:

·hello-jni—asimpleapplicationthatloadsastringfromanativemethodimplementedinasharedlibraryandthendisplaysitintheapplicationUI.

·two-libs—asimpleapplicationthatloadsasharedlibrarydynamicallyandcallsanativemethodprovidedbythelibrary.Inthiscase,themethodisimplementedinastaticlibraryimportedbythesharedlibrary.

·san-angeles—asimpleapplicationthatrenders3DgraphicsthroughthenativeOpenGLESAPIs,whilemanagingactivitylifecyclewithaGLSurfaceViewobject.

·hello-gl2—asimpleapplicationthatrendersatriangleusingOpenGLES2.0vertexandfragmentshaders.

·hello-neon—asimpleapplicationthatshowshowtousethecpufeatureslibrarytocheckCPUcapabilitiesatruntime,thenuseNEONintrinsicsifsupportedbytheCPU.Specifically,theapplicationimplementstwoversionsofatinybenchmarkforaFIRfilterloop,aCversionandaNEON-optimizedversionfordevicesthatsupportit.

·bitmap-plasma—asimpleapplicationthatdemonstrateshowtoaccessthepixelbuffersofAndroidBitmapobjectsfromnativecode,andusesthistogenerateanold-school"plasma"effect.

·native-activity—asimpleapplicationthatdemonstrateshowtousethenative-app-gluestaticlibrarytocreateanativeactivity

·native-plasma—aversionofbitmap-plasmaimplementedwithanativeactivity.

Foreachsample,theNDKincludesthecorrespondingCsourcecodeandthenecessaryAndroid.mkandApplication.mkfiles.Therearelocatedunder<ndk>/samples/<name>/andtheirsourcecodecanbefoundunder<ndk>/samples/<name>/jni/.

Youcanbuildthesharedlibrariesforthesampleappsbygoinginto<ndk>/samples/<name>/thencallingthendk-buildcommand.Thegeneratedsharedlibrarieswillbelocatedunder<ndk>/samples/<name>/libs/armeabi/for(ARMv5TEmachinecode)and/or<ndk>/samples/<name>/libs/armeabi-v7a/for(ARMv7machinecode).

Next,buildthesampleAndroidapplicationsthatusethesharedlibraries:

·IfyouaredevelopinginEclipsewithADT,usetheNewProjectWizardtocreateanewAndroidprojectforeachsample,usingthe"ImportfromExistingSource"optionandimportingthesourcefrom<ndk>/samples/<name>/.Then,setupanAVD,ifnecessary,andbuild/runtheapplicationintheemulator.

·IfyouaredevelopingwithAnt,usetheandroidtooltocreatethebuildfileforeachofthesampleprojectsat<ndk>/samples/<name>/.ThensetupanAVD,ifnecessary,buildyourprojectintheusualway,andrunitintheemulator.

FormoreinformationaboutdevelopingwiththeAndroidSDKtoolsandwhatyouneedtodotocreate,build,andrunyourapplications,seetheOverviewsectionfordevelopingonAndroid.

Exploringthenative-activitySampleApplication

Thenative-activitysampleprovidedwiththeAndroidNDKdemonstrateshowtousetheandroid_native_app_gluestaticlibrary.Thisstaticlibrarymakescreatinganativeactivityeasierbyprovidingyouwithanimplementationthathandlesyourcallbacksinanotherthread,soyoudonothavetoworryaboutthemblockingyourmainUIthread.Themainpartsofthesamplearedescribedbelow:

native-activity实例展示了如何使用android_native_app_glue静态库

·ThefamiliarbasicstructureofanAndroidapplication(anAndroidManifest.xmlfile,asrc/andresdirectories).TheAndroidManifest.xmldeclaresthattheapplicationisnativeandspecifiesthe.sofileofthenativeactivity.SeeNativeActivityforthesourceorseethe<ndk_root>/platforms/samples/native-activity/AndroidManifest.xmlfile.

·Ajni/directorycontainsthenativeactivity,main.c,whichusestheandroid_native_app_glue.hinterfacetoimplementtheactivity.TheAndroid.mkthatdescribesthenativemoduletothebuildsystemalsoexistshere.

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