What Android Is

android之情殇 2013-01-05

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What Android Is

Being an il-lus-trated run through the ba-sics.

What hap-pened was, for our re-cent South Amer-i-can tour I wanted an An-droid ar-chi-tec-ture overview graphic. I ran across, among the An-droid SDK doc-u-men-ta-tion, a page en-ti-tled What is An-droid?, and it’s per-fectly OK. Ex-cept for, I re-ally dis-liked the pic-ture — on purely aes-thetic grounds, just not my kind of let-ter-ing and gra-di-ents and lay-outs — so I de-cided to make an-other one.

I thought I’d run it here and, since I’ve been spend-ing a lot of time re-cently ex-plain-ing What An-droid Is to peo-ple, I thought I’d pro-vide my ver-sion of that as well, in nar-ra-tive rather than point form.

First of all, as Dan Mor-rill mem-o-rably ex-plained in On An-droid Com-pat-i-bil-ity, “An-droid is not a spec-i-fi-ca-tion, or a dis-tri-b-u-tion in the tra-di-tional Linux sense. It’s not a col-lec-tion of re-place-able com-po-nents. An-droid is a chunk of soft-ware that you port to a de-vice.”

Linux · Un-der-neath every-thing is a rea-son-ably up-to-date Linux ker-nel (2.6.32 in my cur-rent Nexus One run-ning Froyo), with some power-sav-ing ex-ten-sions we cooked up; the process of try-ing to merge this stuff into up-stream Linux has been ex-tended and pub-lic and is by no means over. ?

An-droid runs on Linux, but I’d be ner-vous about call-ing it a dis-tro be-cause it leaves out so much that peo-ple ex-pect in one of those: li-braries and shells and ed-i-tors and GUIs and pro-gram-ming frame-works. It’s a pretty naked ker-nel, which be-comes ob-vi-ous the first time you find your-self using a shell on an An-droid de-vice.

If it were a dis-tro it’d be one of the higher-vol-ume ones, ship-ping at 200K units a day in late 2010. But no-body counts these things, and then there are a ton of em-bed-ded fla-vors of Linux ship-ping in un-re-mark-able pieces of con-sumer elec-tron-ics, so there’s a re-fresh-ing ab-sence of any-one claim-ing to be “the most pop-u-lar Linux”. I like that.

Dalvik · The next big piece is Dalvik, com-pris-ing the VM and a whole bunch of basic run-time es-sen-tials. Its de-sign is fairly unique, and judg-ing by re-cent his-tory, seems to be work-ing out pretty well as a mo-bile-de-vice app sub-strate. ?

All the stan-dard APIs that you use to cre-ate An-droid apps are de-fined in terms of Dalvik classes and in-ter-faces and ob-jects and meth-ods. In fact, some of them are thin lay-ers of Dalvik code over na-tive im-ple-men-ta-tions.

It’s pos-si-ble, and com-mon prac-tice, to call back and forth be-tween Dalvik and na-tive code using the JNI pro-to-col, which is a neat trick since what’s run-ning on Dalvik isn’t any-thing like Java byte-codes on a Java VM.

How It’s Gen-er-ated · Na-tive code is cur-rently pro-duced more or less ex-clu-sively by com-pil-ing C or C++ code; but there’s no rea-son it has to be that way. Dalvik code is cur-rently pro-duced by gen-er-at-ing Java byte-codes and trans-lat-ing them; but there’s no rea-son it has to be that way. ?

I want to em-pha-size this point a lit-tle. An-droid apps are de-fined as code that runs on the plat-form and uses the APIs. As long as an app does these things prop-erly, it’s re-ally no-body’s con-cern how it got gen-er-ated.

Spe-cial Apps · The pic-ture is a lit-tle mis-lead-ing, be-cause some of those Dalvik-based apps are pro-vided by Google and some-times are seen as “part of An-droid”. I’m talk-ing about the Di-aler and Con-tacts and Cal-en-dar and Gmail and Chat and so on. Most of them are open-source and re-place-able (and have been re-placed by hand-set mak-ers); a few are closed-source and pro-pri-etary, like Google Maps and An-droid Mar-ket. ?

That Open-Source Thing · In the big pic-ture above, most of the stuff in green is Apache-li-censed. The rest is a mix-ture of GPL and LGPL and BSD, with some Apache in there too. This ex-cludes some low-level de-vice dri-vers and of course the ma-jor-ity of non-Google apps, which are closed-source. ?

The Frame-work · This is the stuff that uniquely de-fines An-droid; more or less every-thing that Google wrote and you wouldn’t ex-pect to find on a rea-son-ably-con-fig-ured GNU/Linux box. Its proper use is the sub-ject of all the many pages on dis-play at developer.?android.?com and of end-less mail-ing lists, sam-ple sites, and a grow-ing num-ber of books. ?

I like it; but you al-ready knew that.

The standard libraries that ship with Android

Li-braries · The word “stan-dard” here means “gen-er-ally avail-able to pro-gram-mers work-ing in an open-source en-vi-ron-ment”. The pic-ture isn’t com-pre-hen-sive. ?

Quite a few peo-ple, in-clud-ing me, have over-em-pha-sized the role of the Har-mony li-braries. To start with, the An-droid se-lec-tion ex-cludes lots of stuff, for ex-am-ple AWT and Swing and OMG CORBA; all su-per-flu-ous for apps using the An-droid frame-work.

Also, just count-ing roughly by code bulk of all the sort of stuff in this pic-ture, the Har-mony code com-prises less than half the total. I don’t want to diss Har-mony, they’re a won-der-ful pro-ject and I’m a huge fan; but it’s in-ac-cu-rate to give the im-pres-sion that An-droid is just Dalvik plus Har-mony.

· · ·

What’s In an App? · An An-droid app lives in what’s called an APK, which is sim-ply a ZIP file, with a par-tic-u-lar in-ter-nal file lay-out that al-lows it to be run in place, with-out un-pack-ing. There’s noth-ing magic about them, you can email them around and drop them on USB keys and ex-tract pieces with unzip. ?

The An-droid Man-i-fest is the in-ter-face be-tween an app and the An-droid sys-tem, and that’s all I’m going to say here be-cause it’s a key piece of the puz-zle and de-serves lengthy dis-cus-sion if any at all.

The re-source bun-dle con-tains your audio and video and graph-ics and so on, the pieces that come with the app as op-posed to being fetched over the net-work.

Na-tive or Not · Most apps these days are writ-ten for Dalvik. When I say “most apps” I mean “every-thing that isn’t a game”; game de-vel-op-ers typ-i-cally want to code in C/C++ and that’s it. Dalvik of-fers a nice fast gate-way to OpenGL and all the phone’s hard-ware, but game devs just don’t want to hear about vir-tual ma-chines, so they use the An-droid NDK. ?

If you’re writ-ing code in the Java pro-gram-ming lan-guage you can use Eclipse and a pretty nice tool-chain that makes the bar-rier to entry re-mark-ably low. If you’re cod-ing to the NDK, you’re going to be doing a lot of the build-time ma-chin-ery your-self and liv-ing with-out some of the nice de-bug-ging and pro-fil-ing candy, not to men-tion sign-ing up to port your code to other CPU ar-chi-tec-tures if they run An-droid and have lots of users. But game de-vel-op-ers revel in pain.

And that’s what An-droid is · Hope you liked the pic-tures. ?

My thanks to Dan Born-stein and Dan Mor-rill for san-ity-check-ing this and of-fer-ing sev-eral sig-nif-i-cant im-prove-ments.
[Up-dated:] A cou-ple of fact-checks con-tributed by Brian Swet-land.

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