<table cellspacing="0" cellpadding="0" border="0"><tr><td valign="top" style="font: inherit;"><p>CET is GMT+2 AFAIK.<br>
BogDan.<br><br><br></p>
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Robert Schuster <r.schuster@tarent.de>; <br>
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necessitas-devel@kde.org <necessitas-devel@kde.org>; <br>
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Java part rework - concept <br>
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Thu, Jul 7, 2011 10:22:12 AM <br>
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<td valign="top" style="font:inherit;">Hi all,<BR>finally I found the time to do a write up of the concept of the Java<BR>part rework that Bogdan, Christian and I talked about at QCS in Berlin.<BR>Attached is (hopefully) a drawing that shows the individual components.<BR>This mail is supposed to be a complete explanation of the whole topic.<BR>For a discussion on IRC later this day it would be nice if you've read<BR>it. :)<BR><BR>Base technique: At QCS we found out that it is very impractical to have<BR>that many Java classes in sourcecode form in each application. Simply<BR>because every class becomes part of the public interface. This will<BR>cause a huge maintenance burden. I'd simply say it would not work. We<BR>need to move large parts of the code into places that are independent<BR>from the application. This however strongly required the need to add<BR>classes from a foreign APK or DEX file to the
application's classpath.<BR>We were not sure whether Android would allow this but it indeed does. As<BR>a proof of concept we wrote a demo[0] application which can load any (!)<BR>APK or DEX file and create an instance of any class and call its<BR>toString() method. The POC worked in the way we wanted on a rooted but<BR>also on a normal locked device.<BR><BR>With this technique at hand we can do the following:<BR>* Move all code that is not really necessary for app startup into<BR>separate APK files<BR>* Define an interface between App and Loader which can handle updates<BR>(that allows for incompatible changes in the App code over time)<BR>* Profit! Err, yeah ... ;)<BR><BR>Let's explain all the little boxes from the drawing:<BR><BR>App:<BR>----<BR>This is the source code that is part of each Android-Qt application. The<BR>user *may* change little pieces of the code as long as it does not<BR>violate the interface to the Loader or Ministro. What the user
can do is<BR>calling any methods of the public interfaces to the loader for example.<BR>We will mark the pieces of code that the user *must* not remove or<BR>modify otherwise.<BR>This whole thing will be as small as possible!<BR>Whenever there is the need to change the App code in an incompatible way<BR>we need to introduce a new Loader interface. E.g. we will have a<BR>LoaderVersion1 interface for now. When it changes the App code will ask<BR>for LoaderVersion2. The Loader's reponsibilities will be explained below.<BR><BR>Loader:<BR>-------<BR>The Loader connects methods from the App with the Android Activity<BR>class. It will implement all available methods in order to gain maximum<BR>control over the App code without letting the App decide by itself. The<BR>Loader is published by us and can as such be updated over time allowing<BR>*us* to fix bugs, introduce new feature to *existing* applications<BR>without changing them.<BR>There might be
applications with different versions of the starter code<BR>in it. Each of those version requires a specific interface in the<BR>Loader. The most up to date version of the Loader will understand (=<BR>implement) and offer all interfaces. That way old and new application<BR>start code is always compatible with the Loader.<BR>If a Loader cannot satisfy the requested interface of an application<BR>then this means that the current version of the Loader is too old and it<BR>will update itself.<BR>(The Loader will technically be part of Ministro, when Ministro is<BR>installed. If the user is a developer and chose 'use local libs' then<BR>the loader will also be copied to a well known directory and directly<BR>included into the Apps classpath. As such at runtime of an App the<BR>Loader APK will be part of the App's process space!)<BR><BR>Ministro:<BR>---------<BR>Ministro does what it does right now: It downloads the libs (if<BR>necessary), provides the
locations of the libraries and tells the app<BR>that it can start. I'm actually thinking of putting the 'satisfy a<BR>certain Loader interface version' into Ministro, too. Because that seems<BR>like the best place for it).<BR>Unlike now the interface between App and Ministro will become upgrade<BR>compatible. While changes will happen less frequently it is very<BR>important that we allow and can deal with incompatible changes to the<BR>interface between Ministro and the App.<BR><BR>Bridge:<BR>-------<BR>These are all the classes that interact in some way with the C++ code of<BR>the Android-Qt port. Unlike now they will be a private interface.<BR>Meaning that we can do *any* change we want in them. For each supported<BR>Qt version (4.9, 5.0 etc.) there will be an accompanying bridge. The<BR>bridge may even consist of multiple APKs if we want to save space (e.g.<BR>if QtMobility is not used, then the classes for it do not need to be<BR>available as
well).<BR><BR>Qt:<BR>---<BR>These are the native Qt libraries. No change needed. Except that any<BR>code can *assume* the existence of certain bridge classes and can<BR>resolve class names at will.<BR><BR>Caveats:<BR>-------<BR>From a development perspective this is quite a task. The Java code<BR>changes are not so difficult (for me at least) but I have no clue how we<BR>can modify the Android-Qt build to also compile Java classes, dexify and<BR>make an APK out of them. Because this is what we need. When you compile<BR>Qt for Android you'll need a Java compiler, the DEX tool and something<BR>that creates an APK for you. IOW this is where I need you help.<BR><BR>The Ministro build also needs to be changed a bit. The Ministro APK<BR>could contain the Loader classes. That would not be a problem. However<BR>somehow QtCreator needs the Loader classes too when using 'use local<BR>libs' flag. I propose that the SDK installer also downloads the
Loader<BR>classes (which might be the same as Ministro but as a library not an<BR>application) from a known location.<BR><BR>I plan to use real Java interfaces in order to define the public<BR>interface. An alternative approach would be to rely on reflection only.<BR>However that will make the interface very opaque and the code will be<BR>difficult to read for anyone with only a small knowledge of Java. Having<BR>interfaces however means we will need to share bits of sourcecode<BR>between the projects (namely the interface file). I'm still<BR>contemplating how to do this properly.<BR><BR>For tonight I am mostly interested in how to solve the caveats.<BR><BR>Let me know what you think. If not we'll see each other at 7pm CET/GMT+1. :)<BR><BR>Regards,<BR>Robert<BR><BR>[0] - <a href="http://paste.debian.net/122188/" target=_blank >http://paste.debian.net/122188/</a><BR><BR>-- <BR>tarent solutions GmbH<BR>Thiemannstr. 36 a, D-12059 Berlin • <a
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