JDK 9 RFR of JDK-8072480: javac should support compilation for a specific platform version

[Maurizio Cimadamore]

Hi Jan,
great work - couple of comments below:

* it seems like some of the routines in Arguments can be simplified 
using lambdas - especially lookupPlatformProvider and checkOptionAllowed
* Why JDKPlatformProvider is not called JDKPlatformProvider*Factory* ?
* JavacProcessingEnvironment.JoiningIterator seems to have commonalities 
with CompoundScopeIterator - any chance that we might refactor this a bit?
* What's the rationale for giving an error if -platform is specified and 
a non-StandardFileManager is provided? Can't we simply swallow that, 
ignore the platform settings and issue a Lint 'options' warning?
* Would it make sense for some of the classfile generation logic in 
CreateSymbols to be moved under the classfile API ?
* I had hard time reconciling some of the comments in CreateSymbols with 
how the files were laid out. I think in the end, what you mean is that 
if you have platforms 7, 8, 9 - you should pick one baseline (i.e. 8) 
and then generate diffs for 9 and 7 against the 8 one. If that's the use 
case, I think the command line can be simplified a bit in order to 
explicitly state which of the platform is the baseline one, and the 
remaining parameters can be inferred from the tool (as the 
<base-platform-for-platform1,2 ... N> seem to be typically all identical 
but one which is set to <none> - the one for the baseline). Maybe the 
inference logic should be different (i.e. use 8 as a baseline for 7 and 
7 as a baseline for 6) - but - whatever the logic, I think it should be 
chosen once and for all, and live implicitly in the tool? Or are there 
reasons as to why it might be handy to customize the baselines?

Maurizio

On 21/05/15 08:01, Jan Lahoda wrote:
> Hi,
>
> This is a patch adding a new option, -platform, to javac.
>
> Patch for the top-level repository is here:
> http://cr.openjdk.java.net/~jlahoda/8072480/webrev.00/top-level/
>
> Patch for the langtools repository is here:
> http://cr.openjdk.java.net/~jlahoda/8072480/webrev.00/langtools/
>
> These changes also require additional langtools changes as their 
> prerequisite:
> http://cr.openjdk.java.net/~jlahoda/8080675/webrev.00/
>
> Overall, one can imagine '-platform N' to have the same effect as 
> '-source N -target N -bootclasspath <APIs-for-N>'. The possible values 
> for 'N' are pluggable in a limited way, though (see 
> langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/platform/PlatformProvider.java). 
> Note that this patch only introduces N=7 and N=8, which correspond to 
> Open JDK 7 and 8 GA.
>
> A tricky problem to solve is where to get the APIs for platform N. The 
> proposal is to include history data in the textual format inside the 
> OpenJDK repositories (the input data), process them at build time and 
> create a lib/ct.sym file holding the data in the JDK image. javac 
> running with the -platform option then compiles against the lib/ct.sym 
> file. The input history data are placed 
> <top-level-repository>/make/data/symbols (the sym.txt files). This 
> patches only includes data for OpenJDK 7 and 8, and only for public 
> APIs (more or less Java SE and JDK @Exported APIs).
>
> The size of the history data is currently ~6MB in the JDK checkout and 
> ~650kB inside the .hg directory (the size could change significantly 
> if additional classes/elements, like non-public API classes, would 
> need to be included). The lib/ct.sym file is currently ~4.5MB.
>
> The ct.sym file is a zip file containing signature files. The 
> signature files are similar to classfiles (so javac can read them as 
> classfiles), but are missing some attributes, most notably the Code 
> attribute. On the top-level, the ct.sym contains directories named 
> "7", "78" and "8". When compiling for version 'N', the bootclasspath 
> for that version is obtained by using directories which have 'N' in 
> their name.
>
> The sigfiles for ct.sym are created by 
> <top-level-repository>/make/tools/symbolgenerator/CreateSymbols.java. 
> The same file can also be used to construct the sym.txt files. Concise 
> instructions are part of the CreateSymbols.java.
>
> I am sending this as one review, as the changes together make one 
> feature, but the langtools and top-level changes are independent to a 
> great degree: the langtools changes add the "-platform" javac; and the 
> top-level changes add the history data and ability to build the ct.sym 
> file. If desired, these could be pushed and/or reviewed independently.
>
> Many thanks go to Jon Gibbons, Joe Darcy, Magnus Ihse Bursie, Alan 
> Bateman and others who have provided advices and help on the matter 
> before.
>
> Any insights/comments are wholeheartedly welcome.
>
> Thanks,
>     Jan