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

[Jan Lahoda]

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