javac: "legacy" mode

[Mandy Chung]

On 1/27/2012 1:37 PM, Jonathan Gibbons wrote:
> Until recently, javac implicitly supported "legacy" compilations 
> (meaning compilations that were valid in JDK 7 or earlier). This was 
> as side-effect by design of "single-module mode", because the default 
> requires if none was given was "jdk at 8-ea".
>
> Now, we have changed the default requires to "java.base@>=8".  This 
> means that without any other changes, a legacy compilation referencing 
> non-base classes will fail to compile.
>
> There are two possible solutions.
>
> 1. Have javac support -Xmode:legacy, which would change the default 
> requires. It would also require everyone to change the way they use 
> javac for legacy compilations.
>

This seems a major compatibility concern but it might worth taking the 
migration story into account.   What if javac -source 7 -target 7 uses 
the entire "jdk" as the default and keep "java.base" as the default for 
-source 8 -target 8 (possibly provide a switch to say using the entire 
"jdk" rather than just the base)?   I'm in two minds and I think it 
deserves some more thought.

> 2. Distinguish between the case of no module compilation units in a 
> compilation and module compilation units involved, but not containing 
> any requires on the base module.  If no module compilation unit is 
> observable, one would be synthesized containing "requires jdk;" or 
> some equivalent.   If any observable module compilation unit does not 
> contain a requires for the platform base, then a "requires 
> java.base>=N;" will be generated, with an appropriate value of N, per 
> the spec.

I did some work to support legacy mode in runtime some time ago [1].  I 
have re-based it with the latest jigsaw forest.   It's just a good 
timing and discuss how legacy mode should work in runtime/compile-time.  
I should restart this work soon.

Mandy
[1] http://mail.openjdk.java.net/pipermail/jigsaw-dev/2010-June/001061.html