PRE-PROPOSAL: Source and Encoding keyword

[Jeremy Manson]

I completely agree with Neal.  Also, you are only discussing the
language, and not the APIs.  An historical problem with javac -source
is that it doesn't actually prevent you from calling APIs that only
exist in later versions, it only restricts you to the earlier version
of the language.  If you use javac -source 5 nowadays, that doesn't
actually prevent you from  putting calls to Deques and
ConcurrentSkipListMaps in your code.

The easiest way to fix this would be aggressive use of the @since
tags, but they are only consistently used in java.lang / java.util,
and that's only because Martin Buchholz made it a priority to go back
and fix all of the non-existent / incorrect ones.

Jeremy

On Sat, Mar 7, 2009 at 9:42 AM, Neal Gafter <neal at gafter.com> wrote:
> I think you'll find that the devil is in the details.  First, you'll
> have to incorporate all the language rules from all the versions of
> the JLS into a new edition of the JLS (depending on the "source"
> setting).  Second, you'll have to describe in precise detail the
> interaction of code written in different source settings when combined
> into a single program.  How are Java 5 bridge methods seen in a Java 4
> program?  What happens in a Java 4 program if you attempt to override
> a method that had a Java 5 covariant return in its superclass?  You'll
> have to spell out these interactions in the language specification in
> gory detail.  That might be useful to do, but it's hard to imagine
> that you'd have time to even scratch the surface, much less complete
> such a specification, in the timeframe of project coin.
>
> On Fri, Mar 6, 2009 at 10:03 PM, Reinier Zwitserloot
> <reinier at zwitserloot.com> wrote:
>> We have written up a proposal for adding a 'source' and 'encoding'
>> keyword (alternatives to the -source and -encoding keywords on the
>> command line; they work pretty much just as you expect). The keywords
>> are context sensitive and must both appear before anything else other
>> than comments to be parsed. In case the benefit isn't obvious: It is a
>> great help when you are trying to port a big project to a new source
>> language compatibility. Leaving half your sourcebase in v1.6 and the
>> other half in v1.7 is pretty much impossible today, it's all-or-
>> nothing. It should also be a much nicer solution to the 'assert in
>> v1.4' dilemma, which I guess is going to happen to v1.7 as well, given
>> that 'module' is most likely going to become a keyword. Finally, it
>> makes java files a lot more portable; you no longer run into your
>> strings looking weird when you move your Windows-1252 codefile java
>> source to a mac, for example.
>>
>> Before we finish it though, some open questions we'd like some
>> feedback on:
>>
>> A) Technically, starting a file with "source 1.4" is obviously silly;
>> javac v1.4 doesn't know about the source keyword and would thus fail
>> immediately. However, practically, its still useful. Example: if
>> you've mostly converted a GWT project to GWT 1.5 (which uses java 1.5
>> syntax), but have a few files remaining on GWT v1.4 (which uses java
>> 1.4 syntax), then tossing a "source 1.4;" in those older files
>> eliminates all the generics warnings and serves as a reminder that you
>> should still convert those at some point. However, it isn't -actually-
>> compatible with a real javac 1.4. We're leaning to making "source
>> 1.6;"  (and below) legal even when using a javac v1.7 or above, but
>> perhaps that's a bridge too far? We could go with magic comments but
>> that seems like a very bad solution.
>>
>> also:
>>
>> Encoding is rather a hairy issue; javac will need to read the file to
>> find the encoding, but to read a file, it needs to know about
>> encoding! Fortunately, *every single* popular encoding on wikipedia's
>> popular encoding list at:
>>
>> http://en.wikipedia.org/wiki/Character_encoding#Popular_character_encodings
>>
>> will encode "encoding own-name-in-that-encoding;" the same as ASCII
>> would, except for KOI-7 and UTF-7, (both 7 bit encodings that I doubt
>> anyone ever uses to program java).
>>
>> Therefore, the proposal includes the following strategy to find the
>> encoding statement in a java source file without knowing the encoding
>> beforehand:
>>
>> An entirely separate parser (the encoding parser) is run repeatedly
>> until the right encoding is found. First it'll decode the input with
>> ISO-8859-1. If that doesn't work, UTF-16 (assume BE if no BOM, as per
>> the java standard), then as UTF-32 (BE if no BOM), then the current
>> behaviour (-encoding parameter's value if any, otherwise platform
>> default encoding). This separate parser works as follows:
>>
>> 1. Ignore any comments and whitespace.
>> 3. Ignore the pattern (regexp-like-syntax, ): source\s+[^\s]+\s*; - if
>> that pattern matches partially but is not correctly completed, that
>> parser run exits without finding an encoding, immediately.
>> 4. Find the pattern: encoding\s+([^\s]+)\s*; - if that pattern matches
>> partially but is not correctly completed, that parser run exists
>> without finding an encoding, immediately. If it does complete, the
>> parser also exists immediately and returns the captured value.
>> 5. If it finds anything else, stop immediately, returning no encoding
>> found.
>>
>> Once it's found something, the 'real' java parser will run using the
>> found encoding (this overrides any -encoding on the command line).
>> Note that the encoding parser stops quickly; For example, if it finds
>> a stray \0 or e.g. the letter 'i' (perhaps the first letter of an
>> import statement), it'll stop immediately.
>>
>> If an encoding is encountered that was not found during the standard
>> decoding strategy (ISO-8859-1, UTF-16, UTF-32), but worked only due to
>> a platform default/command line encoding param, (e.g. a platform that
>> defaults to UTF-16LE without a byte order mark) a warning explaining
>> that the encoding statement isn't doing anything is generated. Of
>> course, if the encoding doesn't match itself, you get an error
>> (putting "encoding UTF-16;" into a UTF-8 encoded file for example). If
>> there is no encoding statement, the 'real' java parser does what it
>> does now: Use the -encoding parameter of javac, and if that wasn't
>> present, the platform default.
>>
>> However, there is 1 major and 1 minor problem with this approach:
>>
>> B) This means javac will need to read every source file many times to
>> compile it.
>>
>> Worst case (no encoding keyword): 5 times.
>> Standard case if an encoding keyword: 2 times (3 times if UTF-16).
>>
>> Fortunately all runs should stop quickly, due to the encoding parser's
>> penchant to quit very early. Javacs out there will either stuff the
>> entire source file into memory, or if not, disk cache should take care
>> of it, but we can't prove beyond a doubt that this repeated parsing
>> will have no significant impact on compile time. Is this a
>> showstopper? Is the need to include a new (but small) parser into
>> javac a showstopper?
>>
>> C) Certain character sets, such as ISO-2022, can make the encoding
>> statement unreadable with the standard strategy if a comment including
>> non-ASCII characters precedes the encoding statement. These situations
>> are very rare (in fact, I haven't managed to find an example), so is
>> it okay to just ignore this issue? If you add the encoding statement
>> after a bunch of comments that make it invisible, and then compile it
>> with the right -encoding parameter, you WILL get a warning that the
>> encoding statement isn't going to help a javac on another platform /
>> without that encoding parameter to figure it out, so you just get the
>> current status quo: your source file won't compile without an explicit
>> -encoding parameter (or if that happens to be the platform default).
>> Should this be mentioned in the proposal? Should the compiler (and the
>> proposal) put effort into generating a useful warning message, such as
>> figuring out if it WOULD parse correctly if the encoding statement is
>> at the very top of the source file, vs. suggesting to recode in UTF-8?
>>
>> and a final dilemma:
>>
>> D) Should we separate the proposals for source and encoding keywords?
>> The source keyword is more useful and a lot simpler overall than the
>> encoding keyword, but they do sort of go together.
>>
>> --Reinier Zwitserloot and Roel Spilker
>>
>>
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