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From "didge" <di...@san.rr.com>
Subject RE: preprocessing java source
Date Mon, 09 Sep 2002 09:24:55 GMT
The downsides I mention are the result of Java's lack of an integrated
preprocessing facility.  I bet the Java authors presumed that since Java was
platform independent, there was no need for a preprocessor and they never
forsaw the growth that Java would undergo nor that it would one day develop
into multiple, slightly incompatible platforms.

There are legitimate problems with preprocessing, but there is sometimes no
other alternative.  Love it or hate it, preprocessing is a necessity.

For some problems, reflection, while it would work, is vastly inferior to
preprocessing for handling platform specific differences, especially with
regard to performance.  And for other problems, reflection is not even an
option, especially where there is a massive api change, change to the
language itself or an change that results in incompatible api.

To wrapper an api with reflection, you must implement a complex dynamic
facade to wrap the platform dependent api, since no platform dependent api
can be directly referenced by the facade itself.  Every delegated method
call is dynamically invoked, with significant overhead.

The facade will have to declare that it supports the all platform dependent
methods declared by the class to be type compatible (and only if the class
is not final!), yet only a subset will ever actually be implemented for any
given platform.  If a client accidentally invokes an unimplemented method on
the facade, a runtime exception is the most likely result.  Ouch!  You'll
may never find out if your api users are missusing your apis until their app
is coverage tested (if you're lucky) or more likely, deployed!

In addition, the're at least a few issues in Java that cannot be resolved
(or at least in a worthwhile fasion) short of preprocessing.
1. Non-backaward compatible language changes.  There have been remarkably
few language changes in Java, for better or worse.  Assertions, however,
will definitely prevent your code from compiling or running with pre-JDK1.4
releases.  Reflection cannot help you here.
2. Package name changes.  What a headache it was when swing went from
com.sun.java.swing to java.awt.swing to javax.awt.swing.  I guess you could
use reflection...
3. You can't use reflection to deal with classes that are referenced in a
method's signature but is unavailable on the target platform.  For example,
say you wanted to wrap ResultSet with a facade for some reason.  As of
JDK1.4.x, java.sql.ResultSet now contains the method java.sql.Ref getRef(int
column).  But java.sql.Ref doesn't exist in JDK1.1.  Reflection can't create
classes that don't exist so your only alternatives to preprocessing are to
create a version of the facade for each target platform (and have your build
system play a shuffle game with the files) or to live with just a subset of
the supported functionality.
4. Another issue similar to 3 is if a method's return type is changed.
Again, reflection can't allow you to declare a facade with two methods
identical except for their return types.  You can either change the method
signatures in the facade (then use reflection) or create two different
facades that you then have to have your build system swap around based on
the target platform.

I would love to hear about any other uses for preprocessing for solving
build problems!

didge

-----Original Message-----
From: Laurie Harper [mailto:zodiac@holoweb.net]
Sent: Sunday, September 08, 2002 10:55 PM
To: Ant Developers List
Subject: Re: preprocessing java source


Re-read your list of downsides; these are all good reasons why Java doesn't
include a pre-processor.

Have you considered factoring out the JDK version dependent code and using
reflection to select the appropriate implementation at run-time? That way,
you eliminate the need for pre-processing to comile on any platform and, as
a bonus, you get to run on any platform -- not just the one you compiled on.

That's the way to deal with platform dependencies in an object oriented
way...

L.

On 9/6/02 8:05 PM, "didge" <didge@foundrylogic.com> wrote:

> Folks,
>
> For some time I've been looking around for a solution to the problem of
> handling multiple JDK platforms.  In C/C++, I would have relied on the
> preprocessor and it would be a done deal.  No such luck in Java.
>
> However, I realized that by combining Ant and Velocity, I could come up
with
> a workaround that might meet my needs.  I'm offering my solution to the
this
> group for possible inclusion as an optional ant task, if you feel that it
is
> generally useful as I think that it is.
>
> Essentially, I subclassed the Javac ant task, and used Velocity to
> preprocess each file as if they were template files. Then, I let Javac
> continue to compile just the preprocessed files.
>
> The net effect is that source files become Velocity templates and you can
> now use VTL to do as you please with your source.  And, as an added bonus,
> the VTL in the source files can access all of the properties known by Ant.
> I simply put the Project.properties into the VelocityContext supplied to
the
> merge().  For example, you can get access to an ant property called foo by
> using this VTL:
> #if ( $ant.get("jdk.target").startsWith("1.4") )
> some code...
> #else ( $ant.get("jdk.target").startsWith("1.3") )
> other code
> #else
> default code
> #end
>
> It is also possible to comment out the preprocessor directives so that
IDEs
> won't choke on the VTL directives.  Since Velocity ignores Java comments,
> Velocity will process its directives regardless, but this can result in
some
> rather wierd situations for IDEs, as you can imagine, since you could end
up
> with a strange mix of code.  Non realtime parsing IDEs probably won't care
> but those that do, such as IDEA, will markup the code with lots of syntax
> errors.  But this should work very well at least for assert like
statements.
>
> On the bummer side, you must use ant to build effectively.  This is
> mitigated a bit by the fact that most IDEs now let you execute your ant
> targets directly, but you have to be smart about debugging.  You can no
> longer debug against the original source, you now have to go against the
> source in the temporary directory, unless your java-commented VTL is very
> carefully written.
>
> An additional bummer is for javadoc comments that reference set methods.
> They usually look like someclass#setFoo, which Velocity interprets as a
> directive.  But by simply escaping the #, e.g. someclass\#setFoo, all will
> be well.
>
>
> I've successfully used this now to write a complete java.sql wrapper
library
> that compiles from version 1.1.x through 1.4.x from a single source.
>
> I'd be interested to know whay you think, if any of you have any time to
> look at it.  See the attached .zip.
>
> To build, you'll need to copy ant.jar (1.4.1 is what I used) and
> velocity-dep-1.3.1-rc2.jar.  (I didn't include them with the zip because
> they're large and you may not want to bother).  Unzip the .zip and copy
the
> aforementioned .jars into the lib directory and exectue 'ant build-g'.  To
> see an example, execute 'ant ppjavac' which will preprocess and compile a
> file under the example directory called PPTest.java.  You'll get a
> sub-directory called 'foo' where the processed file will end up.  It's
> .class will go into build/classes.
>
> The interesting ant target in the build.xml is called ppjavac.  It takes
all
> of javac's attributes and adds two more.  preprocess allows you to turn on
> or off preprocessing.  prepDir is the location of the temporary directory.
>
> enjoy!
>
> didge
>
>
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