Scala, Maven, and preprocessors

Question

I know all of the philosophical arguments against preprocessors and macros in Java. I don\'t agree that just because some may abuse a language feature, it should be excluded

Answer 1

Logging should be a cross-cutting concern, anyway. It's possible to do what you want with aspects.

Answer 2

Update @ralph Well, this is a question from 2+ years ago, but since I'm having the same need for __LINE__ and __FILE__ and you had mentioned SCALA; here are some ideas I'm having, using Scala macros (as of v2.10.0-RC1)

def $currentPosition:String = macro _currentPosition;
def _currentPosition(c:Context):c.Expr[String]={ import c.universe._;
  val pos = c.enclosingPosition;
  c.Expr(Literal(Constant(
    s"${pos.source.path}: line ${pos.line}, column ${pos.column}" )))
}

Macros being evaluated at compile-time, $currentPosition is replaced with a literal string that describes its position in the source code. For example, put in a println at line 13, it displays:

/sandbox/tmp_juno_workspace2/LogMacro_Test/src/test/Trial.scala: line 13, column 15

I have not played with these mechanisms extensively, but by tweaking the thing, one can develop the logging features (s)he requires (I should add that writing macros can be difficult - it is for me!).

Answer 3

IMHO the "correct" way to achieve this would be to write a compiler plugin to perform the substitution, but is it really worth that amount of effort?

Answer 4

You can use the compiler API to do this, but be careful. The "rules" for annotation processors/compiler plugins state that they are not supposed to modify the class being generated. Sun/Oracle may enforce this at anytime.

For now, have a peek at http://projectlombok.org. It uses the compiler API to generate getters, setters and so forth. They have source code that you could use as a model, or you could contribute handlers to them.

you could probably do something like the following:

public class Foo {
  @FileName private static String fileName;
  @LineNumber private static int lineNumber;
  ...
  public void foo() {
     log(fileName, lineNumber, "some message");
  }
}

Then have the annotation processor change the fileName and lineNumber references to the actual file/line.

Just be aware that this could break with later JDK versions. I don't know if Sun/Oracle will actually enforce the "don't modify the class being generated" rule, but they could.