Java runtime annotations are stored in Java bytecode using attributes, as specified by the Java virtual machine specification. So one can use toolkits like the ASM toolkit for creating and reading annotations in addition to using the Java reflection API for reading annotations.
Implementations of languages that compile to Java bytecode can also leverage runtime annotations and it can make for really smooth integration between classes/objects written in the language and frameworks, such as Jersey (JAX-RS), Metro (JAX-WS, JAXB), JUnit and Spring etc. written in Java, that process annotations.
So what's the support for runtime annotations like out there for some common JVM-based language implementations?
Scala has by far the most robust support for annotations. The annotation syntax is very similar to Java. JAX-RS and Jersey specify support for runtime annotations on classes, fields, methods and method parameters. I can annotate Scala classes using JAX-RS annotations and the Jersey runtime correctly processes the thoses classes (as shown here). In fact since the integration between Java and Scala is so smooth and Scala being such a pleasurable language to code in i could not resist including in the Jersey distribution an example written in Scala.
Groovy 1.1 supports runtime annotations and like Scala the syntax is very similar to Java. Originally i played around with Jersey and Groovy 1.1 Beta. This version supported annotations on classes, method and field but not on method parameters. Recently downloaded Groovy 1.1 RC 1 and this version supports annotations on method parameters. So i can annotate Groovy classes and support all the Jersey features as i can in Scala (Groovy 1.1 is not yet final where as Scala 2.6 is so Scala's support is considered more robust). Since Groovy requires no compile step i can script Jersey resources (more on this later in another blog).
Now things start to get a little fuzzy. My understanding is Ruby does not currently support annotations but that it is very easy to do so because of Ruby's meta-programming capability, as shown here. Charles Nutter (one of the JRuby leads) says here:
"JRuby doesn't support annotations because Ruby doesn't support annotations. So what! We can extend Ruby..."
and further says "I've planted the seed here and on the JRuby dev list...let's see if it germinates.", but i don't know if that seed has taken root...
Python has function decorators in Python 2.4, function annotations and class decorators implemented in the subversion repository of python 3.0 (or Python 3000, it says here that it will go final in August 2008). So perhaps Python 3000, when it is ready, would be the right place to start any Java-based annotation support.
The latest stable release of Jython is version 2.2.1 supporting Python 2.2. From the roadmap Jython 2.5 is next (no date given) and this release will target Python 2.5. It says the core developers will be:
"concentrating on making a clean 2.5, while improving the java
So maybe something specific to Jython w.r.t. to annotation support could be implemented. Although the Jython roadmap looks on the right track i suspect the 2.5 release won't be happening any time soon and the 3.0 release is way over the horizon. However, the competition between language implementations is in rude health so perhaps things will zip along faster than I expect.