Annotation model validation

Question

Hi,
I'm trying to build an annotation based framework and I wonder if there is a way to validate the developers' use of the framework's annotations.
For instance :
@TCPConnectionHandler(serverHost = "localhost", serverPort = 9595)
public class MyConnectionHandler {
     @In
     private InputStream in;
     @In
     private Map in2;
     @Out
     private OutputStream out;
     public MyConnectionHandler() {
     @HandleConnection
     public void process() {
          try {
               BufferedReader reader = new BufferedReader(
                         new InputStreamReader(in));
               PrintWriter writer = new PrintWriter(out, true);
               String s = null;
               while (((s = reader.readLine()) != null) && (!s.equals("bye"))) {
                    writer.println(s);
          } catch (Exception e) {
               e.printStackTrace();
}I'd like that at compilation time, an error says "Inside a TCPConnectionHandler, only one member can me marked as @In and it must be an InputStream."
Any idea ?
Thanks.

Answer 1

Hi Bruce,
Thanks for helping. I won't be able to come to
Javaone but I'd be interested in reading a copy of
your slides. Are you implementing static asserts ?
If you mean making assertions about what is (or should be) seen via static analysis, then yes.
Regarding my issue, to be more precise, I'd like the
framework to be available as a simple JAR, containing
the annotations and the utility classes. I don't want
developers having to put something in their code to
validate their use of the framework.Yes you can.
>
Is something like this possible with Mustang :
* launch javac with annotation processing turned onit is on by default.
* execute some custom
javax.lang.model.element.ElementVisitor to
check framework's annotation are used correctly ;Yes, except you place your implementation of javax.annotation.processing.Processor in your jar file, and create a /META-INF/services/javax.annotation.processing.Processor file with the FQN of your Processor in there so that javac can find it automatically.
* throw compilation error if not.
Yep, once you do the static analysis inside your Processor's process() method, and find an error, you get a Messager object from the Processing Environment (which you are passed) and then get the Messager to output the error. The actual compilation phase will not then take place.
Any documentation about that on mustang.dev.java.net
?Start here.
http://download.java.net/jdk6/docs/api/index.html?javax/annotation/processing/package-summary.html
See also
http://blogs.sun.com/roller/page/ahe?entry=throwing_unchecked_exceptions
for an example processor that checks for throwing unchecked exceptions.
That example uses the compile line to explicit call the processor, but for your case the automatic discovery technique makes it seemless for users of your jar.
And I really do mean seamless, they will need the jar file on their classpath so the compiler can find the class files for your annotations they are using, and if that is the case, then the compiler will also find your processor, and they'll get error messages and a failed compile if they are misusing the annotations (according to the processor). Its pretty amazing really.
>
Thanks again.
Thomas