Tuesday, January 09, 2007

Mailet Annotations - Proof of concept.

Well I did it at last, got my hands wet with annotations.

Work being work we're still well stuck in the j2sdk1.4 era because of the stuff we don't have the balls to upgrade too quickly :-)

James being James we tend to stick like glue to the oldest JDK we can until we have a justification for requiring an upgrade, as opposed to just a half-arsed reason for it, which is what we usually manage.

So yesterday, after a few weeks of debate on the mailet-api@james list about how we would deal with satisfying mailets' dependance on arbitrary services, I had a go with annotations.

We'd originally toyed with the idea of the service location pattern using JNDI. I built a POC for that out of the Apache Directory naming stuff, which is pretty comprehensively top-notch in both the "it works" and "its easy" categories. But that really seemed to be a cop-out since James has long espoused IoC. So the feeling on the list which incidentally includes some non-james people interested in the API, was that dependance injection should be specified where possible in the API, and Annotations used to let the container know what the Mailet is expecting.

Working on a sandbox fork of James, which I'm doing so I can get away with this stuff *and* let other people see it before I accidentally delete it, I created a custom annotation type and annotated a mutator ("setter").
The idea being that, in this case, the annotation would contain a String name parameter of a pre-configured datasource (javax.sql.Datasource) and on finding the annotation James' mailet loader would lookup the service (in this case in the JNDI context as it happens) and inject it into the mailet by calling the annotated method.
Although I did it with DataSource's which aren't quite the same as Services will be, it was relatively easy to programme James to sort this stuff out, and extremely nice and easy to annotate the method.

Whats more, and this is the big selling point for me, it worked first time.
I know it counts for nothing but it output this reassuring message to the console...

ToRepository Mailet requires datasource maildb
maildb is org.apache.james.util.dbcp.JdbcDataSource@652552
mailet got datasource :-) org.apache.james.util.dbcp.JdbcDataSource@652552

So now, having proven it, I guess I have to get to grips with the niceties of Annotations and design the annotations we want to use.


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