Overview
Using Java Reflection to generate a list of REST endpoints defined in JAX-RS code
Background – JAX-RS
I’ve been working on a project that uses JAX-RS – the Java API for RESTful web services. If you don’t know JAX-RS, you write web services in Java using annotations to specify what REST endpoint a Java method implements.
For example, you can use @Path annotations on a class to define the root URI for methods in the class, and then use annotations like @GET, @Produces(MediaType.APPLICATION_JSON) and @Path on the individual class methods to define the endpoints that they implement.
The problem?
Reading from code to the web service is straightforward enough. By which I mean, if I’m looking at a Java method, it’s easy enough to look at it and know what endpoint it is implementing.
Going the other way can be a little trickier.
Once a project gets bigger, you can have REST endpoints spread around a large number of classes. And methods can inherit attributes from other classes than the one they’re in, through annotations like @Parent.
What if I’m using one of the project’s REST APIs, and want to look at the source for the method that’s handling it, whether to extend it or fix a bug? How can I remember which method in which class is responsible for the REST endpoint I’m using?
Using Reflection
Documentation is one way. As I develop the code, maintain a list of the mapping of Java methods to web services endpoints. And keep that up-to-date as I make any changes to the code.
But that’s very manual, and doesn’t seem very smart.
This got me thinking yesterday evening. I’d not used Java Reflection before, but thought it must be possible to work it out from the Java annotations in the same way that my JAX-RS provider must.
So I spent a bit of time trying it out and thought it might be useful to share what I came up with. It’s not terribly elegant or efficient. It’s the result of a few hours tinkering. But it shows the basic idea, and that seems useful enough to warrant sharing.