Jersey - The @Context annotation for injection. How does it work?

Question

I was looking at a good REST tutorial using Jersey. Down the page, there is a web resource that is built which is entitled TodoResource which itself contains two instance variables

public class TodoResource {
    @Context
    UriInfo uriInfo;

    @Context
    Request request;

    String id;

    public TodoResource(UriInfo uriInfo, Request request, String id) {
        this.uriInfo = uriInfo;
        this.request = request;
        this.id = id;
    }
}

I was wondering exactly how the UriInfo and Request instance variables are initialized? I know that using the @Context annotation allows for information to be injected, but at what point does this happen? Will this be handled automatically by Jersey?

Answer 1

Jersey doesn't modify the class, but it creates it on every request from the client.

After the class constructor was invoked, the context fields are injected.
(Should you try to access those fields inside the constructor, they will be null)

In your case, the class wouldn't need a specific constructor, so just:

public TodoResource () {
    // in most cases the ctor stays empty.
    // don't do much work here, remember: the ctor is invoked at every client request
}

But inside methods (which represent web-resources) annotated with @POST, @GET, ... you would have access to context fields.