JAX-RS (Jersey) ExceptionMapper - @Context injection into static/singleton class - it works, but why?

I have a single-instance class, implementing ExceptionMapper. It's not a static class, but it's a class for which I know only single instance is created (I checked - constructor is called only once).

My class uses @Context HttpServletRequest, and I can clearly observe that when my ExceptionMapper.toResponse() method is called, the @Context 'request' parameter has a value which is relevant for a request where the exception is thrown.

The doc says this is indeed by-design supported feature and that it's done by using "proxies".

I wonder how exactly this is implemented - how a single instance can have different member variable values simultaneously?

Thank you,
AG

P.S.: here's the test code:

@Provider
public class MyExceptionMapper implements ExceptionMapper<Exception> {

    public MyExceptionMapper() {
        System.out.println("CTOR!!");
    }

    @Context HttpServletRequest req;

    public static boolean done = false;  
    public Response toResponse(Exception ex) {
        if (!done) {
            done = true;
            Thread.sleep(10000);
        }
        System.out.println(req.getRequestURI());
        return null;
    }
}

My REST handler method throws exception, so when I execute the following 2 requests "in parallel" (the sleep above makes sure first one is not finished when second one arrives and IMHO should modify the one-and-only 'req' field):

- http://localhost/app/one
- http://localhost/app/two

my program prints:

CTOR!
http://localhost/app/one
http://localhost/app/two