Java builder pattern accessible from JavaScript in Rhino

I'd like to achieve something like the following (from within JavaScript) in Rhino.

myService.http('http://localhost:9000')
         .path('foo/bar')
         .get();

And then having the final get() return the actual response. It's modeled after Jersey's web request builder in Java.

The Java code I have so far is:

class MyHttpRequestBuilder {
    private final String baseUrl;
    private final StringBuilder path = new StringBuilder();
    private final Map<String, String> queryParams = new HashMap<>();
    private final Map<String, String> headers = new HashMap<>();

    public MyHttpRequestBuilder(String baseUrl) {
        this.baseUrl = baseUrl;
    }

    public MyHttpRequestBuilder path(String path) {
        this.path.append(path);
        return this;
    }

    public MyHttpRequestBuilder header(String headerKey, String headerValue) {
        headers.put(headerKey, headerValue);
        return this;
    }

    public MyHttpRequestBuilder queryParam(String queryKey, String queryValue) {
        queryParams.put(queryKey, queryValue);
        return this;
    }

    public String get() {
        return "Testing - Accessed " + baseUrl + " with path " + path;
    }
}

I've registered this in a service class like so:

public class MyService {
    public MyHttpRequestBuilder http(String baseUrl) {
        return new MyHttpRequestBuilder(baseUrl);
    }
}

I am then adding that into a Scriptable container in the Rhino context, as per various examples.

It works fine if I just call functions directly that simply return a String or another object.

Unfortunately the builder approach with "return this;" (which would allow for a more fluent code on the JS side) does not want to play ball. Is this even possible with Rhino?

I am using Java 8 and Rhino 1.7.7.1.