XAgents - Web Agents XPages style

In Domino Agents are the Swiss Army Knife of development. They can be called from the client, triggered on schedule, automatically run on an event, be associated with a web action (Open, Close) or directly called from an URL using ?OpenAgent. The ?OpenAgent use of agents is subject of this blog entry. A agent triggered from an URL can use Print statements to output results back to the browser. When the first line of your print statements is "Content-Type:..." you also can serve XML or JSON or whatever exotic format your agent logic was able to conceive. Agents come with the usual drawback: the runtime environment is intitialized new for every agent run, so you need to update a document if you want to keep track (Replication conflicts anyone?)
XPages don't have a notion of agents (you can call them in the backend, but you won't get anything back from the print statements), so it seems a step back. On closer inspection however one can see, that XPages offer a greater flexibility for the same pattern of use. Every element on an XPages has a "render" attribute which determines if the element should be rendered. "Every" includes the page itself. So when you set the page's render property to false, well the XPage engine renders nothing. So you get an empty canvas you can paint on. XPages still provides you with the page event and access to scoped variables, so you can code better performing "agents" since you can keep lookups in memory. To get the equivalent to the LotusScript print you need the ResponseWriter from the externalContext. To set the content type you use the response.setContentType also to be found in the external Context. A sample snippet you could put in the afterRenderResponse event of your page would look like this (error handling omitted):

// The external context gives access to the servlet environment
var exCon = facesContext.getExternalContext();

// The writer is the closest you get to a PRINT statement
// If you need to output binary data, use the stream instead
var writer = facesContext.getResponseWriter();

// The servlet's response, check the J2EE documentation what you can do
var response = exCon.getResponse();

// In this example we want to deliver xml and make sure it doesn't get cached
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");

// Here all your output will be written
writer.write("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<christmas date\"24Dec\" />");

// We tell the writer we are through
writer.endDocument();
facesContext.responseComplete();
writer.close();

// Update:On 8.5.2 or later writer.close() seems not be necessary/be rather harmful. Read the revisited entry
More information about the FacesContext can be found in the Domino Developer Wiki.

Bonus Tip: Since you do not render anything from the XPage you just used, why not take that empty page and write the documentation for that XAgent?
Update: Other posts on rendering your own output in XPages:

• Rendering ODF documents in your XPages
• Rendering your own output in XPages revisited

Have fun writing XAgents!