How to open a new window from an Action

Another frequent question on the mailing lists is how to open a new browser window from an Action. This one won't take too long:

YOU CAN'T!

Strictly-speaking, this is true... code running on the server has no way to tell the browser to open the response in a new window. That being said, there ARE a couple of ways to get the desired effect.

Perhaps the easiest way is to set the target of the form you submit to "_new". This will open a new window in which the results returned by the server will be shown. Note that this will ALWAYS happen, there is no standard way to only use this target some of the time. You can however use Javascript to variably set and clear the target attribute if you need to sometimes open a window and sometimes not, perhaps depending on whether a given checkbox is checked or not.
You can manually open a window using Javascript, and from there submit the form. This will usually involve copying the values from the form in the parent document to a copy of it in the child window, and then programmatically submitting it. This can be a good option if you are essentially spawning a navigational subbranch in your application and not simply showing results (i.e., a parallel task the user can perform vs. just showing a generated PDF).
A third option is to use AJAX. You may not know it, but you can return Javascript from the server via an AJAX call and execute it. What you can do is return something along these lines (this would literally be the HTTP response text):

   s = "";
   s += "<html><head><title>Results</title></head><body>";
   s += "These are my results";
   s += "</body></html>";
   w = window.open("", "Results", "resizable,height=200,width=200");
   w.document.open();
   w.document.write(s);
   w.document.close();

Imagine generating this in an Action, where the string "These are my results" is replaced by a full HTML page (properly escaped of course). Yes, you could generated the response with a JSP, which is what I would recommend. That might be confusing, so let me explain... a JSP does NOT have to render a complete HTML document. It can instead render just a snippet of HTML, or just some Javascript, or just some comma-separated data, there is no limitation. If you are doing AJAX, whatever is most appropriate can be done via JSPs (like XML too!). This saved you from writing a lot of println's in your Actions to generate a response.

Now, if you make an AJAX call to this Action, and return this content and execute it, you will get your response opened in a new window. Neat, huh?!? If you decide to go this route, I suggest checking out the AjaxParts Taglib in the Java Web Parts project (http://javawebparts.sourceforge.net). It will allow you to put a single custom tag on your page and save you from having to write the AJAX code or the code to execute the returned results (although you will still have to write the code in the Action).

Takeaway point

The important point to take away from all this is that aside from the target attribute on the form tag, Javascript is involved. There is currently no good way to avoid that (and someone can correct me if I missed something). If you are working in an environment where scripting is a problem, you may want to revisit your design and see if you really need a new window at all.