How to display the response it a different window (a.k.a. How to change form target in action?)
By Frank Zammetti, extracted from StrutsSolutions page.
Simply put, there is no way to do this. Opening a new window is a strictly client-side activity. You have to "fake it". There's two ways to do it...
First, you could simply target your HTML form using the target="_blank" attribute. This will result in whatever response the server sends back appearing in a new window. The down-side to this is you have to know before-hand to do this. If you want a variable solution, you have to get into scripting... You can set the target attribute's value via client-side scripting as appropiate before submitting the form.
Second, you could have a check in all JSP's that looks for some flag in the request object. When present, the JSP inserts an onLoad Javascript event handler that opens the new window and redirects to the URL where the content can be found.
Another possibility if you are using frames is to target all form submissions to a hidden frame. Include in all pages some Javascript that performs some checks and handles things accordingly... I use this in one system I wrote... The checks I'm speaking of are based on some flag value in the request object (just insert the value into a page-level Javascript variable)... The flag may direct the page to copy itself to the main display frame, or to a new window. The former is a very nice way to have a "Please Wait" screen while the server is processing. The later is the same basic idea, but in a new window. This also allows you to do some nice error handling things, such as simply hiding the Please Wait layer in the main frame to expose the page as it existed before the form was submitted. Nice way to debuild a page without incurring any server time. But now I'm getting off on a tangent 
The bottom-line here is this: There is no way to direct the browser to open the response in a new window from an Action. You either have to indicate you want this behavior when the form is submitted, or make it happen with scripting once the response is back at the browser. At least, this is true within the confines of plain old HTML... You could always pull an applet out and do something like this!