This isn't a decision you can make at the server side.
When you click the link on the page, it has to know at that time where it will load the response into.
All the server does is send back a response, and leaves it to the client to put the response where it will - replace the current page, in a new window etc etc.
You can do in
struts by setting a target="_blank" on your html:form or html:link tag.