First off, I assume you already know how to return the data Java side, in which case this is really just a javascript question. To do simple post-gets, I usually use Ajax, like so:
new Ajax.Request(myUrlToCall, {
method:'POST',
onComplete: function(transport) {
window.location.replace(theRedirectUrl);
}
});
This will call whatever url you wanted to call Java side, then on the completion of this call, will redirect you to whatever url you want to go to. If you need to send some parameters to the java side, all you need to add is:
parameters:myParamVar,
after the method:'POST' line, and if you want your java side to return the url, then all you'd need to do is send it as json or similar in the response, then do:
window.location.replace(transport.responseJSON["theRedirectUrl"])
Hope that helped!