Let's assume that you have 2 divs inside of your html file.
<div id="div1">some text</div>
<div id="div2">some other text</div>
The java program itself can't update the content of the html file because the html is related to the client, meanwhile java is related to the back-end.
You can, however, communicate between the server (the back-end) and the client.
What we're talking about is AJAX, which you achieve using JavaScript, I recommend using jQuery which is a common JavaScript library.
Let's assume you want to refresh the page every constant interval, then you can use the interval function to repeat the same action every x time.
setInterval(function()
{
alert("hi");
}, 30000);
You could also do it like this:
setTimeout(foo, 30000);
Whereea foo is a function.
Instead of the alert("hi") you can perform the AJAX request, which sends a request to the server and receives some information (for example the new text) which you can use to load into the div.
A classic AJAX looks like this:
var fetch = true;
var url = 'someurl.java';
$.ajax(
{
// Post the variable fetch to url.
type : 'post',
url : url,
dataType : 'json', // expected returned data format.
data :
{
'fetch' : fetch // You might want to indicate what you're requesting.
},
success : function(data)
{
// This happens AFTER the backend has returned an JSON array (or other object type)
var res1, res2;
for(var i = 0; i < data.length; i++)
{
// Parse through the JSON array which was returned.
// A proper error handling should be added here (check if
// everything went successful or not)
res1 = data[i].res1;
res2 = data[i].res2;
// Do something with the returned data
$('#div1').html(res1);
}
},
complete : function(data)
{
// do something, not critical.
}
});
Wherea the backend is able to receive POST'ed data and is able to return a data object of information, for example (and very preferrable) JSON, there are many tutorials out there with how to do so, GSON from Google is something that I used a while back, you could take a look into it.
I'm not professional with Java POST receiving and JSON returning of that sort so I'm not going to give you an example with that but I hope this is a decent start.