Accessing HTML DOM elements from Java

I'm developing (with Java) a P2P application. One of the features includes a chat service. When a user sends a message to all of the application users, each user gets the message and updates its chat HTML page.

How can I access, from my Java code, the DOM of this page and change it, without the need to refresh the page in order to see the new message?

Is there any object in Java that can get me this access? For example, can I call a JavaScript function that inserts the new message?

If by from Java you mean applet then: You can define some javascript functions in your HTML page to return/modify what you want and then call the javascripts from the applet. Look at here.

If by Java you mean Web server then you have to use some AJAX solution, you can look for example at JQuery

What you're really looking for is a technology known as Comet. Comet is Reverse Ajax. It's a technique that uses long-lived HTTP connections to hold a connection open from a client browser to a server so that the server can push updates back to the client browser.

The basic flow is that the server pushes a command back to the browser in the response, and JavaScript parses the response via a callback function, and then the JavaScript updates the DOM, all without reloading the page.

You can learn more about Comet on the CometD Website, and if you're developing on Google App Engine, this blog post on the ChannelAPI will be helpful.