JavaServer Pages, 3rd Edition

Mixing Client-Side and Server-Side Code

I touched on the difference between server-side code and client-side code in Chapter 3. JSP is a server-side technology, so all JSP elements, such as actions and scriptlets, execute on the server before the resulting page is sent to the browser. A page can also contain client-side code, such as JavaScript code or Java applets, to provide a more interactive user interface. This code is executed by the browser itself.

A JSP page can generate JavaScript code dynamically the same way it generates HTML, WML, or any other type of text contents. Therefore, you can add client-side scripting code to your JSP pages. The important thing to keep in mind here is that even though you can include JavaScript code in your JSP page, the container doesn’t see it as code at all. It treats it as template text and just sends it to the browser together with the rest of the response. Also remember that the only way a browser can invoke a JSP page is to send an HTTP request; there is no way that a JavaScript event handler such as onClick or onChange can directly invoke a JSP element such as an action, a scriptlet, or a Java method declared with a JSP declaration in a JSP page. A client-side script can ask the browser to make a request for a complete page, but there is no way that the script can process the response and use it to do something such as populate a selection list with the data.

Applets can make your pages more interesting and provide an easier-to-use interface ...