Adding dynamic content via expressions

As we saw in the previous section, any HTML file can be turned into a JSP file by changing its extension to .jsp. Of course, what makes JSP useful is the ability to embed Java. Put the following text in a file with .jsp extension (let us call it hello.jsp), place it in your JSP directory, and view it in a browser.



Hello! The time is now



Notice that each time you reload the page in the browser, it comes up with the current time.

The character sequences enclose Java expressions, which are evaluated at run time.

This is what makes it possible to use JSP to generate dyamic HTML pages that change in response to user actions or vary from user to user.

Exercise: Write a JSP to output the values returned by System.getProperty for various system properties such as java.version, java.home, os.name, user.name, user.home, user.dir etc.

Your First JSP

JSP simply puts Java inside HTML pages. You can take any existing HTML page and change itsextension to ".jsp" instead of ".html". In fact, this is the perfect exercise for your first JSP.

Take the HTML file you used in the previous exercise. Change its extension from ".html" to ".jsp". Now load the new file, with the ".jsp" extension, in your browser.

You will see the same output, but it will take longer! But only the first time. If you reload it again, it will load normally.

What is happening behind the scenes is that your JSP is being turned into a Java file, compiled and loaded. This compilation only happens once, so after the first load, the file doesn't take long to load anymore. (But everytime you change the JSP file, it will be re-compiled again.)

Of course, it is not very useful to just write HTML pages with a .jsp extension! We now proceed to see what makes JSP so useful.