O'Reilly Book Excerpts: Java Cookbook: Solutions and Examples for Java Developers
Web Server Java -- Servlets and JSP
This excerpt is Chapter 18 from Java Cookbook: Solutions and Examples for Java Developers, published in June 2001 by O'Reilly.
This chapter covers Web Server Java, but you won't find anything about writing CGI programs in Java here. Although it would be entirely possible to do so, it would not be efficient. The whole notion of CGI programs is pretty much passe. Every time a CGI program is invoked, the web server has to create a new heavyweight process in which to run it; this is inefficient. If it's interpreted in Java, the program has to be translated into machine code each time; this is even more inefficient.
Today's trend is toward building functionality into the web server: Microsoft ASP, PHP3, Java servlets, and JavaServer Pages? (JSP) are examples of this. None of these normally requires a separate process to be created for each request; the Java-based solutions run in a thread (see Chapter 24) inside the web server, and the Java bytecode need only be translated into machine code once in a long while, assuming a just-in-time (JIT) runtime system. Naturally, this book concentrates on the Java solutions.
We'll use two examples in this chapter. Consider the task of displaying a web page with five randomly chosen integer numbers (lottery players love this sort of thing). The Java code you need is simple:
// Part of file netweb/servlets_jsp/FiveInts.java Random r = new Random( ); for (int i=0; i<5; i++) System.out.println(r.nextInt( ));
But of course you can't just run that and save its output into
an HTML file because you want each person seeing the page to get a different
set of numbers. If you wanted to mix that into a web page, you'd have to write
println( ) a bit of HTML. This would be a
The servlet code could get messy, however, since you'd have to escape double quotes inside strings. Worse, if the webmaster wanted to change the HTML, he'd have to approach the programmer's sanctified source code and plead to have it changed. Imagine if you could give the webmaster a page containing a bit of HTML and the Java code you need, and have it magically compiled into Java whenever the HTML was changed. Imagine no longer, says the marketer, for that capability is here now, with JavaServer Pages.
The second example is a dictionary (list of terms); I'll present this both as a servlet and as a JSP.
I won't talk about how you get your servlet engine installed, nor exactly how you install your servlet. If you don't already have a servlet engine, though, I'd recommend downloading Tomcat from http://jakarta.apache.org/. Tomcat is the official reference implementation--so designated by Sun--for the servlet and JSP standard. It is also (as you can infer from the URL) the official servlet engine for the ever-popular Apache web server.