The JSP Files (part 1): Purple Pigs In A Fruitbasket - Java In A Teacup (
Page 3 of 7 )
In order to begin
working on JSP, you need to get yourself copies of Sun's Java Development Kit,
Apache's httpd Web server and mod_jserv module, and the Tomcat servlet engine,
and configure them so that they're all working together. This tutorial assumes
that you've got a JSP development environment set up - in case you don't, take a
look at "Slapping Together A JSP Development Environment" at , a tutorial which
will guide you through the process.
With that out of the way, let's get
down to the nitty-gritty of actually creating a JSP page. Open up a new file in
your favourite text editor and type in the following lines of code:
<html>
<head>
</head>
<body>
<%
// asking for it!
out.println("Waiter, can I have a cup of Java, please?");
%>
</body>
</html>
Save this file with the extension .jsp - for example,
"coffee.jsp" - in an appropriate location and then view it by pointing your
browser to it - for example, http://localhost/jsp/coffee.jsp . You should see
something like this:
<html>
<head>
</head>
<body>
Waiter, can I have a cup of Java, please?
</body>
</html>
</font>
And that, grasshopper, is your first scriptlet!
In
JSP-lingo, a "scriptlet" is a block of code executed by the JSP engine when the
user requests a JSP page. All scriptlets are enclosed within <%...%> tags
(similar to ASP and PHP code), like this:
<%
... JSP code ...
out.println("Waiter, can I have a cup of Java, please?");
... JSP code ...
%>
Every JSP statement ends in a semi-colon - this convention is
identical to that used in Perl, and omitting the semi-colon is one of the most
common mistakes newbies make. Just as an example, here's what happens when you
omit the semi-colon from the example above:
Error: 500
Location: /jsp/coffee.jsp
Internal Servlet Error:
org.apache.jasper.JasperException: Unable to compile class: Invalid type
expression.
out.println("Waiter, can I have a cup of Java, please?")
^
: Invalid declaration.
out.write("\r\n\r\n\r\n");
^
2 errors
at org.apache.jasper.compiler.Compiler.compile(Compiler.java, Compiled
Code)
at org.apache.jasper.servlet.JspServlet.doLoadJSP(JspServlet.java:462)
at
org.apache.jasper.servlet.JasperLoader12.loadJSP(JasperLoader12.java:146)
at org.apache.jasper.servlet.JspServlet.loadJSP(JspServlet.java:433)
at
org.apache.jasper.servlet.JspServlet$JspServletWrapper.loadIfNecessary(JspSe
rvlet.java:152)
at
org.apache.jasper.servlet.JspServlet$JspServletWrapper.service(JspServlet.ja
va:164)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:318)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java, Compiled
Code)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
at org.apache.tomcat.core.ServletWrapper.doService(ServletWrapper.java:404)
at org.apache.tomcat.core.Handler.service(Handler.java:286)
at org.apache.tomcat.core.ServletWrapper.service(ServletWrapper.java:372)
at
org.apache.tomcat.core.ContextManager.internalService(ContextManager.java:79
7)
at org.apache.tomcat.core.ContextManager.service(ContextManager.java:743)
at
org.apache.tomcat.service.connector.Ajp12ConnectionHandler.processConnection
(Ajp12ConnectionHandler.java:166)
at org.apache.tomcat.service.TcpWorkerThread.runIt(PoolTcpEndpoint.java,
Compiled Code)
at org.apache.tomcat.util.ThreadPool$ControlRunnable.run(ThreadPool.java,
Compiled Code)
at java.lang.Thread.run(Thread.java, Compiled Code)
Whoops!
It's also possible to add comments to your JSP
code, as in the example above. JSP supports both single-line and multi-line
comment blocks - take a look:
<%
// this is a single-line comment
/* and this is a
multi-line
comment */
%>
Like PHP and Perl, white space is ignored in
JSP.
Finally, the statement which actually prints output to the browser -
as you'll see, this is done using the "out" object. Since JSP is based on Java,
and Java is an object-oriented language, most of your JSP statements will
include object references such as this one.
/ 9 
