Java In A Teacup - Java

Get to grips with Java Server Pages with this introductorytutorial and find out how to use one of the more powerful server-sidelanguages around. This first part explains the history and basics of JSPdocuments, and also illustrates variables, includes and the String object.

TABLE OF CONTENTS:

By: Vikram Vaswani and Harish Kamath, (c) Melonfire

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February 07, 2001

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TOOLS YOU CAN USE

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.

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