Examining Java Server Pages in detail

Overview
In this article we will study the anatomy of a JSP page and how you can use each of these elements for your own use.

Anatomy of a JSP page
If you rename a .html file to a .jsp file then that page is a valid JSP page and will compile and run successfully. But this .jsp page will be using default features provided to a JSP page and will not be doing anything useful. One other thing to keep in mind is that all JSP pages are compiled to Java Servlet classes, so inherently JSP pages are actually Java Servlets.

A JSP page consists of following parts :

• Directives
• Scripting Elements
• JSP Tags

We will now take each one of them in detail.

Directives
Directives as their name implies are compile time control tags. They allow you to customize how your JSP pages are compiled to Java Servlets. There are currently of three types :

• Page Directive
• Taglib Directive
• Include Directive

i. The Page Directive
This directive is placed at the top of a JSP page and allows you to customize different features of your JSP page. It's syntax is as follows :

<%@ page attribute-name="value" %>

All JSP directives, scripting elements and expressions lie in between the <% and %> tags. Following are the different attributes available to you along with the values you can use in your JSP page.

ii. The Taglib Directive
It is used to incorporate custom tags in a JSP page. It requires two attributes and it's syntax is as follows :

<%@ taglib uri="tag-lib-uri" prefix="tag-prefix" %>

The uri attribute contains the location of the tag library TLD ( Tag Library Descriptor ) file, while prefix is the tag prefix you want to use for your custom tag e.g,

<%@ taglib uri="/WEB-INF/tlds/mytag.tld" prefix="star" %>

<star:time />

Above code describes a tag with prefix 'star' which displays current time. We will learn more about these custom JSP tags in some later tutorial.

iii. The Include Directive
The include directive allows insertion of the content of another file into the JSP page at compilation time. It's syntax is as follows :

<%@ include file="localOrAbsoluteURL" %>

During compilation the content of the file specified in the file attribute will be added to the current JSP page e.g,

	... Some HTML and JSP code above

<%@ include file="/misc/copyright.txt" %>

</body>
</html>

Above code adds the copyright info to the current JSP page.

Scripting Elements
The scripting elements in a JSP page comprise of four types :

• Comments
• Class level declarations
• Expression
• Scriptlets

Lets look at all of them one by one.

i. Comments
Like other Java classes you can add comments in a JSP page. Comments are useful as it makes your code more understandable. The syntax of JSP comment is as follows :

<%-- This is a JSP Comment --%>

ii. Class Level Declarations
This feature allows you to add your own class level methods and variables which can be accessed by the scriptlets. It's syntax is as follows :

<%! Declaration Statement %>

For example :

<%! String author = "Faisal Khan"; %>

Above code declares a class level variable named author with a value of "Faisal Khan".

iii. Expression
It is a short hand for writing output directly to the output stream. It's syntax is as follows :

<%= statement %>

The value of the Java statement is displayed to the user. e.g,

<%= new java.util.Date().toString() %>

Above code displays the current date and time to the user.

iv. Scriptlets
Within scriptlets you can add whatever Java code you like. All this code will go inside the _jspService() method, so you cannot declare class level methods or variables within scriptlets ( see above ). It's syntax is as follows :

<% statements %>

For example :

<%
	String name = "Faisal Khan";

	out.println("My name is " + name);
%>

Above code first declares a name variable with value of "Faisal Khan" and then displays it to the user screen.

If you have been following some of my tutorials on accessing database with Java Servlets then you can just put all that JDBC code inside JSP Scriptlets and it will work. We will look into accessing database with JSP in detail in some other article.

JSP Tags
As we saw earlier we can create and use our own custom tags inside a JSP page using the taglib directive. With that there are quite a few standard JSP tags which are part and parcel of the JSP specification and provide really useful functionality :

<%@ page attribute-name="value" %>

<%@ taglib uri="tag-lib-uri" prefix="tag-prefix" %>

<%@ taglib uri="/WEB-INF/tlds/mytag.tld" prefix="star" %>

<star:time />

<%@ include file="localOrAbsoluteURL" %>

	... Some HTML and JSP code above

<%@ include file="/misc/copyright.txt" %>

</body>
</html>

<%-- This is a JSP Comment --%>

<%! Declaration Statement %>

<%! String author = "Faisal Khan"; %>

<%= statement %>

<%= new java.util.Date().toString() %>

<% statements %>

<%
	String name = "Faisal Khan";

	out.println("My name is " + name);
%>