docs:programming:jsp:elements_of_jsp [hartungdesign.net wiki]

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====== Elements of JSP ======


===== How it works =====
Whenever a .jsp is requested for the first time, the server does the following:

  - Translates the .jsp page into a servlet
  - Compiles the servlet into a class file
  - Executes the servlet (response is sent to the client)

Subsequent requests (as long as the .jsp page is unchanged) use the same loaded class file.

===== Anatomy of a JSP Page =====
A JSP page is a mixture of standard HTML tags, web page content, and some dynamic content that is specified using JSP constructs. Everything except the JSP constructs is called Template Text.

 
===== JSP Constructs =====

==== JSP Comments: Different from HTML comments ====
<code>
		<!-- an HTML comment -->
		<%-- a JSP comment --%>
</code>

JSP comments are used for documenting JSP code and are not visible client-side (using browser's View Source option) where as HTML comments are visible.

==== (JAVA) Expressions ====
<code>
		<%= some_java_expression %>
</code>

Example:
<code>
<%= new java.util.Date().toString() %>
</code>

Output of the expression is placed in the HTML template at the same location.

There are some pre-defined Java variables/objects available for use in expressions (provide access to important servlet functionality):

request
This is the same object as HttpServletRequest parameter in th get/post methods. Same methods (like, getParameter, getAttribute, etc) can be applied to it.

  * **out**
    * The servlet printwriter.

  * **session**
    * Same as servlet session object.

Example:
<code>
   <HTML>
    <HEAD>
     <TITLE>JSP Expressions: Predefined Objects</TITLE>
    </HEAD>
    <BODY>
     <H1>Using Predefined Objects</H1>
     <UL>
      <LI> Your Hostname: <%= request.getRemoteHost() %>
      <LI> Your Session ID: <%= session.getId() %>
      <LI> The value of INFO parameter: <%= request.getParameter("INFO") %>
    </BODY>
   </HTML>
</code>

==== Scriptlets ====
Scriptlets are arbitrary pieces of Java code inserted in the page using the format:
<code>
		<% some_java_code %>
</code>

Example 1
<code>
   <HTML>
    <HEAD>
     <TITLE>JSP: Scriptlets</TITLE>
    </HEAD>
     <%
        String bgColor = request.getParameter("COLOR");
       
        if (bgColor == null)
           bgColor = "WHITE";
        
     %>
    <BODY BGCOLOR="<%= bgColor %>" >
     <H1>Example Scriptlet: Sets background color</H1>
       
    </BODY>
   </HTML>
</code>
 
Example 2
<code>
   <HTML>
    <HEAD>
     <TITLE>JSP: Scriptlets 2</TITLE>
    </HEAD>
     <% String bgColor = request.getParameter("COLOR"); %>
       
     <% if (bgColor == null) { %>
           <BODY BGCOLOR="FFFFFF" >
     <% } else { %>
           <BODY BGCOLOR="<%= bgColor %>" >
     <% } %>
       
     <H1>Example Scriptlet: Conditionally sets background color</H1>
     <% if (bgColor == null) { %>
           You did not supply a color, I used white.
     <% } else { %>
           Here is the color you requested.
     <% } %>
       
    </BODY>
   </HTML>
</code>