A Brief Introduction To JSP

A Brief Introduction to Java Server Page
K. Yue April 2001

Introduction

Comparable to ASP. Allows embedding Java code inside HTML contents.
Used to be called 'Page Compilation.'
JRun supports Java Servlet 2.2 and JSP 1.1.
Java Server Web Development Kit from Sun supports JSP 1.1.
JSP files end with the extension .jsp.
Use XML tags.

Architecture

A JSP file request is usually handled by the JSP-compliant server the first time it is accessed since last modification as below:

A special servlet (such as JSPServlet in WebLogic) should have been configured to handle all .jsp file.
If there is no compilation error, the .jsp file is compiled to a servlet class file (for example, in WebLogic, hello.jsp is compiled to _hello.class, which is stored in a special directory). The servlet class file implements javax.servlet.jsp.JspPage.
The generated JspPage servlet class is invoked.

JSP Core Syntax

Page directives are used to define attributes that apply to an entire JSP page.

Example: HelloWorld.jsp

<%@ page info="A Hello World JSP Page" %>
<% for (int i=0; i<10; i++) {
out.println("Hello, World.");
}
%>

Scriplets contain code fragments valid in the page scripting language.
Include directives are used to include a file of text or code in the JSP source file.

Example: IncludeIncorrect.jsp

<%@ page info="A Simple Hello World Page"
contentType="text/html;charset=ISO-8859-1"
%>
<BODY BGCOLOR="#CCCCFF">
<%@ include file="HelloWorld.jsp" %>
</BODY>

Note that this may cause trouble in some JSP server as the page directive is now duplicated.

Regular comments (sent to the client): .
Hidden comments (not sent to the client): <%-- and --%>

Example: ParameterSnoop.jsp

<%@ page import = "java.util.*" %>
<%-- The directive import allows importing Java packages. --%>

<%
   out.println("Request Parameters:");
   Enumeration enum = request.getParameterNames();
   while (enum.hasMoreElements()) {
      String name = (String) enum.nextElement();
      String values[] = request.getParameterValues(name);
      if (values != null) {
         for (int i = 0; i < values.length; i++) {
            out.println(name + " (" + i + "): " + values[i]);
         }
      }
   }
%>

Declaration directives variables or methods valid in the page scripting language.
Expression directives contains expressions valid in the page scripting language.

Example: TempCount.jsp

<%@ page isThreadSafe="true"
info="Simple useless non-permanent counter" %>

<%-- Define a static variable count --%>
<%! int count = 0; %>

<BODY BGCOLOR="ccccff">
The number of visitors since this JSP page is loaded:
<%= ++count %>.
</BODY>

Important built-in objects:

JspWriter out -- to write to the browser
HttpServletRequest request -- the request object.
HttpServletResponse response -- the response object.
PageContext pageContext -- the page context for this JSP
HttpSession session -- the session object for the client (if any)
ServletContext application -- The servlet (application) context
ServletConfig config -- The ServletConfig for this JSP
Object page -- the instance of this page's implementation class (i.e., 'this')

The built-in objects out and request are used most frequently.
Many of these objects (e.g., out, response and request) are not available within the declaration directives.

Using Java Beans

Very Basic Steps:

Create the Java Beans to be used.
Put the class files of the Java Beans in the appropriate path. To be safe, that should be in your Java classpath, and not in your vendor classpath.
In JRun in voyager, that is the directory youraccount/JRun/Web-INF/classes.
Use the tag <jsp:useBean> to declare beans to be used within your jsp programs.
Use the tags <jsp:setProperty> and <getProperty> to set and get properties of the Java beans.

<jsp:useBean>:

To specify the beans to be used.
Useful attributes:

id: bean id (name) to refer to.
scope: scope of bean existence; values: page, session, application and request.
class: initiate the bean from a class.

Note that the useBean tag can include other tags, such as setProperty.

<jsp:setProperty>

to set a property of the named bean.
Useful attributes:

name: bean id.
property: property name.
value: property value; may use expression directive as values.
param: setting property value from HTTP user parameters.

<jsp:getProperty>

to get a property of the named bean.
Useful attributes:

name: bean id.
property: property name.

Limitation:

Only Java Bean, not EJB.
Cannot get indexed properties.

Example: HelloBean.jsp

<jsp:useBean id="hello" class="temp.HelloBean" />
<BODY BGCOLOR=#CCCCFF>
Hello,
<SPAN STYLE="COLOR:BLUE"><%= hello.getMessage() %></SPAN>.
</BODY>

temp/HelloBean.java (an 'ugly' version)

package temp;
public class HelloBean {
   private String message = "You are the sunshine of my life.";
   public String getMessage() {
      return message;
   }
}