JSP basics - Java Server Pages

 The JSP - Java Server Pages

• JSP extension of the servlet technology which offers both scripting ( as in HTML) as well as programming (as in Servlets) 
• When a JSP page is requested, the JSP container ( also called page compiler) compiles the jsp page into a servlet class. 
• The JSP API that handles all the JSP technology has been bundled under the following two packages: 
• javax.servlet.jsp 
• javax.servlet.jsp.tagext 
  

JSP Syntax

A JSP page is all about JSP tags and some template data ( such as HTML tags). That's pretty much of it. It always feels good learning about JSP as we get to see the very well-know HTML tags (I presumed that you already know HTML at this point).

JSP consists of three types of elements.

• Directive elements 
• Scripting elements 
• Action elements 

DIRECTIVES

<%@ directiveName (attributeName="atrributeValue") %>

These are the messages/information to the JSP container on how it should translate the JSP page into the corresponding servlet.

Again, there are three types of directives:

• Page directives
• Include directives
• Tag library directives 

1. Page Directive
<%@ page (attributeName="atrributeValue") %>

You might have encountered some "import" statements at the beginning of some JSP pages. Those import statements falls under this category. E.g.

<%@ page import="java.util.Date, java.util.Enumeration" language="java" %>
where,
          page => represents directiveName
          import/language => represents attributeName
          and the values enclosed by quotes represents atrributeValue

The following is the list of available Page directive attributes:

Attribute	Value Type	Default Value
language	Scripting language name	"java"
info	String	Depends on the JSP container
contentType	MIME type, character set	"text/html;charset=ISO-8859-1"
extends	Class name	None
import	Fully qualified class name or package name	None
buffer	Buffer size or false	8Kb
autoFlush	Boolean	"true"
session	Boolean	"true"
isThreadSafe	Boolean	"true"
errorPage	URL	None
isErrorPage	Boolean	"false"

NOTE:

Except for the import attribute, JSP does not allow to repeat the same attribute within a page directive or in multiple directives on the same page.

Page Attributes a little in-depth:

• language 
• <%@ page language="java" %>
• The only supported language by Tomcat JSP Container
• Other JSP containers might accept different languages
• info 
• <%@ page info="any string here" %>
• Can be retrieved by calling the method getServletInfo() from anywhere withing the page
• contentType 
• <%@ page contentType="application/json; charset=UTF-8" %> 
• Defines MIME type of the HTTP response sent to the client 
• extends 
• <%@ page extends="yourClass" %>
• Defines the parent class that will be inherited by the generated servlet of the current page
• This attribute is less common
• import 
• <%@ page import="java.io.*" %>
• One of the most common attributes used
• Same as we do in java classes/interfaces
• By default, tomcat imports the following packages:
• javax.servlet.* 
• javax.servlet.http.* 
• javax.servlet.jsp.* 
• javax.servlet.jsp.tagext.*
• org.apache.jasper.runtime.*
• buffer 
• <%@ page buffer="none" %>
• <%@ page buffer="20kb" %>
• <%@ page buffer="20" %>
• buffer="20" is the same as "20kb"
• But it's up to the JSP container to use its discretion to increase the buffer in order for the improved performance
• autoFlush 
• <%@ page autoFlush="false" %>
• If true, the JSP container automatically flushes the buffer when it's full.
• If false, the buffer can be flushed manually by the following statement: 
• out.flush()
• session 
• <%@ page session="true" %>
• By default, the session is set "true", meaning the JSP page takes part in session management
• It is wise and strongly advised not to set it true unless you specifically require it because this consumes resources.
• isThreadSafe 
• <%@ page isThreadSafe="true" %>
• True indicates that the simultaneous access to this page is safe
• False indicates that the simultaneous access to this page is not safe. In that case, the JSP container puts multiple requests in a queue and serves them only one at a time.
• errorPage 
• <%@ page errorPage="MyErrorPage.jsp" %>
• Indicates the error page which is displayed if an uncaught exception occurs in the current page
• The error page must have the page attribute isErrorPage="true"
• isErrorPage 
• <%@ page isErrorPage="true" %>
• If true, then only be set as an error page in other JSP pages
• If true, this page has an access to the exception implicit object

2. Include directives

<%@ include file="includes/Footer.html" %>

• As the name suggest, Include Directive enables us to include the contents of other files in the current JSP page.
• The included page can be static HTML page as well as dynamic JSP page.
• Very commonly used to include Header page, Footer page or a page containing import statements and so on.

2. Taglib directives

• The taglib, or tag library, directive can be used to extend JSP functionality. It's a broad topic so it's not covered here at this moment. It will be covered under it's own topic later soon.

SCRIPTING ELEMENTS

Scripting elements allow you to insert Java code in your JSP pages. There are three types of scripting elements:

• Scriptlets
• Declarations
• Expressions 

1. Scriptlets

<% ... %>

• Syntax looks similar to Directives. But don't get confused, Directives starts with <%@ i.e. <%@ ... %>

<%   try {     Class.forName("com.mysql.jdbc.Driver");     System.out.println("JDBC driver loaded");   }   catch (ClassNotFoundException e) {     System.out.println(e.toString());   } %> ... ... <% out.println("You can do anything here"); String str = "some string"; str = str + " string concatenation"; out.println( str ); %>

2. Declarations

<%! ... %>

• Used to declare methods and variables that can be used from any point in the JSP page.
• Can appear anywhere throughout the page. For instance, a method declaration can appear above a page directive that imports a class, even though the class is used in the method.

<%!   String getSystemTime() {     return Calendar.getInstance().getTime().toString();   } %> <%@ page import="java.util.Calendar" %> <%   out.println("Current Time: " + getSystemTime()); %>

3. Expressions 
<%= ... %>

• Expressions get evaluated when the JSP page is requested and their results are converted into a String and fed to the print method of the out implicit object. 
• If the result cannot be converted into a String, an error will be raised at translation time. If this is not detected at translation time, at request-processing time, a ClassCastException will be raised.
• Don't need to add a semicolon at the end of an expression

Below is the two equivalent statements:

<!-- Expression --> <%= java.util.Calendar.getInstance().getTime() %> <!-- Equivalent Scriptlet --> <% out.print(java.util.Calendar.getInstance().getTime()); %>