Quick learn JSP (Java Server Pages)

What is JavaServer Pages?

JavaServer Pages (JSP) is a technology for developing web pages that support dynamic content which helps developers insert java code in HTML pages by making use of special JSP tags, most of which start with <% and end with %>.
A JavaServer Pages component is a type of Java servlet that is designed to fulfill the role of a user interface for a Java web application. Web developers write JSPs as text files that combine HTML or XHTML code, XML elements, and embedded JSP actions and commands.
Using JSP, you can collect input from users through web page forms, present records from a database or another source, and create web pages dynamically.
JSP tags can be used for a variety of purposes, such as retrieving information from a database or registering user preferences, accessing JavaBeans components, passing control between pages and sharing information between requests, pages etc.

Why Use JSP?

JavaServer Pages often serve the same purpose as programs implemented using the Common Gateway Interface (CGI). But JSP offer several advantages in comparison with the CGI.

• Performance is significantly better because JSP allows embedding Dynamic Elements in HTML Pages itself instead of having a separate CGI files.
• JSP are always compiled before it's processed by the server unlike CGI/Perl which requires the server to load an interpreter and the target script each time the page is requested.
• JavaServer Pages are built on top of the Java Servlets API, so like Servlets, JSP also has access to all the powerful Enterprise Java APIs, including JDBC, JNDI, EJB, JAXP etc.
• JSP pages can be used in combination with servlets that handle the business logic, the model supported by Java servlet template engines.
  
   Finally, JSP is an integral part of J2EE, a complete platform for enterprise class applications. This means that JSP can play a part in the simplest applications to the most complex and demanding.
  
  

  Setting up JSP Environment

  This step involves downloading an implementation of the Java Software Development Kit (SDK) and setting up PATH environment variable appropriately.
  You can downloaded SDK from Oracle's Java site: Java SE Downloads.
  Once you download your Java implementation, follow the given instructions to install and configure the setup. Finally set PATH and JAVA_HOME environment variables to refer to the directory that contains java and javac, typically java_install_dir/bin and java_install_dir respectively.
  If you are running Windows and installed the SDK in C:\jdk1.5.0_20, you would put the following line in your C:\autoexec.bat file

set PATH=C:\jdk1.5.0_20\bin;%PATH%
set JAVA_HOME=C:\jdk1.5.0_20

Alternatively, on Windows NT/2000/XP, you could also right-click on My Computer, select Properties, then Advanced, then Environment Variables. Then, you would update the PATH value and press the OK button.
On Unix (Solaris, Linux, etc.), if the SDK is installed in /usr/local/jdk1.5.0_20 and you use the C shell, you would put the following into your .cshrc file.

setenv PATH /usr/local/jdk1.5.0_20/bin:$PATH
setenv JAVA_HOME /usr/local/jdk1.5.0_20

Alternatively, if you use an Integrated Development Environment (IDE) like Borland JBuilder, Eclipse, IntelliJ IDEA, or Sun ONE Studio, compile and run a simple program to confirm that the IDE knows where you installed Java.

Setting up Web Server: Tomcat

A number of Web Servers that support JavaServer Pages and Servlets development are available in the market. Some web servers are freely downloadable and Tomcat is one of them.
Apache Tomcat is an open source software implementation of the JavaServer Pages and Servlet technologies and can act as a standalone server for testing JSP and Servlets and can be integrated with the Apache Web Server. Here are the steps to setup Tomcat on your machine:

• Download latest version of Tomcat from http://tomcat.apache.org/.
• Once you downloaded the installation, unpack the binary distribution into a convenient location. For example in C:\apache-tomcat-5.5.29 on windows, or /usr/local/apache-tomcat-5.5.29 on Linux/Unix and create CATALINA_HOME environment variable pointing to these locations.

Tomcat can be started by executing the following commands on windows machine

%CATALINA_HOME%\bin\startup.bat
 
 or
 
 C:\apache-tomcat-5.5.29\bin\startup.bat

 Tomcat can be started by executing the following commands on Unix (Solaris, Linux, etc.) machine:

$CATALINA_HOME/bin/startup.sh
 
or
 
/usr/local/apache-tomcat-5.5.29/bin/startup.sh

 After a successful startup, the default web applications included with Tomcat will be available by visiting http://localhost:8080/.

Further information about configuring and running Tomcat can be found in the documentation included here, as well as on the Tomcat web site: http://tomcat.apache.org

JSP Processing:

The following steps explain how the web server creates the web page using JSP:

• As with a normal page, your browser sends an HTTP request to the web server.
• The web server recognizes that the HTTP request is for a JSP page and forwards it to a JSP engine. This is done by using the URL or JSP page which ends with .jsp instead of .html.
• The JSP engine loads the JSP page from disk and converts it into a servlet content. This conversion is very simple in which all template text is converted to println( ) statements and all JSP elements are converted to Java code that implements the corresponding dynamic behavior of the page.
• The JSP engine compiles the servlet into an executable class and forwards the original request to a servlet engine.
• A part of the web server called the servlet engine loads the Servlet class and executes it. During execution, the servlet produces an output in HTML format, which the servlet engine passes to the web server inside an HTTP response.
• The web server forwards the HTTP response to your browser in terms of static HTML content.
• Finally web browser handles the dynamically generated HTML page inside the HTTP response exactly as if it were a static page.

All the above mentioned steps can be shown below in the following diagram:

The Scriptlet:

A scriptlet can contain any number of JAVA language statements, variable or method declarations, or expressions that are valid in the page scripting language.
Following is the syntax of Scriptlet:

<% code fragment %> 

You can write XML equivalent of the above syntax as follows:

<jsp:scriptlet>
   code fragment
</jsp:scriptlet>

 Any text, HTML tags, or JSP elements you write must be outside the scriptlet. Following is the simple and first example for JSP:

NOTE: Assuming that Apache Tomcat is installed in C:\apache-tomcat-7.0.2 and your environment is setup as per environment setup tutorial.
Let us keep above code in JSP file hello.jsp and put this file in C:\apache-tomcat-7.0.2\webapps\ROOT directory and try to browse it by giving URL http://localhost:8080/hello.jsp. This would generate following result:

JSP Declarations:

A declaration declares one or more variables or methods that you can use in Java code later in the JSP file. You must declare the variable or method before you use it in the JSP file.
Following is the syntax of JSP Declarations:

<%! declaration; [ declaration; ]+ ... %>

 You can write XML equivalent of the above syntax as follows:

<jsp:declaration>
   code fragment
</jsp:declaration>

Following is the simple example for JSP Declarations:

<%! int i = 0; %> 
<%! int a, b, c; %> 
<%! Circle a = new Circle(2.0); %> 

JSP Expression:

A JSP expression element contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file.
Because the value of an expression is converted to a String, you can use an expression within a line of text, whether or not it is tagged with HTML, in a JSP file.
The expression element can contain any expression that is valid according to the Java Language Specification but you cannot use a semicolon to end an expression.
Following is the syntax of JSP Expression:

<%= expression %>

You can write XML equivalent of the above syntax as follows:

<jsp:expression>
   expression
</jsp:expression>

Following is the simple example for JSP Expression:

<html> 
<head><title>A Comment Test</title></head> 
<body>
<p>
   Today's date: <%= (new java.util.Date()).toLocaleString()%>
</p>
</body> 
</html> 

This would generate following result:

Today's date: 19-june-2014 21:24:25