Thursday, December 10, 2009

Introduction to JSTL

1. What is JSTL?

JSTL stands for JSP
Standard Tag Library. JSTL has been standardized and is being one of the most
important technologies in implementing J2EE Web Application. The main objective
of the JSTL is basically to simplify the Java codes within JSP (scriptlets) as
well as to increase the level of reusability within our J2EE web application. Before
JSTL is introduced, J2EE Web Applications (especially in the presentation layer
– JSP) are extremely complex and are very tough to be maintained. It is true
that the new developer may take some time to understand all the underlying
codes within J2EE Web Application This is where JSTL should help.

Here is a simple JSTL flow concept; JSTL is
compiled into a servlets (Java codes) before being displayed to JSP. Some
classes of standard.jar are required to parse and translate these JSTL tags
into servlets (Java codes). Lastly but not least, the servlet that has been
compiled will be executed accordingly.

There are many more advantages
of using JSTL compared to scriptlets. Therefore, it is recommended to replace
scriptlets with JSTL in the presentation layer (JSP).

There
are 5 major types of JSTL tags:
  1. JSTL Core tags,
    prefixed with c
  2. JSTL Format tags,
    prefixed with fmt
  3. JSTL Function
    tags, prefixed with fn
  4. JSTL Database
    tags, prefixed with sql
  5. JSTL XML tags,
    prefixed with x


JSTL Core Tags
<%@ taglib
uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
Mainly used for replacement of scriptlet logical
tags as well as basic URL handling tag such as catch, choose, if, forEach,
param, when, redirect, import, url, etc.


JSTL Format Tags
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt"
prefix="fmt" %>
Mainly used for displaying number and date time format. This could be
used for internationalization support as well. Tags examples are setLocale,
setTimeZone, setBundle, formatNumber, formatDate, etc.


JSTL Function Tags
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions"
prefix="fn" %>
Very useful JSTL tags. Most are used in
conjunction with JSTL core tags. These tags are designed for manipulating
string.


JSTL Database Tags
<%@ taglib uri="http://java.sun.com/jsp/jstl/sql"
prefix="sql" %>
Tags are used to interact with database level. With database tags you
could do transaction, update and query the database from your UI level.
Personally, I do not prefer these tags. The MVC design pattern should always be
retained.


JSTL XML tags
<%@ taglib uri="http://java.sun.com/jsp/jstl/xml" prefix="x"
%>
Similar to core tags, except xml tags will
deal with xml stuffs like parsing xml documents, validating xml documents, output
an xpath and etc.

For depth details to all JSTL tags, you can
find more information within your NetBean’s installation folder i.e. installation_netbeans_folder\enterprise1\docs\

Additionally, JSTL accepts the conditional operators
like ‘eq’, ‘ne’, ’==’, ’null’, ’empty’, ’not’, ’!=’, ’>=’, ’<=’, ’and’, ’&&’,
’or’, ’||’ all are valid.

Here is the mapping of relational and
logical operators with JSP Notations.

Operators

JSP
Notation

>

gt

<

Lt

>=

ge

<=

le

==

eq

!=

ne

&&

and

||

or

!

not

‘’

empty

/

div

%

mod
While other arithmetic operators such as +,
-, and * can also be used together with the JSTL tag as well.


Join my groups to share knowledge at
http://groups.google.com/group/rjetla?hl=en
http://rjetla.blogspot.com/

Thanks & Regards

l¯¯l)¯¯)
l__l\__\aju
.     l¯¯l
(¯¯(/__/etla

"falling down is not a crime, staying down is a crime"





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Monday, December 7, 2009

Beans and Form processing


Forms are a very common method of interactions in web sites.  JSP makes forms processing specially easy.
The standard way of handling forms in JSP is to define a "bean". This is not a full Java bean.  You just need to define a class that has a field corresponding to each field in the form.  The class fields must have "setters" that match the names of the form fields.  For instance, let us modify our GetName.html to also collect email
address and age.

The new version of GetName.html is

<HTML>
<BODY>
<FORM METHOD=POST ACTION="SaveName.jsp">
What's your name? <INPUT TYPE=TEXT NAME=username SIZE=20><BR>
What's your e-mail address? <INPUT TYPE=TEXT NAME=email SIZE=20><BR>
What's your age? <INPUT TYPE=TEXT NAME=age SIZE=4>
<P><INPUT TYPE=SUBMIT>
</FORM>
</BODY>
</HTML>

To collect this data, we define a Java class with fields "username", "email" and "age" and we provide setter methods "setUsername", "setEmail" and "setAge", as shown.  A "setter" method is just a method that starts with "set" followed by the name of the field.  The first character of the field name is upper-cased.
So if the field is "email", its "setter" method will be "setEmail".  Getter methods are defined similarly, with "get" instead of "set".   Note that the setters  (and getters) must be public.

package user;
public class UserData {

    String username;
    String email;
    int age;
 
    public void setUsername( String value )
    {
        username = value;
    }
 
    public void setEmail( String value )
    {
        email = value;
    }
 
    public void setAge( int value )
    {
        age = value;
    }
 
    public String getUsername() { return username; }
 
    public String getEmail() { return email; }
 
    public int getAge() { return age; }

}

The method names must be exactly as shown.  Once you have defined the class, compile it and make sure it is available in the web-server's classpath.  The server may also define special folders where you can
place bean classes, e.g. with Blazix you can place them in the "classes" folder.  If you have to change the classpath, the web-server would need to be stopped and restarted if it is already running.  Note that we are using the package name user, therefore the file UserData.class must be placed in a folder named
user under the classpath entry. Now let us change "SaveName.jsp" to use a bean to collect the
data.

<jsp:useBean id="user" class="user.UserData" scope="session"/>
<jsp:setProperty name="user" property="*"/>
<HTML>
<BODY>
<A HREF="NextPage.jsp">Continue</A>
</BODY>
</HTML>

All we need to do now is to add the jsp:useBean tag and the jsp:setProperty tag!  The useBean tag will look for an instance of the "user.UserData" in the session.  If the instance is already there, it will update the old instance.  Otherwise, it will create a new instance of user.UserData (the instance of the user.UserData is called a bean), and put it in the session. The setProperty tag will automatically collect the input data, match names against the bean method names, and place the data in the bean!

Let us modify NextPage.jsp to retrieve the data from bean..
<jsp:useBean id="user" class="user.UserData" scope="session"/> 
<HTML>
<BODY>
You entered<BR>
Name: <%= user.getUsername() %><BR>
Email: <%= user.getEmail() %><BR>
Age: <%= user.getAge() %><BR>
</BODY>
</HTML>

Notice that the same useBean tag is repeated.  The bean is available as the variable named "user" of class "user.UserData". The data entered by the user is all collected in the bean. We do not actually need the "SaveName.jsp", the target of GetName.html could have been NextPage.jsp, and the data would still be available the same way as long as we added a jsp:setProperty tag. But in the next tutorial, we will actually use SaveName.jsp as an error handler that automatically forwards the request to NextPage.jsp,
or asks the user to correct the erroneous data.

Exercise:  1)  Write a JSP/HTML set that allows a user to enter the name of a system property, and then displays the value returned by System.getProperty for that property name (handle errors appripriately.)
2)  Go back to the exercises where you manually modified boolean variables. Instead of a boolean variable, make these come from a HIDDEN form field that can be set to true or false.

Join my groups to share knowledge at
http://groups.google.com/group/rjetla?hl=en
http://rjetla.blogspot.com


Thanks & Regards


l¯¯l)¯¯)
l__l\__\aju
       l¯¯l
(¯¯(/__/etla
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