Change Background color using Custom Tag

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Change Background color using Custom Tag
Posted on: June 29, 2010 at 12:00 AM

In this program, We will create a Custom Tag which changes the background color.

Change Background color using Custom Tag

In this program, We will create a Custom Tag which changes the background color.

JSP Custom Tags :

Provide a mechanism to a Web programmer to reuse and encapsulate complex recurring code in a JSP application.
Provide simplicity and reusability of Java code.
Enable you to perform various functions, such as:

Accessing all implicit variables of a JSP page, such as request, response, in, and out.
Modifying the response generated by a calling JSP page.
Initializing and instantiating a JavaBean component.

Types of Custom Tags

The various types of custom tags that you can develop in JSP are :

Empty tags: Refer to the custom tags that do not have any attribute or body. The following code snippet shows an empty custom tag:

<td:welcome />

Tags with attributes: Refer to custom tags for which you can define attributes to customize the behavior of the custom tag. The following code snippet shows a custom tag with an attribute color:

<td: welcome color=?blue?></td:welcome>

Tags with a body: Refer to the custom tag within which you can define nested custom tags, scripting elements, actions, HTML text, and JSP directives. The following code snippet shows a custom tag that contains a JSP scripting element as its body:

<td: welcome>


 <%=today_date%>

</td:welcome>

Nested tags: Refer to the set of custom tags in which one custom tag encloses one or more custom tags. The following code snippet shows a nested custom tag:

<td1:ifTag condition ?<%=eval>? >


     <td2:valueTrue>
       The expression evaluates to true
     </td2:valueTrue>

</td1:ifTag>

Development of Custom Tag :

To develop a custom tag, you need to perform following steps:

Develop a tag handler or class file(.java)
Develop the Tag Library Descriptor (TLD) file
Include the Tag Library in a JSP page
Deploy the application

EXAMPLE : In this Example, We will create a Custom Tag which changes the background color .When you enter your Name in Html form & select color -color changes according to your choice, this background color change is due to the custom tag, which we have created with the help of tag handler class :-

Tag Handler File : Build a class that implements the javax.servlet.jsp.tagext tag interface as follows. Compile it and place it under the web-inf/classes directory (in the appropriate package structure).

HelloTag.java


package hello;

/* Import packages that are required for implementing custom tag */

import java.io.*;

import javax.servlet.jsp.*;

import javax.servlet.jsp.tagext.*;

import java.util.GregorianCalendar;

/* Extending the BodyTagSupport interface */

public class HelloTag extends BodyTagSupport

{

    private String name;

    private PageContext pageContext;

    private Tag parent;

    String color;

  /* Declaring the constructor for the class */

  public HelloTag()

  {

    super();

  }

  /* Initializing the name */

  public void setName(String name)

  {

        this.name = name;

  }

  /* Initializing the color */

  public void setColor(String color)

  {

    this.color = color;

  }

  public int doAfterBody() throws JspException

  {

    try

    {

      /* Creating an instance of the BodyContent class */

      // Get the bodycontent as string

		      BodyContent bc = getBodyContent();

      // GetJspWriter to output content 

      String body = bc.getString();   

      /* Creating an instance of the JspWriter class */

      JspWriter out = bc.getEnclosingWriter();

      String dt;//Declaring the variable for system date

      /* Creating an instance of the GregorianCalendar class */

      GregorianCalendar now = new GregorianCalendar();

      /* Invoking the getTime() method */

      dt = now.getTime().toString();

      /* Finding the substring of the date type variable dt */

      String dt1 = dt.substring(11,16);

      /* Setting the color as selected by the user */

      out.print("<body bgcolor="+color+">");

      /* Displaying the custom message to the user according to 

         the system time */

      if(body != null)

        out.print("<CENTER>Hi"+body+"! </CENTER>");

      if(dt1.compareTo("12.00")<0)

        out.print("<CENTER>Good Morning to You.</CENTER>");

      if((dt1.compareTo("12.00")>0) && (dt1.compareTo("16.00")<0))

        out.print("<CENTER>Good Afternoon to You.</CENTER>");

      if((dt1.compareTo("16.00")>0) && (dt1.compareTo("24.00")<0))

        out.print("<CENTER>Good Evening to You.</CENTER>");

      out.print("<BR>");

      out.print("<CENTER>Welcome to ABC Inc.</CENTER>");

      out.print("<BR>");

      out.print("<CENTER>The current time: "+dt1+"</CENTER>");

      out.print("</body>");

    }

    catch(IOException ioe)

    {

             throw new JspException("Error:"+ioe.getMessage());

    }

    return SKIP_BODY;

  }

}

Tag Library Descriptor (TLD) file : Now we need to describe the tag, so create a file called Mytaglib.tld and place it under the web-inf directory.

<taglib>
<tlib-version>1.0</tlib-version>
<jsp-version>1.2</jsp-version>
<short-name>first</short-name>

<uri>/Mytaglib.tld</uri>

<info>A simple tag library for the examples</info>
<tag>
<name>hello</name>
<tag-class>hello.HelloTag</tag-class>
<body-content>JSP</body-content>
<attribute>
<name>color</name>
<rtexprvalue>true</rtexprvalue>
<required>true</required>
</attribute>
</tag>
</taglib>

HTML File :

AcceptInput.html

<html>
<body>
<h2>Select a color of your choice </h2>
<font size=4 face="Verdana" color=#112244>
<form method="post" action="helloCustomTag.jsp">
<br>
<input type="radio" name="r1" value="G" checked="false">Green
<br>
<input type="radio" name="r1" value="Y" checked="false">Yellow
<br>
<input type="radio" name="r1" value="R" checked="false">Pink
<br>
<input type="radio" name="r1" value="B" checked="false">Blue
<br>
<p>Enter Name <input type="type" name="uname">
<br>
<br>
<input type="Submit" value= "Submit" name="b1">
</p>
</form>
</body>
</html>

JSP File :

helloCustomTag.jsp


<%-- Providing the location of the Mytaglib directive --%>

<%@ taglib uri="/Mytaglib.tld" prefix="first" %>

<HTML>

<HEAD>

<TITLE>Customized Tag</TITLE>

</HEAD>

<%

  /* Setting the color parameter selected by the user */

  String str = request.getParameter("r1");

  /* Comparing the color and then invoking the custom

     tag with attribute color and body */

  if(str.compareTo("G")==0)

  {

%>  

    <first:hello color="LIGHTGREEN">

      <%=request.getParameter("uname")%>

    </first:hello>

<%    

  }  

  if(str.compareTo("R")==0)

  {

%>

    <first:hello color="RED">

      <%=request.getParameter("uname")%>

    </first:hello>



<%

  }

  if(str.compareTo("Y")==0)

  {

%>

    <first:hello color="LIGHTYELLOW">

      <%=request.getParameter("uname")%>

    </first:hello>

<%  

  }

  if(str.compareTo("B")==0)

  {

%>

    <first:hello color="LIGHTBLUE">

      <%=request.getParameter("uname")%>

    </first:hello>

<%  

  }

%>

<BR>

<B>

</BODY>

</HTML>

Deployment of application (OUTPUT) :

After Submitting the values :

Download Source Code

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