JSP - Custom Tags

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In this chapter, we will discuss the Custom Tags in JSP. A custom tag is a user-defined JSP language element. When a JSP page containing a custom tag is translated into a servlet, the tag is converted to operations on an object called a tag handler. The Web container then invokes those operations when the JSP page's servlet is executed.

JSP tag extensions lets you create new tags that you can insert directly into a JavaServer Page. The JSP 2.0 specification introduced the Simple Tag Handlers for writing these custom tags.

To write a custom tag, you can simply extend SimpleTagSupport class and override the doTag() method, where you can place your code to generate content for the tag.

Create "Hello" Tag

Consider you want to define a custom tag named <ex:Hello> and you want to use it in the following fashion without a body −

<ex:Hello />

To create a custom JSP tag, you must first create a Java class that acts as a tag handler. Let us now create the HelloTag class as follows −

package com.howcodex;

import javax.servlet.jsp.tagext.*;
import javax.servlet.jsp.*;
import java.io.*;

public class HelloTag extends SimpleTagSupport {
   public void doTag() throws JspException, IOException {
      JspWriter out = getJspContext().getOut();
      out.println("Hello Custom Tag!");
   }
}

The above code has simple coding where the doTag() method takes the current JspContext object using the getJspContext() method and uses it to send "Hello Custom Tag!" to the current JspWriter object

Let us compile the above class and copy it in a directory available in the environment variable CLASSPATH. Finally, create the following tag library file: <Tomcat-Installation-Directory>webapps\ROOT\WEB-INF\custom.tld.

<taglib>
   <tlib-version>1.0</tlib-version>
   <jsp-version>2.0</jsp-version>
   <short-name>Example TLD</short-name>
   
   <tag>
      <name>Hello</name>
      <tag-class>com.howcodex.HelloTag</tag-class>
      <body-content>empty</body-content>
   </tag>
</taglib>

Let us now use the above defined custom tag Hello in our JSP program as follows −

<%@ taglib prefix = "ex" uri = "WEB-INF/custom.tld"%>

<html>
   <head>
      <title>A sample custom tag</title>
   </head>
   
   <body>
      <ex:Hello/>
   </body>
</html>

Call the above JSP and this should produce the following result −

Hello Custom Tag!

Accessing the Tag Body

You can include a message in the body of the tag as you have seen with standard tags. Consider you want to define a custom tag named <ex:Hello> and you want to use it in the following fashion with a body −

<ex:Hello>
   This is message body
</ex:Hello>

Let us make the following changes in the above tag code to process the body of the tag −

package com.howcodex;

import javax.servlet.jsp.tagext.*;
import javax.servlet.jsp.*;
import java.io.*;

public class HelloTag extends SimpleTagSupport {
   StringWriter sw = new StringWriter();
   public void doTag()
   
   throws JspException, IOException {
      getJspBody().invoke(sw);
      getJspContext().getOut().println(sw.toString());
   }
}

Here, the output resulting from the invocation is first captured into a StringWriter before being written to the JspWriter associated with the tag. We need to change TLD file as follows −

<taglib>
   <tlib-version>1.0</tlib-version>
   <jsp-version>2.0</jsp-version>
   <short-name>Example TLD with Body</short-name>
   
   <tag>
      <name>Hello</name>
      <tag-class>com.howcodex.HelloTag</tag-class>
      <body-content>scriptless</body-content>
   </tag>
</taglib>

Let us now call the above tag with proper body as follows −

<%@ taglib prefix = "ex" uri = "WEB-INF/custom.tld"%>

<html>
   <head>
      <title>A sample custom tag</title>
   </head>
   
   <body>
      <ex:Hello>
         This is message body
      </ex:Hello>
   </body>
</html>

You will receive the following result −

This is message body

Custom Tag Attributes

You can use various attributes along with your custom tags. To accept an attribute value, a custom tag class needs to implement the setter methods, identical to the JavaBean setter methods as shown below −

package com.howcodex;

import javax.servlet.jsp.tagext.*;
import javax.servlet.jsp.*;
import java.io.*;

public class HelloTag extends SimpleTagSupport {
   private String message;

   public void setMessage(String msg) {
      this.message = msg;
   }
   StringWriter sw = new StringWriter();
   public void doTag()
   
   throws JspException, IOException {
      if (message != null) {
         /* Use message from attribute */
         JspWriter out = getJspContext().getOut();
         out.println( message );
      } else {
         /* use message from the body */
         getJspBody().invoke(sw);
         getJspContext().getOut().println(sw.toString());
      }
   }
}

The attribute's name is "message", so the setter method is setMessage(). Let us now add this attribute in the TLD file using the <attribute> element as follows −

<taglib>
   <tlib-version>1.0</tlib-version>
   <jsp-version>2.0</jsp-version>
   <short-name>Example TLD with Body</short-name>
   
   <tag>
      <name>Hello</name>
      <tag-class>com.howcodex.HelloTag</tag-class>
      <body-content>scriptless</body-content>
      
      <attribute>
         <name>message</name>
      </attribute>
   
   </tag>
</taglib>

Let us follow JSP with message attribute as follows −

<%@ taglib prefix = "ex" uri = "WEB-INF/custom.tld"%>

<html>
   <head>
      <title>A sample custom tag</title>
   </head>
   
   <body>
      <ex:Hello message = "This is custom tag" />
   </body>
</html>

This will produce following result −

This is custom tag

Consider including the following properties for an attribute −

S.No. Property & Purpose
1

name

The name element defines the name of an attribute. Each attribute name must be unique for a particular tag.
2

required

This specifies if this attribute is required or is an optional one. It would be false for optional.
3

rtexprvalue

Declares if a runtime expression value for a tag attribute is valid
4

type

Defines the Java class-type of this attribute. By default it is assumed as String
5

description

Informational description can be provided.
6

fragment

Declares if this attribute value should be treated as a JspFragment.

Following is the example to specify properties related to an attribute −

.....
   <attribute>
      <name>attribute_name</name>
      <required>false</required>
      <type>java.util.Date</type>
      <fragment>false</fragment>
   </attribute>
.....

If you are using two attributes, then you can modify your TLD as follows −

.....
   <attribute>
      <name>attribute_name1</name>
      <required>false</required>
      <type>java.util.Boolean</type>
      <fragment>false</fragment>
   </attribute>
   
   <attribute>
      <name>attribute_name2</name>
      <required>true</required>
      <type>java.util.Date</type>
   </attribute>
.....
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