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JSP - Custom Tags

2015-09-01 08:20 453 查看
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 let you create new tags that you can insert directly into a JavaServer Page just The JSP 2.0 specification introduced Simple Tag Handlers for writing these custom tags.

To write a customer tab 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. So let us create 
HelloTag
class as follows:

package com.tutorialspoint;

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!");
}
}


Above code has simple coding where 
doTag()
method takes the current 
JspContext
object using 
getJspContext()
method and uses it to send “
Hello Custom Tag!
” to the current 
JspWriter
object.

Let us compile above class and copy it in a directory available in environment variable 
CLASSPATH
. Finally create 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.tutorialspoint.HelloTag</tag-class>
<body-content>empty</body-content>
</tag>
</taglib>


Now it’s time to use 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>


Try to call above JSP and this should produce 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 following changes in above our tag code to process the body of the tag:

package com.tutorialspoint;

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());
}

}


In this case, the output resulting from the invocation is first captured into a 
StringWriter
before being written to the 
JspWriter
associated with the tag. Now accordingly 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.tutorialspoint.HelloTag</tag-class>
<body-content>scriptless</body-content>
</tag>
</taglib>


Now let us call 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>


This will produce 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 setter methods, identical to JavaBean setter methods as shown below:

package com.tutorialspoint;

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()
. Now let us add this attribute in TLD file using 
<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.tutorialspoint.HelloTag</tag-class>
<body-content>scriptless</body-content>
<attribute>
<name>message</name>
</attribute>
</tag>
</taglib>


Now let us try following 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


Hope above example makes sense for you. It would be worth to note that you can include following properties for an attribute:

PropertyPurpose
nameThe name element defines the name of an attribute. Each attribute name must be unique for a particular tag.
requiredThis specifies if this attribute is required or optional. It would be false for optional.
rtexprvalueDeclares if a runtime expression value for a tag attribute is valid
typeDefines the Java class-type of this attribute. By default it is assumed as String
descriptionInformational description can be provided.
fragmentDeclares 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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