Logic Present Tag (<logic:equal>...</logic:equal>)
empty tag - If
the requested variable is either null or an empty string then we use this tag to
evaluate the contents contained in the nested body parts of this tag.
Tag evaluation of the nested body content occurs only
if the specified value is either absent (i.e. null), an empty
string (i.e. a java.lang.String with a length of zero), or an empty
java.util.Collection or java.util.Map (tested by
the isEmpty() method on the respective interface).
Attributes of Empty Tag
| Attribute Name |
Description |
| name |
This attribute specifies a JSP bean
variable. This variable is needed for comparison.
|
| property |
The variable to be compared is the property
of the bean. This is specified by the name
attribute. The property reference can be simple, nested, and/or
indexed.
|
| scope |
The bean scope within which to search for the
bean is named by the name property, or "any
scope" if not specified.
|
Logic notEmpty Tag (<logic:notEmpty>...</logic:notEmpty>)
notEmpty
- If the requested variable is neither null nor an empty string then we use
this tag to evaluate the contents contained in the nested body parts of this
tag.
Tag evaluation of the nested body content
occurs only if the specified value is available (i.e.not null)
and is not an empty string (i.e. a java.lang.String with a length
of zero).
Attributes of notEmpty Tag
| Attribute Name |
Description |
| name |
This attribute specifies a JSP bean
variable. This variable is needed for comparison.
|
| property |
The variable to be compared is the property
of the bean. This is specified by the name
attribute. The property reference can be simple, nested, and/or
indexed.
|
| scope |
The bean scope within which to search for the
bean is named by the name property, or "any
scope" if not specified.
|
Example Illustrating the use of the Empty<logic:empty>
and the notEmpty <logic:notEmpty> logic tags.
Here you will learn to use the Struts Logic tags.
We will cover an example that will show a comparison between the two
logic tags (ie..<logic:empty> and the <logic:notEmpty>).
Example code
Creating an Action
Class
Develop a simple action class LogicAction.java.
package roseindia.net;
import java.io.*;
import java.util.*;
/**
* @author Amit Gupta
* @Web http://www.roseindia.net
* @Email struts@roseindia.net
**/
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import org.apache.struts.action.*;
public class LogicAction extends Action
{
public ActionForward execute(ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException {
return mapping.findForward("success");
}
}
|
|
Creating Form Bean
Our form bean class contains only one property text.
Here is the code of FormBean (LogicForm.java)
package roseindia.net;
import org.apache.struts.action.*;
/**
* @author Amit Gupta
* @Web http://www.roseindia.net
* @Email struts@roseindia.net
**/
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class EmptyForm extends ActionForm
{
private String text = "";
public String getText()
{
return text;
}
public void setText(String text)
{
this.text=text;
}
}
|
|
Defining form Bean in struts-config.xml file
Add the following entry in the struts-config.xml file
for defining the form bean
<form-bean name="LogicForm"
type="roseindia.net.LogicForm" /> |
Developing the Action Mapping in the struts-config.xml
Here, Action mapping helps to select the method from the Action class for specific
requests.
<action path="/LogicAction"
type="roseindia.net.LogicAction"
name="LogicForm"
scope="request"
input="/pages/InputLogic.jsp">
<forward name="success" path="/pages/output.jsp"/>
</action> |
Developing the InputLogic.jsp
page
<%@ taglib uri="/tags/struts-html" prefix="html" %>
<%@ taglib uri="/tags/struts-logic" prefix="logic" %>
<%@ taglib uri="/tags/struts-bean" prefix="bean" %>
<html:html>
<head>
<title>Using <logic> Tags</title>
</head>
<body>
<h1>Using <logic> Tags</h1>
<html:form action="/LogicAction" method ="post">
<h2>Enter text :</h2>
<html:text property="text"/>
<html:submit value="Submit"/>
<html:cancel/>
</html:form>
</body>
</html:html>
|
Developing the output.jsp page
<%@ taglib uri="/tags/struts-bean" prefix="bean" %>
<%@ taglib uri="/tags/struts-logic" prefix="logic" %>
<HTML>
<HEAD>
<TITLE>Here's Your Data...</TITLE>
</HEAD>
<BODY>
<H4>Here's Your Data...</H4>
<h4>The text entered is:
<bean:write name="LogicForm1" property="text"/></h4>
<logic:present name="LogicForm1" property="text" >
<h5>Using the tag <logic:present > </h5>
Result: Successfully supported
</logic:present>
<logic:notPresent name="LogicForm1" property="text1" >
<h5>Using the tag<logic:notPresent></h5>
Result: Support for a Variable other than a String is not Present
</logic:notPresent>
</BODY>
</HTML> |
Add the following line in the index.jsp to call the
form.
<li>
<html:link page="/pages/InputLogic.jsp">Struts File Upload</html:link>
<br>
Example demonstrates how LogicAction Class works.
</li>
Building and Testing the Example
To build and deploy the application go to Struts\Strutstutorial
directory and type ant on the command prompt. This will deploy the application.
Open the browser and navigate to the InputLogic.jsp page. Your browser
displays the following page.
Writing nothing to the InputLogic.jsp page
displays the working of the Empty Logic tag <logic:equal >
Writing nothing displays the following out.jsp
page
Now write any data to the InputLogic.jsp to see
the working of the notEqual logic tag <logic:notEqual>
The <logic:notEqual
> evaluated here displays the output.jsp
as
Download example code
|
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