Recipe3.12.Handling Unchecked Checkboxes

Recipe 3.12. Handling Unchecked Checkboxes

Problem

You need to ensure that a Boolean ActionForm property, corresponding to an HTML checkbox, is set to false when the checkbox is unchecked.

Solution

Create a checkbox input field that uses JavaScript to set the value of a hidden Boolean field. Use the logic:equal tag to set the checked property of the checkbox if the value for the hidden field is true. The JSP page (checkbox_test.jsp) in Example 3-18 uses this approach to guarantee a true or false value is always submitted.

Example 3-18. Guaranteeing checkbox settings

<%@ page contentType="text/html;charset=UTF-8"  language="java" %> <%@ taglib uri="http://jakarta.apache.org/struts/tags-html"  prefix="html" %> <%@ taglib uri="http://jakarta.apache.org/struts/tags-logic"  prefix="logic" %> <html> <head>   <title>Struts Cookbook - Chapter 4 : Checkbox Test</title> </head> <body>   <html:form method="get" action="/ProcessCheckbox">     <input type="checkbox" name="foo_"       onclick="javascript:elements['foo'].value=this.checked;"       <logic:equal name="CheckboxForm" property="foo" value="true">          checked        </logic:equal>     >     <html:hidden property="foo"/>     <html:submit/>   </html:form> </body> </html>

Discussion

For such a common little field, the HTML checkbox can cause trouble. If a checkbox is unchecked and the form is submitted, no value for that field will be sent in the request. Suppose you have a form with one checkbox on it:

<html:form method="get" action="ProcessFoo">   <html:checkbox property="foo"/>   <html:submit/> </html:form>

If the checkbox is checked, then the resultant request URL looks something like this:

http://localhost/jsc-ch04/ProcessFoo?foo=on

When processed by Struts, your ActionForm is populated by BeanUtils.populate( ) method. If foo is a boolean property, its value is set to true.

The problem occurs when you uncheck the checkbox with the intention of setting the property value to false. If the checkbox is unchecked, the resultant URL looks something like this:

http://localhost/jsc-ch04/ProcessFoo?

Where did the property value go? One would expect the request query string to contain "foo=off" or "foo=". Unfortunately, no request parameter is generated for an unchecked checkbox. When BeanUtils.populate( ) is called, it doesn't know to set the property value.

This problem can usually be handled by implementing the reset() method in your ActionForm. The Struts request processor calls this method before the ActionForm is populated. The method gives you a chance to set the form properties to desired default values. If the HTTP request doesn't contain a name/value pair for a property, then the property retains the value set in the reset( ) method. For checkboxes, set the value to false, as shown here:

public void reset( ActionMapping mapping,                     HttpServletRequest request )  {     foo = false; }

However, the reset( ) method can't always solve the problem. If you're using a form in session scope in a wizard-style interface, then implementing reset() will clear the form when you don't want it to. You need an alternative that guarantees that a value will be sent when the form is submitted. The Solution does that by implementing two fields on the form. The actual form field corresponding to the Boolean ActionForm property isn't the checkbox, but is a hidden field generated using the html:hidden tag. The checkbox is then created using normal HTML (input type="checkbox"). A JavaScript onclick event handler is defined for this control:

javascript:elements['foo'].value=this.checked;

When a user clicks the checkbox, the value of the hidden field is set. If the checkbox is checked, the value is set to true; otherwise, it is set to false. To ensure that the checkbox renders correctly when the form is initially displayed, the logic:equal tag is used to render the checked attribute for the field.

Recipe3.12.Handling Unchecked Checkboxes