JavaServer Pages, 3rd Edition

Unexpected <jsp:setProperty> Behavior

The <jsp:setProperty> action can automatically set all properties in a bean with names matching the names of the parameters received with the request. This is a great feature that’s used in many of the examples in this book, but unless you know how it works behind the scenes, you can be in for a surprise.

When the <jsp:setProperty> code is invoked, it gets a list of all request parameter names and uses bean introspection to find the corresponding property setter methods. It then calls all setter methods to set the properties to the values of the parameters. This means that if you have a property in your bean that doesn’t match a parameter, the setter method for this property is not called. In most cases, this is not surprising. If the parameter is present in some requests but not in others, however, things may get a bit confusing. This is the case with parameters corresponding to checkbox, radio button, and selection list elements in an HTML form. If this type of element is selected, the browser sends a parameter with the element’s name and the value of the selected item. If the element is not selected, it doesn’t send a parameter at all.

For example, let’s say you have a bean with an indexed property, such as the projects property in the com.ora.jsp.beans.emp.EmployeeBean used in Chapter 13. This bean is kept in the session scope. The user can change the value of the property through a group of checkboxes in a form. To unregister all projects, ...