Actions in JSP

The jsp:setProperty Action

We use jsp:setProperty to give values to properties of beans that have been referenced earlier. We can do this in two contexts. First, we can use jsp:setProperty after, but outside of, a jsp:useBean element, as below:

<jsp:useBean id="myName" ... /> ... <jsp:setProperty name="myName" property="someProperty" ... />

In this case, the jsp:setProperty is executed regardless of whether a new bean was instantiated or an existing bean was found.

A second context in which jsp:setProperty can appear is inside the body of a jsp:useBean element, as below:

<jsp:useBean id="myName" ... > ... <jsp:setProperty name="myName" property="someProperty" ... /> </jsp:useBean>

Here, the jsp:setProperty is executed only if a new object was instantiated, not if an existing one was found.

There are four possible attributes of jsp:setProperty :

Attribute Usage name This required attribute designates the bean whose property will be set. The jsp:useBean element must appear before the jsp:setProperty element. property This required attribute indicates the property you want to set. However, there is one special case: a value of "*" means that all request parameters whose names match bean property names will be passed to the appropriate setter methods. value This optional attribute specifies the value for the property. String values are automatically converted to numbers, boolean, Boolean, byte, Byte, char, and Character via the standard valueOf method in the target or wrapper class. For example, a value of "true" for a boolean or Boolean property will be converted via Boolean.valueOf, and a value of "42" for an int or Integer property will be converted via Integer.valueOf. You can't use both value and param, but it is permissible to use neither. See the discussion of param below. param This optional attribute designates the request parameter from which the property should be derived. If the current request has no such parameter, nothing is done: the system does not pass null to the setter method of the property. Thus, you can let the bean itself supply default values, overriding them only when the request parameters say to do so. For example, the following snippet says "set the numberOfItems property to whatever the value of the numItems request parameter is, if there is such a request parameter. Otherwise don't do anything." <jsp:setProperty name="orderBean" property="numberOfItems" param="numItems" /> If you omit both value and param, it is the same as if you supplied a param name that matches the property name. You can take this idea of automatically using the request property whose name matches the property one step further by supplying a property name of "*" and omitting both value and param. In this case, the server iterates through available properties and request parameters, matching up ones with identical names.