Setting JavaBeans Component Properties

Setting JavaBeans Component Properties

There are two ways to set JavaBeans component properties in a JSP page:

• With the jsp:setProperty element

• With a scriptlet: <% beanName .setPropName (value ); %>

The syntax of the jsp:setProperty element depends on the source of the property value. Figure summarizes the various ways to set a property of a JavaBeans component using the jsp:setProperty element.

A property set from a constant string or request parameter must have a type listed in Figure. Since both a constant and request parameter are strings, the Web container automatically converts the value to the property's type; the conversion applied is shown in the table. String values can be used to assign values to a property that has a PropertyEditor class. When that is the case, the setAsText(String) method is used. A conversion failure arises if the method throws an IllegalArgumentException. The value assigned to an indexed property must be an array, and the rules just described apply to the elements.

You would use a runtime expression to set the value of a property whose type is a compound Java programming language type. Recall from Expressions (page 415) that a JSP expression is used to insert the value of a scripting language expression, converted into a String, into the stream returned to the client. When used within a setProperty element, an expression simply returns its value; no automatic conversion is performed. As a consequence, the type returned from an expression must match or be castable to the type of the property.

The Duke's Bookstore application demonstrates how to use the setPropertyelement and a scriptlet to set the current book for the database helper bean. For example, bookstore3/bookdetails.jsp uses the form:

<jsp:setProperty name="bookDB" property="bookId"/>

while bookstore2/bookdetails.jsp uses the form:

<% bookDB.setBookId(bookId); %>

The following fragments from the page bookstore3/showcart.jsp illustrate how to initialize a currency bean with a Locale object and amount determined by evaluating request-time expressions. Because the first initialization is nested in a useBean element, it is only executed when the bean is created.

<jsp:useBean id="currency" class="util.Currency" 
    scope="session"> 
    <jsp:setProperty name="currency" property="locale" 
        value="<%= request.getLocale() %>"/> 
</jsp:useBean> 
<jsp:setProperty name="currency" property="amount" 
    value="<%=cart.getTotal()%>"/>