Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.

13.1 Basic Bean Use

289

This usually means  instantiate an object of the class specified by 

Class

,

and bind it to a variable with the name specified by 

id

.  So, for example, the

JSP action

can normally be thought of as equivalent to the scriptlet

<% coreservlets.Book book1 = new coreservlets.Book(); %>

Although it is convenient to think of 

jsp:useBean

 as being equivalent to

building an object, 

jsp:useBean

 has additional options that make it more

powerful. As we'll see in Section 13.4 (Sharing Beans), you can specify a

scope

 attribute that makes the bean associated with more than just the cur 

rent page. If beans can be shared, it is useful to obtain references to existing

beans, so the 

jsp:useBean

 action specifies that a new object is instantiated

only if there is no existing one with the same 

id

 and 

scope

. 

Rather than using the 

class

 attribute, you are permitted to use 

beanName

instead. The difference is that 

beanName

 can refer either to a class or to a file

containing a serialized bean object. The value of the 

beanName

 attribute is

passed to the 

instantiate

 method of 

java.beans.Bean

.

In most cases, you want the local variable to have the same type as the

object being created. In a few cases, however, you might want the variable to

be declared to have a type that is a superclass of the actual bean type or is an

interface that the bean implements. Use the 

type

 attribute to control this, as

in the following example:

This use results in code similar to the following being inserted into the

_jspService

 method:

Runnable thread1 = new MyClass();

Note that since 

jsp:useBean

 uses XML syntax, the format differs in three

ways from HTML syntax: the attribute names are case sensitive, either single

or double quotes can be used (but one or the other must be used), and the

end of the tag is marked with 

/>

, not just 

>

. The first two syntactic differ 

ences apply to all JSP elements that look like 

jsp:xxx

. The third difference

applies unless the element is a container with a separate start and end tag. 

Core Warning

Syntax for 

jsp:xxx

 elements differs in three ways from HTML syntax: 

attribute names are case sensitive, you must enclose the value in single or 

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.