useBean tag

Question

Hi,

I'm really confused in the following two lines of Head First servlets & JSP book of page no. 349:

1. The is a way of declaring and initializing the actual bean object you're using in .

2.Declare and intializea bean attribute with

<jsp:useBean> <jsp:useBean id="person"class="foo.Person" scope="request"/>

In the first line, why they've called an attribute as an object?

Since attribute is name/value pair bound to scope,like request, session.

Answer 1

<jsp:useBean id="person" class="foo.Person" scope="request"/>

In this line, the attribute "person" in the request scope will be an object instance of type "foo.Person".

The Apache Tomcat 6 implementation translates the tag to this Java code:

  foo.Person person = null;
  synchronized (request) {
    person = (foo.Person) _jspx_page_context.getAttribute("person", 
                            PageContext.REQUEST_SCOPE);
    if (person == null){
      person = new foo.Person();
      _jspx_page_context.setAttribute("person", person,
                            PageContext.REQUEST_SCOPE);
    }
  }

_jspx_page_context is an instance of PageContext.

Answer 2

<jsp:useBean id="person"class="foo.Person" scope="request"/>

This calls the default constructor for foo.Person

The id "person" allows you to reference the Bean on your jsp page

<div>   
    <c:out value="${person.name}" />
</div>

The scope is the scope for the Bean foo.Person

JSP syntax reference for useBean has definitions for each scope.

So your JavaBean would look something like this

package foo;

public class Person {

    private String name;

    public Person() {
        this.name = "jack"
    }

    public String getName() {
       return name;
    }

    public void setName(String n) {
        this.name = n;
    }

}

If the Person Bean has already been instantiated in your referenced scope, the useBean will locate and make available the Bean for use in expressions and scriplets on your JSP page.