useBean doubt

[Venkata Kumar]

Hello All,

I am reading the book Head First Servlets and Jsp. I came across the following useBean question in "scripless jsp" chapter.

jsp looks like (only relavant code is given)

<jsp:useBean id="person" type="foo.Employee" >
   <jsp:setProperty name="person" property="name" />
</jsp:useBean>
Name is : <jsp:getProperty name="person" property="name"/> 

foo.Person is abstract class
1) what happens if the servlet code looks like this

foo.Person p = new foo.Employee();
p.setName("evan");
request.setAttribute("person",p);

given answer: fails at request time
2) what happens if the servlet code looks like

foo.Employee p =  new foo.Employee();
p.setName("evan");
request.setAttribute("person",p);

given answer: sucess and prints "name is evan"
i don't understand the 2nd answer. The person attribute has request scope. As useBean didn't mention any scope page scope is considered as default.
Then 2nd scenario also should fail. Am i right?
In the 1st case object is of type Person. It won't match with the type attribute(foo.Employee) of useBean tag. Suppose if scopes match will this work.


Thanks,
Venkat

[Chinmaya Chowdary]

Hi Venkat.

Then 2nd scenario also should fail. Am i right?

Right. It should fail.

In the 1st case object is of type Person. It won't match with the type attribute(foo.Employee) of useBean tag. Suppose if scopes match will this work.

Yes, if scopes match this will work.

[Venkata Kumar]

Thanks Chinmaya.

As per my understanding the use of type field is that bean is of given type and we can call functions declared in the given type class

Please see the scwcd faq link http://faq.javaranch.com/java/JspParameterStandardActions

The bean "per" is of type bean.Person. How can we print a attribute of bean.Employee. Then what is the use of type attribute in jsp:usebean.


Thanks,
Venkat

[Chinmaya Chowdary]

Hi Venkat.

Then what is the use of type attribute in jsp:usebean.

'type' is used if the corresponding bean object already available and it was set in particular scope. If our jsp invokes

<jsp:useBean id="person" type="foo.Employee" />

, container first sees is the bean is available or not? Like

foo.Employee person=(foo.Employee)pageContext.getAttribute("person",person);

if it finds, it will use that, if not it will not create another bean object and body will not be executed(if present).

[Christophe Verré]

You mean

foo.Employee person=(foo.Employee)pageContext.getAttribute("person");  

[Chinmaya Chowdary]

Hi Christophe. I am sorry, instead of this

foo.Employee person=(foo.Employee)pageContext.getAttribute("person");  

in Tomcat, it is

person=(foo.Employee) _jspx_page_context.getAttribute("person", PageContext.PAGE_SCOPE);

[Venkata Kumar]

I am confused about inheritance effect on type field. I want to know which getter/setter methods are called if we put a bean object in request scope and mention only type (super class of actual bean object) in <jsp:useBean> tag

Suppose see this example. I took the code from http://faq.javaranch.com/java/JspParameterStandardActions and put it different way.

public class Person {
    private String name;

    public Person() {
        super();
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

public class Employee extends Person {
    private String empID;

    public Employee() {
        super();
    }
    public String getEmpID() {
        return empID;
    }
   public void setEmpID(String id) {
        empID = id;
    }
}

In the servlet code i create a bean object like this

bean.Employee  person = new bean.Employee();
//populate person data
request.setAttribute("person",person);

jsp code looks like

<jsp:useBean id="person" type="bean.Person" scope = "request">
  <jsp:setProperty name="person" property = "*"/>
</jsp:useBean>

Person Name:<jsp:getProperty name="person" property="name"/>
Employee ID:<jsp:getProperty name="person" property="empID"/>

Here Class attribute is not mentioned and person bean is available in request scope so container won't execute the usebean body.
Container looks for the person bean object like this

bean.Person person=(bean.Person)pageContext.getAttribute("person",person);  

now can we get the property empID of bean.Employee class. will this work

Employee ID:<jsp:getProperty name="person" property="empID"/>

As person is of type bean.Person can we access the properties of bean.Employee class in jsp?

[Christophe Verré]

This has been extensively discussed in the past.
Here is one.

[Venkata Kumar]

Thanks Christophe for providing the link. That is useful to clarify my doubts and bring peace to my mind.