Doubts in Standard Actions

[Micheal John]

I have a doubt in <jsp:useBean> standard action.

How the <jsp:useBean> will create a bean..

In HFSJ states that:

If the <jsp:useBean>? can't find an attribute object named "person", it can make one!

It checks for a bean based on the values of the id and scope in the tag, and if it doesn't get one, it makes an instance of the class specified in the class, assigns the object to the id variable, then sets it as an attribute in the scope you defined in the tag.

I don't understand the above two quotes..can anyone explain it..?

Example:

I have a bean class Person:

package com.example.model;
 
public class Person{
private String empname;
 
public void setEmpname(String empname){
this.empname = empname;
}
 
public String getEmpname(){
return empname;
}
 
}

And in the JSP:

<jsp:useBean id="person" class="com.example.model.Person" scope="request"/>
Emp Name: <jsp:getProperty name="person" property="empname"/>

Whether the quotes states that, if I am not having the Person bean class, the <jsp:useBean> will create a new bean class with getter and setter..?

<jsp:useBean id="person" class="com.example.model.Person" scope="request"/> is eqivalent to request.setAttribute("person",p);

So <jsp:useBean> is set the attribute person...

person = (com.example.model.Person)_jspx_page_context.getAttribute("person",PageContext.REQUEST_SCOPE");

The abive thing is retrieving the attribute

Then how comes when retrieving the attribute, it will be null..?

Pls explain..? some where I have confused a lot..

[Tha�s Manfrim Firmino]

You wrote so many things that I'm confused. Anyway, have you already studied taglibs? If yes ( and I hope so):
<jsp:useBean id="person" class="com.example.model.Person" scope="request"/>
will do something like:
Person person = (Person)request.getAttribute("person");
if (person==null)//doesn't exists
person = new Person();

Does it answer your question?

[Sayak Banerjee]

Its very simple....don't get confused.....if there is no attribute(we're talkin' about a bean set as an attribute) by the name specified by id="...." in the jsp:useBean tag (the container finds it out if the getAttribute() method returns null) then a new bean(the same class as class="...." in the jsp:useBean tag) is instantiated and set as an attribute by the name as in id="...." in the scope specified(or page scope if scope is not specified)

Thats it....HTH
[ January 03, 2007: Message edited by: Sayak Banerjee ]

[Micheal John]

Thanks for the reply..

Person person = (Person)request.getAttribute("person");
if (person==null)//doesn't exists
person = new Person();

My doubt is in the above thing, who is setting the attribute for person...
because we are not setting the attribute for person like request.setAttribute("person",p);
So always Person person = (Person)request.getAttribute("person"); will be null!!
If possible..can you show one example on it..pls?

[ January 04, 2007: Message edited by: Micheal John ]
[ January 04, 2007: Message edited by: Micheal John ]

[Renu Radhika]

if your jsp looks like this

Person p=new Person();
p.setName("name1");
request.setAttribute("person",p);
<jsp:useBean id="person" scope="request">
<jsp:getProperty name="person" property="name">

This will print name1 as there is a request attribute with the name person and no new bean is created.Hope its clear now

[Renu Radhika]

Please read the above as this,I have added whatever I missed out

<%
myPackage.Person p=new myPackage.Person();
p.setName("name1");
request.setAttribute("person",p);
%>
<jsp:useBean id="person" scope="request" type="myPackage.Person">
<jsp:getProperty name="person" property="name">

[Micheal John]

Yes.. it's clear now renu.. thanks for your explanation..

if your jsp looks like this
Person p=new Person();
p.setName("name1");
request.setAttribute("person",p);
<jsp:useBean id="person" scope="request">
<jsp:getProperty name="person" property="name">

so if in my jsp, I am using only the code below, without creating the instance of person and set the name for it.. then the <jsp:useBean> will created an instance of the person automatically and set the arrtibute wuth the specified scope..right..?

<jsp:useBean id="person" scope="request">
<jsp:getProperty name="person" property="name">

[Renu Radhika]

To create one and set the value for name you have to write like this

<jsp:useBean id="person" scope="request" class="myPackage.Person">
<jsp:setProperty name="person" property="name" value="name1">
</jsp:usebean>

note that here both 'class' and body-content for jsp:useBean is important,then only it will know which class instance should be assigned to person reference and to set any property if a new bean is getting created then include it in the body