<jsp:useBean> standard action type and class confusion

[Luis Centeno]

Hi Ranchers...

I've been looking at standard actions and <jsp:useBean> is really confusing to me...
I wrote some code to clear my doubts but I am confused about type and class attributes.
Pleas take a look at my example code:

I wrote three classes:

package com.lagc.bean;

public interface Movable{

	public void setVehicleType(String vehicleType);
	public String getVehicleType();
}

package com.lagc.bean;

public abstract class Mammal{	
	private String height;
	private String width;
	protected String foodEatenInKgs;
	
	public String getHeight(){
		return height;
	}
	
	public void setHeight(String height){
		this.height = height;
	}
	
	public String getWidth(){
		return width;
	}
	
	public void setWidth(String width){
		this.width = width;
	}
	
	public abstract String getFoodEatenInKgs();
	
	public abstract void setFoodEatenInKgs(String fooEatenInKgs);
}

package com.lagc.bean;
public class Person extends Mammal implements Movable{
	private String id;
	private String name;
	private String lastName;
	private String vehicleType;
	
	public void setId(String id){
		this.id = id;
	}
	
	public String getId(){
		return id;
	}
	
	public void setName(String name){
		this.name = name;
	}
	
	public String getName(){
		return name;
	}
	
	public void setLastName(String lastName){
		this.lastName = lastName;
	}
	
	public String getLastName(){
		return lastName;
	}
	
	public void setFoodEatenInKgs(String foodEatenInKgs){
		this.foodEatenInKgs = "I eat " + foodEatenInKgs + " kgs of food";
	}
	
	public String getFoodEatenInKgs(){
		return foodEatenInKgs;
	}
	
	public void setVehicleType(String vehicleType){
		this.vehicleType = "I can move on " + vehicleType;
	}
	
	public String getVehicleType(){
		return vehicleType;
	}
}

And a servlet in order to instantiate the Person class and send different references to the jsp as a request-scoped attribute.

		
		Person p1 = new Person();
		p1.setId(txtId);
		p1.setName(txtName);
		p1.setLastName(txtLastName);
		p1.setHeight(txtHeight);
		p1.setWidth(txtWidth);
		p1.setFoodEatenInKgs(txtFoodEatenInKgs);
		p1.setVehicleType(txtVehicle);
		
		Movable m1 = p1; 
		Mammal mm1 = p1; 
		
		request.setAttribute("person",p1); //1
		request.setAttribute("person2",m1); //2
		request.setAttribute("person3",mm1); //3
		
		RequestDispatcher dispatcher = null;
		
		dispatcher = request.getRequestDispatcher(
			request.getParameter("show") != null ? "show.jsp" :
			request.getParameter("setAllProperties") != null ? "setAllProperties.jsp" :
			request.getParameter("optionalProperties") != null ? "optionalProperties.jsp" :
			request.getParameter("showAllProperties") != null ? "showAllProperties.jsp" :
			"");

		dispatcher.forward(request, response);

Now...I wrote a jsp for the fields to be shown in the view and I noticed particular behaviors...

When I used

<jsp:useBean id="person3" class="com.lagc.bean.Person" scope="request" />
<jsp:getProperty name="person3" property="id"/>
<jsp:getProperty name="person3" property="vehicleType"/>

I got both values in the view and the following code is generated by the container

	com.lagc.bean.Person person3 = null;
      synchronized (request) {
        person3 = (com.lagc.bean.Person) _jspx_page_context.getAttribute("person3", PageContext.REQUEST_SCOPE);
        if (person3 == null){
          person3 = new com.lagc.bean.Person();
          _jspx_page_context.setAttribute("person3", person3, PageContext.REQUEST_SCOPE);
        }
      }
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getId())));
      out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getVehicleType())));

When I used

<jsp:useBean id="person3" type="com.lagc.bean.Movable" scope="request" />
<jsp:getProperty name="person3" property="id"/>
<jsp:getProperty name="person3" property="vehicleType"/>

I got an error since id field is not part of Movable interface...

When I used...

<jsp:useBean id="person3" class="com.lagc.bean.Person" type="com.lagc.bean.Movable" scope="request" />
<jsp:getProperty name="person3" property="id"/>
<jsp:getProperty name="person3" property="vehicleType"/>

I got both values in the view and the following code is generated by the container

com.lagc.bean.Movable person3 = null;
      synchronized (request) {
        person3 = (com.lagc.bean.Movable) _jspx_page_context.getAttribute("person3", PageContext.REQUEST_SCOPE);
        if (person3 == null){
          person3 = new com.lagc.bean.Person();
          _jspx_page_context.setAttribute("person3", person3, PageContext.REQUEST_SCOPE);
        }
      }
out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getId())));
out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getVehicleType())));

My doubt is about the way the container handles type and class attributes.

I noticed that when class attribute is defined, its value is used to cast the attribute found by _jspx_page_context.findAttribute("person3"), even though the attribute bound to the request is not of this type. And when class attribute is not defined, it tries to use the type attribute value, in my code example it does not work though...

Is this the expected behavior from the container? Is class attribute value intended to do this casting as well? Am I missing something?

[Ankit Garg]

Hmm this is interesting. I read the JSP spec 2 times regarding this and it mentions that if you specify both type and class attributes of jsp:useBean tag, and if the object is found, then the object must be of the type specified by the type attribute, there is no compulsion that the object be of type specified by the class attribute. So basically if I have a tag

<jsp:useBean id="movable" type="com.lagc.bean.Movable" class="com.lagc.bean.Person" />

Now if there is an attribute named movable in the page scope, then its not compulsory that it must be of type com.lagc.bean.Person. So the container should allow you to only access properties of Movable class and not Person class as the actual object might not be of type Person. Maybe I missed something or the container which you are using (I'm using Tomcat 6 and the behavior is the same as your container) is not showing the correct behavior...

[Seetharaman Venkatasamy]

I agree with Ankit

[Luis Centeno]

I read the JSP spec 2 times regarding this and it mentions that if you specify both type and class attributes of jsp:useBean tag, and if the object is found, then the object must be of the type specified by the type attribute, there is no compulsion that the object be of type specified by the class attribute...

...So the container should allow you to only access properties of Movable class and not Person class as the actual object might not be of type Person.

I absoultely agree with you and this was the way I had understood this concept, but if this is true, my container (Tomcat 5.5) is not doing what it is supposed to do.
Unless, this concept is regarding the actual object, I think the container checks the object class (based on class attribute value) bound to the scope (request in this case) rather than relying on the type attribute value.
Hence, this case is valid since I am binding a Movable reference type referring to a Person object on the heap. This could justify this behavior from the container (shown below) but, again, it is just a supposition.

out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getId())));   
out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getVehicleType())));

What do you guys think?

[Chinmaya Chowdary]

Hi, Luis.

During translation time, jsp:setProperty and jsp:getProperty checks the jsp:useBean attributes and translates accordingly.
1. Suppose if we use 'type' attribute without using 'class' attribute,

    <jsp:useBean id="person3" type="com.lagc.bean.Movable" scope="request" />  

the translation for jsp:useBean will be

    com.lagc.bean.Movable person3 = null;
      synchronized (request) {
        person3 = (com.lagc.bean.Movable) _jspx_page_context.getAttribute("person3", PageContext.REQUEST_SCOPE);
        if (person3 == null){
          throw new java.lang.InstantiationException("bean person3 not found within scope");
        }

In the above gets the 'person3' attribute that was set in 'request' scope. If 'person3' object is not available, it throws 'InstantiationException' indicating 'bean person3 not found within scope'.

Suppose if we use

    <jsp:getProperty name="person3" property="vehicleType"/>  

then page will be translates into

    out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Movable)_jspx_page_context.findAttribute("person3")).getVehicleType())));

Here casting will be done by using 'type' attribute 'com.lagc.bean.Movable'. Suppose if the container casts it with 'com.lagc.bean.Person', there is no meaning it will never available.

2. If we use both 'type' and 'class' attribute:

    <jsp:useBean id="person3" type="com.lagc.bean.Movable" class="com.lagc.bean.Person" scope="request" /> 

it wil be translates into

synchronized (request) {
        person3 = (com.lagc.bean.Movable) _jspx_page_context.getAttribute("person3", PageContext.REQUEST_SCOPE);
        if (person3 == null){
          person3 = new com.lagc.bean.Person();
          _jspx_page_context.setAttribute("person3", person3, PageContext.REQUEST_SCOPE);
        }    

Suppose if we use jsp:getProperty like

    <jsp:getProperty name="person3" property="vehicleType"/>  

then page will be translates into

    out.write(org.apache.jasper.runtime.JspRuntimeLibrary.toString((((com.lagc.bean.Person)_jspx_page_context.findAttribute("person3")).getVehicleType())));

The important point to notice here is that it will not cast it with 'com.lagc.bean.Movable' instead it casted with 'com.lagc.bean.Person' eventhough the jsp:useBean uses 'type' attribute. Because there is a chance of jsp:getProperty to get the 'person3' attribute. In the above translated code if 'person3' attribute is not available, there is chance of creating Person object. Because of this possibility jsp:getProperty casts it with 'Person' class. It behaves intelligently according to jsp:useBean attributes. I tested this in Tomcat5.5.9. I think(I am guessing) this behaviour is guranateed. This makes sense.

[Luis Centeno]

Thank you guys for your help...

Chinmaya: Now it is all clearer, I had not thought about the object creation using the class attribute vaue and consecuences at translation time as well as differences when you only specify one or both attributes of <jsp:useBean>. You are right it all makes sense now, when the object is not found in the scope, it is created with the class attribute value, so it is retrieved with this value as well...

[Prashant Shiralkar]

Hello,

This is regarding an example from the "Be the Container" from Head First Servlets & JSP Edition 2, page#: 358, 420
Person is an abstract class with setName() and getName(). Employee is a subclass of Person.

I tried implementing the following code (I excluded the package):

In a test servlet:

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

In JSP that tries to access this attribute bean:

<jsp: useBean id="person" type="Employee" scope="request">
     <jsp:setProperty name="person" property="name" value="Fred">
</jsp:useBean>
Name is : <jsp:getProperty name="person" property="name"/>

This code works fine and also follows the concept as explained by Chinmay. But the answer in the book says that this code fails at request time. Hence, this makes me wonder if its an error in the book or I am implementing & interpreting it in an incorrect way. Can someone help me clarify this?

[Frits Walraven]

That is just a trick question: the tag is used wrongly, no space is allowed between the jsp: and useBean
so:

 <jsp: useBean id="person" type="Employee" scope="request">   

should be:

 <jsp:useBean id="person" type="Employee" scope="request">   

regards,
Frits

[Chinmaya Chowdary]

Hi, Prashant.

This works fine. See errata .

[Prashant Shiralkar]

Thanks Frits and Chinmay!

While implementing the problem, I put Person.class and Employee.class in /WEB-INF/classes directory. When I request the JSP page for the first time, I can see that the container (Tomcat 5.5) generates the corresponding servlet (.java) file, but it fails to compile it giving an exception:

org.apache.JasperException: Unable to compile class for JSP:
An error occurred at line: X in the jsp file: /Y.jsp
Employee cannot be resolved to a type
...

(X & Y are placeholders here for line# and jsp page respectively.)

This implies that the container fails to locate the Employee.class file. I don't see a reason why it cannot find it in the directory mentioned above. Earlier, I got around this by manually placing the .class files in the same place where the container puts the translated servlet (just in order to focus on the main problem first). Any idea for container's this kinda behavior?