<jsp:useBean> unexpected behavior

[Shwetha Sharma]

In servlet if we have

request.setAttribute("NULL_PERSON",null);

jsp code :

<jsp:useBean type="com.example.model.Person" id="NULL_PERSON" scope="request"/>

In the generated servlet:

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

Runtime exception is thrown



and in case we have something apart from null in the request attribute:

Person person = new Person();
		person.setName("AA");
		person.setAge(12);
		
		request.setAttribute("Person1", person);

the generated servlet has:

  
com.example.model.Person Person1 = null;
      synchronized (request) {
        Person1 = (com.example.model.Person) _jspx_page_context.getAttribute("Person1", PageContext.REQUEST_SCOPE);
        if (Person1 == null){
          Person1 = new com.example.model.Person();
          _jspx_page_context.setAttribute("Person1", Person1, PageContext.REQUEST_SCOPE);
        }
      }
  

Why the null condition has diffeent behaviour in both cases in the generated servlet?

[Royston Monteiro]

Whooaah! Very strange.

From what I have learnt, I think the generated servlet for a given jsp should be the same irrespective of what some other servlet does before calling it.

Could someone please clarify.

ps: which container are you using?

[Chinmaya Chowdary]

Hi, Shwetha.

   and in case we have something apart from null in the request attribute:

    Person person = new Person();  
           person.setName("AA");  
            person.setAge(12);  
              
            request.setAttribute("Person1", person);  

Person person = new Person(); person.setName("AA"); person.setAge(12); request.setAttribute("Person1", person);

Can you please post, how you are accessing jsp:useBean in the above case?

[Shwetha Sharma]

I missed the difference in usage.

If we dont specify the class attribute for the jsp:use bean, then Instanciation Exception is thrown in case the bean is not present in session or its null

   <jsp:useBean type="com.example.model.Person" id="NULL_PERSON" scope="request"/>
  

results in

  
if (NULL_PERSON == null){
          throw new java.lang.InstantiationException("bean NULL_PERSON not found within scope");
        }
  

and in case we give class attribute

  <jsp:useBean type="com.example.model.Person"  class="com.example.model.Person"  id="NULL_PERSON" scope="request"/>  

results in

  
if (NULL_PERSON == null){
          NULL_PERSON = new com.example.model.Person();
          _jspx_page_context.setAttribute("NULL_PERSON", NULL_PERSON, PageContext.REQUEST_SCOPE);
        }
  

[Adeel Ansari]

From the syntax ref:

The <jsp:useBean> element locates or instantiates a JavaBeans component. <jsp:useBean> first attempts to locate an instance of the Bean. If the Bean does not exist, <jsp:useBean> instantiates it from a class or serialized template.