A 

Distributed Container

 is one capable of distributing Web components that are tagged as 

distributable

 across different multiple Java Virtual Machines, perhaps running in different 

hosts. A 

Distributable Application

 is one tagged as such in its Web deployment descriptor.  A 

Distributable JSP Page

 is one in a Distributable Application.

By default, there must be only one instance of a JSP page implementation class per JSP page  

definition in a container (Section 3.2 of Servlet 2.2 spec).  By default, an instance of a Web 

Application must only be run on one Java Virtual Machine at any one time.  This behavior 

can be overridden by declaring the Application to be Distributable (Chapter 9 of Servlet 2.2. 

specification).

A 

single threaded JSP page

 is one with a 

false 

value for its 

isThreadSafe

 attribute of a 

page

directive.  If 

isThreadSafe= false 

, the JSP page implementation shall implement 

javax.servlet.SingleThreadModel

, thus indicating that all requests dispatched to 

that instance shall be delivered serially to the 

service()

 method of the page implementation 

class (Section 3.3.3.1 of Servlet 2.2 spec). 

However, some implementation(s) may additionally use a pool consisting of multiple page 

implementation class instances to do load balancing. Therefore, even when indicating that 

the page is not thread safe, a page author cannot assume that all requests mapped to a 

particular JSP page shall be delivered to the same instance of that page's implementation 

class. The consequence of this is that an author must assume that any mutable resources not 

private/unique to a particular page's instance may be accessed/updated simultaneously by 2 

or more instances; thus any static field values, objects with 

session

 or 

application

scope, or objects shared through some other (unspecified) mechanism by such instances must 

be accessed appropriately synchronized to avoid non deterministic behaviors (Section 3.2.1 

of Servlet 2.2 spec).

In the case of a Distributable JSP page, there is one instance of its JSP page implementation 

class per web component definition per Java Virtual Machine in a Distributed Container 

(Section 3.2 of Servlet 2.2 spec).

If multiple web component definitions in the deployment descriptor indicate the same JSP 

page, there will be multiple instances of the JSP page implementation class, with different 

initialization parameters (Section 3.3.1 of Servlet 2.2 spec).

There is only one instance of the 

ServletContext

 interface associated with each Web 

Application deployed into a Web Container. In cases where the container is distributed over 

many Java Virtual Machines, there is one instance per web application per Java Virtual 

Machine (Section 4.1 of Servlet 2.2 spec).

Context Attributes exist locally to the Java Virtual Machine in which they were created and 

placed.  This prevents the ServletContext from being used as a distributed shared memory 

store.  If information needs to be shared between servlets running in a distributed 

environment, that information should be placed in a session, a database, or in an Enterprise 

JavaBean (Section 4.3.1 of Servlet 2.2 spec).

Chapter 2

Standard Syntax and Semantics

48