Re: Storing global data in the servletContext

[Craig McClanahan]

Jacob Weber wrote:

I have a lot of data that needs to be available to all users, at any 
time (for example, the contents of drop-down menus, which I loaded from 
an XML file). From all the posts I've seen here, the recommended place 
to store global data seems to be the servletContext. Most people are 
setting up a ServletContextListener, and loading this data in the 
contextInitialized method.

But according to the servlet spec 
(http://jcp.org/aboutJava/communityprocess/final/jsr154/index.html), 
page 32:

"Context attributes are local to the JVM in which they were created. 
This prevents ServletContext attributes from being a shared memory store 
in a distributed container. When information needs to be shared between 
servlets running in a distributed environment, the information should be 
placed into a session (See ChapterSRV.7, ³Sessions²), stored in a 
database, or set in an Enterprise JavaBeans TM component."

I'm going to be running in a cluster...does this mean that I can't use 
the servletContext?

Not necessarily.  When you deploy your application on a cluster, the 
contextInitialized() method is going to get fired on each node, so that 
if your data is read only you'll be making a copy of the same 
information inside each node's memory space.  The bytes are not actually 
shared, but the net effect is the same ... requests being handled on 
different nodes see exactly the same data in application scope.  This 
scenario would seem to cover use cases like your caching the contents of 
dropdown menus.

If you need the shared data to be modified, and also ensure that those 
modifications are propogated, you'll need to use one of the other 
techniques.

If that's the case, I have two other questions:
- When do I first put the data into the session? I don't think I have 
access to it in the ServletContextListener.
 

You can't put anything into a session until it is first created.  In 
general, there will be a session for each user, created automatically 
the first time that user requests a resource that declares it wants to 
participate in the session (which JSP pages declare by default, for 
example).

Sessions are not a solution to your "share this data with all users" 
requirement, because by definition they are specific to individual users.

- How do I prevent it from being replicated for all users?

 

Use a different technique (context attributes are fine for data that 
doesn't change; otherwise store things in databases or shared files or 
EJBs or whatever technology you have that is accessible to all nodes in 
the cluster).

Another question: I may have 50 or so pieces of data. When I store them 
(in either the session or the servlet context), would you recommend 
putting them all in as separate attributes, or wrapping them all into 
one bean and just storing that one bean as an attribute? Or does it not 
matter?

 

It doesn't matter enough to matter :-) from a performance perspective 
... I'd go with what makes life easiest to (a) maintain over the long 
term, and (b) access using property or EL expressions.  The latter issue 
would encourage having more beans (as opposed to more properties on a 
single bean, or nested trees of beans) because the accessor expressions 
will be shorter.

Thanks for any help.
Jacob
 

Craig McClanahan

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