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From "Jukka Zitting" <jukka.zitt...@gmail.com>
Subject Proposal for managing JCR resources in webapps
Date Tue, 15 May 2007 21:53:26 GMT

I must confess that I don't really like the RepositoryStartupServlet
and RepositoryAccessServlet mechanisms we currently have in
jackrabbit-webapp. The classes are complex and correctly configuring
them is non-trivial. To simplify things I propose an alternative
approach to managing JCR resources (Repositories, Sessions, Items,
etc.) in webapps, including the jackrabbit-webapp.

The proposal is based on using the standard servlet scope and
attribute model for managing JCR resources. The JCR repository used by
a webapp would be placed in the servlet context as the
"javax.jcr.Repository" attribute. If a webapp uses multiple
repositories, they can be managed as "javax.jcr.Repository.<name>"
attributes. A JCR session would typically be managed as the
"javax.jcr.Session" attribute of a servlet session or a request.

The canonical code to access the repository would be something like this:

    String name = getInitParameter(Repository.class.getName());
    if (name == null) {
        name = Repository.class.getName();
    Repository repository = getServletContext().getAttribute(name);

Using the standard attribute model allows nice separation of concerns
and plays very well with various tools available in the servlet
environment. For example I could implement the Repository startup or
lookup code as a ServletContextListener that manages the repository
lifecycle. Separate ServletContextAttributeListeners could be used to
optionally expose the repository instance through RMI or JNDI. I could
also easily mix and match things, for example to configure a
ServetContextAttributeListener that automatically provides a decorated
version of the repository as a separate attribute, which could again
be further processed by other attribute listeners or servlets.

The same options also apply to JCR sessions on a servlet session or
request level. A typical approach could be to use a session or request
listener or a filter to associate a JCR session with the processing of
a request. The servlet or JSP page that ends up processing the request
can simply look up the respective attribute to get the JCR session.

What this would mean for jackrabbit-webapp? First I would move the RMI
and JNDI binding code from RepositoryStartupServlet into separate
ServletContextAttributeListeners. Then I would replace the
RepositoryStartupServlet dependencies in the other serlets like
SimpleWebdavServlet with the standard servlet context attributes.
Finally I'd split the configuration part of the
RepositoryStartupServlet into a separate class and perhaps turn the
remaining RepositoryStartupServlet code into a
ServletContext(Attribute)Listener class.


Jukka Zitting

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