Date: 2004-11-29T02:45:13
Editor: JohannesTextor <[EMAIL PROTECTED]>
Wiki: Cocoon Wiki
Page: CocoonAndHibernateTutorial
URL: http://wiki.apache.org/cocoon/CocoonAndHibernateTutorial
no comment
Change Log:
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@@ -19,8 +19,14 @@
=== Your basic skills ===
+Time. If you are new to the topic of O/R mapping, you will need a lot
+of time and patience, because it is not an easy topic. Learning Cocoon already
+made you re-think your concept of a Web Application; learning Hibernate, you
+will have to do it again.
+
You should have basic Cocoon knowledge, i.e. about the Sitemap, Flowscript,
-and JX Templates. The samples included will also use CForms.
+and JX Templates. The samples included will also use CForms. Did I mention
+Flowscript ? That one is especially important.
If you want to use Hibernate you should also be fluent in Java. Be warned that
you are going beyond the "no programming required" - statement of the Cocoon
@@ -29,6 +35,10 @@
takes care of Hibernate Session, so if you don't know what a Servlet Filter is
get your favourite book or search engine and read up :)
+You will have a basic notion of what Hibernate is and what it does, otherwise
+you would not have come to this page :) But make sure you have understood what
+lazy collection initialization is and what it does.
+
=== Technical prerequisites ===
A running cocoon installation. I tried this on Cocoon 2.1.6. It might work
@@ -270,11 +280,47 @@
or later experience the (in)famous Lazy Initialization Problem (tm). The
solution for this problem is to
create a servlet filter to manage Hibernate Sessions, so if you are'nt fed up
on Java yet read on :)
-= A Servlet Filter for Managing Hibernate Sessions =
+== A Servlet Filter for Managing Hibernate Sessions ==
+
+=== Why ? ===
+
+(If you just want to continue installing and are not bothering about the Why,
skip this part and come
+back later ...)
+
+If you're getting serious about Hibernate, sooner or later you will want to
use lazy collection initialization
+(Read the corresponding section in GettingStartedWithCocoonAndHibernate for an
introduction on this topic).
+Say you are accessing Hibernate sessions from flowscript, i.e. as follows:
+
+{{{
+ var factory = cocoon.getComponent(Packages.org.test.PersistenceFactory.ROLE);
+ var hs = factory.createSession();
+
+ var data = hs.find("from org.test.Data");
+
+ cocoon.sendPage("data-pipe", {data: data} );
+
+ hs.close();
+}}}
-== Why ? ==
+This will work for simple data objects and also for complex ones, i.e. objects
that contain collections, as long
+as you are not lazily initializing them. If not, you might experience the
following problem: Say your object contains
+a mapped collection {{{otherData}}}. In the flow snippet above, you did not
access this collection, so Hibernate
+won't load its items from the database. But it is still possible that your
view pipeline wants to access {{{otherData}}}.
+
+Now the problem is that the flowscript will just continue to process right
after it has invoked the view pipeline,
+so it is possible that the {{{hs.close()}}} will occur BEFORE your view has
rendered completely. So when the view
+will access {{{otherData}}} after that, you will get an exception telling you
that Hibernate failed to lazily
+initialize the collection. A classical Race Condition.
+
+Obviously that's no good. The solution is to move Hibernate session management
out of the flowscript layer and even
+outside cocoon. So we will create a servlet filter. A servlet filer is invoked
BEFORE the request is passed to cocoon
+and will be notified when the request is processed completely. At that point
we can safely close the Hibernate session,
+given we will not continue to work with it from flowscript after the view
pipeline has been invoked. However, invoking
+the view pipeline should be the last thing you do in a flow script anyway, so
that won't be too much of a problem.
+=== Creating the filter ===
+=== Installing the filter ===
== Appendix ==
