I'm beginning to realize that EJB 2.0 specification has a serious limitation.
It's the EJB QL.
Take for example a simple problem: How to create a finder or select method
that would return a set of entities that a user specifies by for example 4
different attributes where each of them could be missing (null) and would
therefore not account into the query result? Create 16 different finders for
every combination of 4 attributes and a switch statement? What if there were
10 different attributes? Create 1024 different finders?
Another example: the "LIKE pattern_value" construct where "pattern_value" can
only be a literal value and not a parameter. I understand that this
limitation arises from the similar limitation in the SQL from which EJB QL
was derived so that mapping from EJB QL to SQL is easy...
What is missing from the EJB 2.0 spec and would greatly improve the
"fullness" of the specification is execution of EJB QL queries that are
composed programatically at runtime.
So what am I doing on this list? Read on...
As I understand it, the EJB 2.0 spec has provided app server vendors with a
possibility to create proprietary extensions to finder/select method
processing with the following (taken from the ejb-jar_2_0.dtd):
"Queries that are expressible in EJB QL must use the ejb-ql element to
specify the query. If a query is not expressible in EJB QL, the
description element should be used to describe the semantics of the
query and the ejb-ql element should be empty."
...and the interpretation of the description element is up to the container
provider...
I'm proposing (and I believe that it would not be hard to implement in JBoss)
something like the following:
1. bosscmp-jdbc.xml: add optional <ejb-ql-composer-class> element as a child
of <entity> elements:
<entity id="...">
...
<ejb-ql-composer-class>the.composer.Class</ejb-ql-composer-class>
...
</entity>
the.composer.Class would implement the following interface:
public interface EjbQlComposer {
// return the EJB QL query string
public String composeEjbQlQuery(String description, Object[]
parameters)
throws FinderException;
}
Before executing any finder/select method with an empty <ejb-ql/> element in
the particular entity an instance of the specified <ejb-ql-composer-class>
would be created, the composeEjbQlQuery method called with the <description>
element content from the <query> being executed and the parameters to the
finder/select method passed in Object[] array. The method would compose and
return a string consisting of EJB QL query which the container would compile
and execute.
What do you think? Would that be feasible? I'm interested in implementing
this and contributing to JBoss...
Peter
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