I have a problem where a general software product needs minor customizations for different customers. A core entity class (let's say "Article") is 90% common to all clients, but each customer needs a few additional customized fields.

I would very much like to keep the core of the application 100% identical for all customers and to facilitate customizations by way of plugins and configuration files. Also I would like to keep the database schema 100% identical for all customers.

I was thinking that in addition to the common properties (eg title, author, publishDate, etc) which can be accessed via the traditional getter/setter methods there would be a bundle of Properties which are defined by a configuration file. Eg:

<custom-properties>
<property name="article.familyFriendly" datatype="boolean" />
<property name="article.commentsAllowed" datatype="boolean" />
<property name="article.cost" datatype="integer" />
</custom-properties>

In the code, to access a custom property I would use something like this:

Map<String,PropertyValue> properties = myArticle.getProperties();
and
Integer cost = (Integer)myArticle.getPropertyValue("article.cost");

In the database I would have a properties table with columns for entity_id, property_name, property_value


If properties were strings and only strings this would be trivial to implement in Hibernate. However there are complications:

a) Custom properties can have multiple data types (boolean, integer, string and possibly other types such as date, enumarations etc)
b) I must be able to query for objects using these properties as criteria in the query eg
from Article as article
join article.properties as property
where property.name='article.cost'
and property.value<45
c) There may be a requirement to add property value constraints (eg integer min and max values etc)

I have a partial solution implemented in Hibernate, but some of the implementation details are ugly. So before I invest much time in possible wheel reinvention, has this been done before? Or, if not, can anyone suggest other elegant solutions to this general problem?

Thanks in advance,

Joe.