A decent description of the "Configuration By Exception" concept is the
following [*]:
"Java EE 5 introduced the idea of configuration by exception (sometimes
referred to as programming by exception or convention over configuration). This
means, unless specified differently, the container or provider should apply the
default rules. In other words, having to supply a configuration is the
exception to the rule. This allows you to write the minimum amount of code to
get your application running, relying on the container and provider defaults."
To some extent the OFBiz framework already applied this pattern is some areas
(you may remember when we made the entity group-name "org.ofbiz" the default
value and then were able to remove a bunch of configuration files); however I
see a lot of room for further apply it, starting from the ofbiz-component.xml
files.
For example, considering the ofbiz-component.xml file in framework/example:
<ofbiz-component name="example"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:noNamespaceSchemaLocation="http://ofbiz.apache.org/dtds/ofbiz-component.xsd";>
<!-- define resource loaders; most common is to use the component resource
loader -->
<resource-loader name="main" type="component"/>
<!-- place the config directory on the classpath to access configuration
files -->
<classpath type="dir" location="config"/>
<classpath type="dir" location="dtd"/>
<!-- load single or multiple external libraries -->
<classpath type="jar" location="build/lib/*"/>
<!-- entity resources: model(s), eca(s), group, and data definitions -->
<entity-resource type="model" reader-name="main" loader="main"
location="entitydef/entitymodel.xml"/>
<entity-resource type="model" reader-name="main" loader="main"
location="entitydef/entitymodel_view.xml"/>
<!-- <entity-resource type="eca" reader-name="main" loader="main"
location="entitydef/eecas.xml"/> -->
<entity-resource type="data" reader-name="seed" loader="main"
location="data/ExampleTypeData.xml"/>
<entity-resource type="data" reader-name="seed" loader="main"
location="data/ExampleSecurityData.xml"/>
<entity-resource type="data" reader-name="seed" loader="main"
location="data/ExamplePortletData.xml"/>
<entity-resource type="data" reader-name="seed" loader="main"
location="data/ExamplePortalSeedData.xml"/>
<entity-resource type="data" reader-name="seed" loader="main"
location="data/ExampleHelpData.xml"/>
<entity-resource type="data" reader-name="demo" loader="main"
location="data/ExampleDemoData.xml"/>
<!-- service resources: model(s), eca(s) and group definitions -->
<service-resource type="model" loader="main"
location="servicedef/services.xml"/>
<service-resource type="eca" loader="main" location="servicedef/secas.xml"/>
<!--
<service-resource type="group" loader="main"
location="servicedef/groups.xml"/>
-->
<test-suite loader="main" location="testdef/tests.xml"/>
<!-- web applications; will be mounted when using the embedded container -->
<webapp name="example"
title="Example"
menu-name="secondary"
server="default-server"
location="webapp/example"
base-permission="OFBTOOLS,EXAMPLE"
mount-point="/example"/>
<webapp name="birt"
title="BIRT"
server="default-server"
location="webapp/birt"
base-permission="OFBTOOLS,WEBTOOLS"
mount-point="/birt"
app-bar-display="false"/>
</ofbiz-component>
If we assume that the "default" layout of an OFBiz component is the one used by
the "example" component (and actually by all our components) we could simply
have:
<ofbiz-component name="example"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
xsi:noNamespaceSchemaLocation="http://ofbiz.apache.org/dtds/ofbiz-component.xsd";>
<!-- define resource loaders; most common is to use the component resource
loader so we use it; if you want a different one then use the resource-loader
element-->
<!-- the following folders, if present, will be added to the classpath:
config, dtd; use the "classpath" element with attribute type="dir" to add more
-->
<!-- all the jar files in the following folders, if present, will be added
to the classpath: build/lib, lib; use the "classpath" element with attribute
type="jar" to add more -->
<!-- all the entity definitions files and eca definition files and group
definition files in the folder "entitydef" will be loaded using the
reader-name="main" and loader="main"; use the element "entity-resource" to
specify different resources -->
<!-- the data files in the "data" subfolders will be loaded considering the
subfolder name as the "reader-name": for example data/seed/ will be loaded when
seed data are loaded, data/demo will be loaded when demo data is loaded; you
can create a new reader-name simply adding a new subfolder -->
<!-- service definition files, eca(s) and group definitions will be loaded
from the servicedef folder; use the service-resource element ...-->
<!-- all the test suites in the testdef folder will be executed using the
loader="main"; use "test-suite" to specify a different resource -->
<test-suite loader="main" location="testdef/tests.xml"/>
<!-- web applications; will be mounted when using the embedded container -->
<!-- I am pretty sure we could simplify these ones as well, but at this
point you get the idea... -->
<webapp name="example"
title="Example"
menu-name="secondary"
server="default-server"
location="webapp/example"
base-permission="OFBTOOLS,EXAMPLE"
mount-point="/example"/>
<webapp name="birt"
title="BIRT"
server="default-server"
location="webapp/birt"
base-permission="OFBTOOLS,WEBTOOLS"
mount-point="/birt"
app-bar-display="false"/>
</ofbiz-component>
It should be possible to create a component that uses default resource names
and locations without the need to define the ofbiz-component.xml file; for
special components that require non-standard configurations then the
ofbiz-component.xml file will be used as we are doing now.
What do you think? If we start to think in this direction I am pretty sure we
could be able to simplify a lot of the OFBiz configuration files (service
definitions, entity definitions etc...).
Kind regards,
Jacopo
[*]: http://moon26.blogspot.com/2011/04/configuration-by-exception_13.html
