Fuse ESB Enterprise - Deploying into the Container - Using OSGi Configuration Properties

Overview

The OSGi Configuration Admin service defines a mechanism for passing configuration settings to an OSGi bundle. You do not have to use this service for configuration, but it is typically the most convenient way of configuring applications deployed in Fuse ESB Enterprise (including FABs and OSGi bundles).

Persistent ID

In the OSGi Configuration Admin service, a persistent ID is a name that identifies a group of related configuration properties. In Fuse ESB Enterprise, every persistent ID, PersistentID, is implicitly associated with a file named PersistentID.cfg in the ESBInstallDir/etc/ directory. If the corresponding file exists, it can be used to initialize the values of properties belonging to the PersistentID property group.

For example, the etc/org.ops4j.pax.url.mvn.cfg file is used to set the properties associated with the org.ops4j.pax.url.mvn persistent ID (for the PAX Mvn URL handler).

Using OSGi configuration properties in Spring

Spring DM provides support for OSGi configuration, enabling you to substitute variables in a Spring XML file using values obtained from the OSGi Configuration Admin service.

Spring example

Example 1 shows how to pass the value of the prefix variable to the constructor of the myTransform bean in Spring XML, where the value of prefix is set by the OSGi Configuration Admin service.

Example 1. Using OSGi Configuration Properties in Spring XML

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:ctx="http://www.springframework.org/schema/context"
       xmlns:osgi="http://camel.apache.org/schema/osgi"
       xmlns:osgix="http://www.springframework.org/schema/osgi-compendium"
       ... >
  ...
  <bean id="myTransform" class="org.fusesource.example.MyTransform">
    <property name="prefix" value="${prefix}"/>
  </bean>
   
  <osgix:cm-properties id="preProps" persistent-id="org.fusesource.example">
    <prop key="prefix">MyTransform</prop>
  </osgix:cm-properties>

  <ctx:property-placeholder properties-ref="preProps" />

</beans>

The syntax, ${prefix}, substitutes the value of the prefix variable into the Spring XML file. The OSGi properties are set up using the following XML elements:

osgix:cm-properties

To integrate Spring properties with the properties from the OSGi Configuration Admin service, insert an osgix:cm-properties element into the Spring XML file. This element creates a bean that gets injected with all of the properties from the OSGi ManagedService instance that is identified by the persistent-id attribute. The minimal configuration consists of an empty osgix:cm-properties element that sets the persistent-id attribute and the id attribute—for example:

<osgix:cm-properties id="preProps" persistent-id="org.fusesource.example"/>

For an example of how the persistent ID relates to OSGi configuration settings, see the example in Add OSGi configurations to the feature.

If you want to define defaults for some of the properties in the Spring XML file, add prop elements as children of the osgix:cm-properties element, as shown in Example 1.

ctx:property-placeholder

Property placeholder is a Spring mechanism that enables you to use the syntax, ${PropName}, to substitute variables in a Spring XML file. By defining a ctx:property-placeholder element with a reference to the preProps bean (as in Example 1), you enable the property placeholder mechanism to substitute any of the variables from the preProps bean (which encapsulates the OSGi configuration properties) into the Spring XML file.

Blueprint example

Example 2 shows how to pass the value of the prefix variable to the constructor of the myTransform bean in blueprint XML, where the value of prefix is set by the OSGi Configuration Admin service.

Example 2. Using OSGi Configuration Properties in Blueprint

<blueprint
    xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
    xmlns:cm="http://aries.apache.org/blueprint/xmlns/blueprint-cm/v1.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    ... >
    ...
    <cm:property-placeholder persistent-id="org.fusesource.example">
        <cm:default-properties>
            <cm:property name="prefix" value="Blueprint-Example"/>
        </cm:default-properties>
    </cm:property-placeholder>

    <bean id="myTransform" class="org.apache.servicemix.examples.camel.MyTransform">
        <property name="prefix" value="{{prefix}}" />
    </bean>

</blueprint>

The syntax, {{prefix}}, substitutes the value of the prefix variable into the blueprint XML file. The OSGi properties are set up using the following XML elements:

cm:property-placeholder: This element gives you access to the properties associated with the specified persistent ID. After defining this element, you can use the syntax, {{PropName}}, to substitute variables belonging to the specified persistent ID.
cm:property-placeholder/cm:default-properties: You can optionally specify default values for properties by defining cm:property elements inside the cm:default-properties element. If the corresponding etc/PersistentID.cfg file defines property values, however, these will be used instead.