2010/12/15 - Apache Excalibur has been retired.

For more information, please explore the Attic.

What is Meta Information?

Meta Information is information about your components. The information that Fortress needs to properly manage your components is fairly small. It needs to know which classes are components, which services that component implements, what its dependencies are, and what type of lifestyle the component was designed for. Using this information, Fortress can determine if there are some circular dependencies as well as manage other aspects of your component.

Your container has to find the meta information in some way. If you are used to the Excalibur Component Manager (ECM), then you supplied the meta information based on a "roles" file and certain marker interfaces that your component implemented. There are several drawbacks to this approach, some of which include inheritance of conflicting meta information and having to manage your information in separate files.

Fortress has adopted a practice that is available in both Phoenix and Merlin to make managing meta information a breeze. It is the use of JavaDoc tags to mark the meta information right with the source code itself. This scheme works much better, and unlocks some features to you. One of them is for Fortress to tell you that you have a circular dependency (some components that require each other to work) at initialization time instead of at run time.

How does the Meta Info Tags Look?

The best way to demonstrate how these tags look and behave is to show some sample code. Fortress follows the AMTAGS proposal on the WIKI, and also supports a couple extra tags. Specifically, the set of tags that Fortress supports are "avalon.dependency", "avalon.service", "x-avalon.info", "x-avalon.lifestyle", "avalon.component", "fortress.name", "fortress.handler". The two Fortress specific tags are recognized, but are not encouraged for routine use.

Everything except for the "avalon.dependency" must be specified in the component's class JavaDoc. Below is a snippet:

 * My component implementation.
 * @avalon.component
 * @avalon.service type="RoleInterface"
 * @x-avalon.info name="my-component"
 * @x-avalon.lifestyle type="pooled"
public class MyComponent implements RoleInterface
    // ....

The "avalon.component" is a marker tag, and it is required to mark a class as a component. You must have one or more "avalon.service" tags (one per interface type). The value in the "type" attribute must be a resolvable class name. If the type of "RoleInterface" is in the same package, or in one of the imports, then all is well. The "x-avalon.info" tag is used to provide a configuration name for the component. In this case, the configuration name is "my-component". If you do not supply this tag, Fortress will create one for you based on the class name of the component. It is best if you supply your own. The "x-avalon.lifestyle" tag is used to choose how this component is managed.

Dependencies are declared in the service() method for the component. The component needs to implement the Serviceable interface. Fortress knows how to look at the code hierarchy to collect all the dependencies as long as you have all your source code available. The dependency declarations look like this:

 * Get all the dependencies.
 * @avalon.dependency type="OtherService"
 * @avalon.dependency type="Foo"
public void service( ServiceManager manager )
    // ...

All the "avalon.dependency" entries are collected by the same rules as the "avalon.service" entries. In other words, the "type" attribute is evaluated based on the same rules as your Java compiler evaluates them. Fortress will take care of resolving them properly.

Collecting the Meta Information

Fortress provides an ANT task to collect all your meta information and generate the extra files that your JAR files need to have for Fortress to use the meta information at run time. You include the ANT task in your build.xml file like this:

  <!-- ... -->

  <taskdef name="collect-metainfo"
    <classpath refid="tools.class.path"/>

  <target name="compile" depends="jar-tools" description="Compiles the source code">
    <mkdir dir="${build.classes}"/>

    <!-- Compile all classes excluding the tests. -->
    <javac srcdir="${java.dir}"
        <path refid="tools.class.path"/>
        <pathelement location="${loader.classes}"/>
      <include name="**/*.java"/>

    <collect-metainfo destdir="${build.classes}">
      <fileset dir="${java.dir}"/>

  <!-- ... -->

Once you include all the generated files in your JAR for the components, you don't have to do anything more than include the JAR in your classpath for Fortress. Everything else is taken care of.

The x-avalon.lifestyle Tag

The "x-avalon.lifestyle" tag needs a little further explanation. It can have one of four values in the "type" attribute:

  • singleton One instance of the component shared with all clients.
  • thread One instance of the component per thread of execution.
  • pooled One instance of the component per request, with instances reused from a pool.
  • transient One instance of the component per request, with instances destroyed on release.