instantiation

A component may declare either a null constructor (framework version 4.1 and earlier), or, a single constructor with any lifecycle artifact as a parameter argument where arguments may be in any combination or order (framework 4.2 and later). Recognized lifecycle artificats include Logger, Context, Parameters, ServiceManager, and/or Configuration.

NOTE: A component implementation may not duplicate constructor injection of lifecycle artifacts with the equivalent lifecycle stage.

NOTE: The Merlin container also supports substitution of the Context object with a custom context interface and implementation. Using this approach a component author may develop components without dependence on the framework API.

Example:

/**
 * Creation of a new widget.
 *
 * @param logger a logging channel supplied by the container
 * @param manager a service manager supplied by the container from 
 *   which dependent services may be resolved
 * @avalon.dependency type="tutorial.Gizmo" key="gizmo"
 */
 public DefaultWidget( Logger logger, ServiceManager manager ) 
   throws ServiceException
 {
     m_logger = logger;
     m_logger.info( "hello" );
     m_gizmo = (Gizmo) manager.lookup( "gizmo" );
 }

Optional Logger delivery strategy. A component implementation may implement the LogEnabled interface. The container will supply the component root logger via the enableLogging operation.

Example:

/**
 * Supply of a logging channel by the container to this 
 * component.
 *
 * @param logger the logging channel
 */
 public void enableLogging( Logger logger )
 {
     m_logger = logger;
 }

Optional context delivery strategy. A component implementation may implement the Contextualizable interface. The container will supply a component context via the contextualize operation. The context instance will be pre-populated by the container will all requested entries.

Example:

/**
 * Supply of a context object to the component.
 *
 * @param context the component context
 * @avalon.entry key="urn:avalon:home" type="java.io.File"
 * @exception ContextException if an error occurs during 
 *   context entry resolution
 */
 public void contextualize( Context context ) 
   throws ContextException
 {
     m_home = (File) context.get( "urn:avalon:home" );
 }

Optional service manager delivery strategy. A component implementation may implement the Serviceable interface. The container will supply a service manager supporting all declared service dependencies.

Example:

/**
 * Supply of the service manager to the component from which 
 * dependent services may be accessed relative to a service key.
 *
 * @param manager the supplied service manager
 * @avalon.dependency type="tutorial.Gizmo" key="gizmo"
 * @avalon.dependency type="tutorial.Widget" key="widget"
 * @exception ServiceException if an error occurs during 
 *   service resolution
 */
 public void service( ServiceManager manager ) 
   throws ServiceException
 {
     m_gizmo = (Gizmo) manager.lookup( "gizmo" );
     m_widget = (Widget) manager.lookup( "widget" );
 }

Note: the usage of the key attribute on a service dependency tag is optional. If not supplied, an implementation my request the service using the type argument (i.e. the service classname). The recommended practice is to declare a local key.

Optional configuration delivery strategy. A component implementation may implement the Configurable interface. The container will supply a component configuration via the configure operation.

Example:

/**
 * Supply of the component configuration by the container.
 *
 * @param config the component configuration
 * @exception ConfigurationException if an error occurs during 
 *   configuration handling
 */
 public void configure( Configuration config ) 
   throws ConfigurationException
 {
     Configuration location = config.getChild( "location" );
     m_address = location.getAttribute( "address" );
 }

Optional parameters delivery strategy. A component may implement the Parameterizable interface. The container will supply a parameters instance via the parameterize operation.

Example:

/**
 * Supply of parameters to the component by the container
 *
 * @param params the component parameters
 * @exception ParameterException if an error occurs during 
 *   parameter handling
 */
 public void parameterize( Parameters params ) 
   throws ParameterException
 {
     m_secure = params.getParameterAsBoolean( "secure" );
     m_count = params.getParameterAsInteger( "count" );
 }

Optional custom lifecycle stage. A component may implement zero or more custom lifecycle stages. Stages will be applied to the component in the order of avalon.stage declarations.

Example:

/**
 * Component declaring a custom stage dependency.
 *
 * @avalon.component name="demo" lifestyle="singleton" version="1.0"
 * @avalon.stage id="urn:demo:demonstratable"
 */
public class HelloComponent extends AbstractLogEnabled 
  implements Demonstratable
{
    /**
     * A custom lifecycle stage implementation.
     *
     * @param message a message from the custom lifecycle 
     *    stage handler
     */
    public void demo( String message )
    {
        getLogger().info( "extension said: " + message );
    }
}

Optional initialization stage. A component may implement the Initializable interface. The container will invoke initialization following completion of the delivery of lifecycle artifacts and any custom lifecycle stages.

Example:

/**
 * Initialization of the component by the container.
 *
 * @exception Exception if an error occurs during 
 *   the initialization phase
 */
 public void initialize() throws Exception
 {
     ...
 }

Optional execution stage. A component may implement either the Startable or Executable interfaces. If the component implements Executable the execute method will be invoked before the component instance is exposed to any other component. If the component implements the Startable interface the container will invoke the start operation. An implementation is responsible for establishing a working thread and returned from the start operation promptly.

The Startable interface is used by any component that is constantly running for the duration of its life. The interface defines two methods: start and stop. Neither method has any parameters.

The contract surrounding this interface is that the start method is called once after the component is fully initialized, and the stop method is called once before the component is disposed of. Neither method will be called more than once, and start will always be called before stop.

Furthermore, the method must return, it can not go into a endless loop. The container may include a deployment timeout and try to regain the thread control. Details of this functionality is container specific.

From this follows that the component should create a thread and call the Thread.start() method inside this method to start the continous processing. The stop() method should interrupt the thread in a safe manner. This should be done by variables and the Thread.interrupt() method, slightly depending on how the thread is operating. Implications of using this interface require that the start and stop methods be conducted safely (unlike the Thread.stop method) and not render the system unstable.

Executable Example:

/**
 * Execute implementation.
 */
 public void execute()
 {
     ...
 }

Startable Example:

/**
 * Start the component.
 */
 public void start() throws Exception
 {
     ...
 }