Using Merlin

Managing Context

This tutorial presents information about the management of the runtime context supplied to your component.

Resources supporting this tutorial are contained in the tutorials/context/avalon/ package.

Merlin Standard Context Entries

In order to receive a runtime context we need to update the HelloComponent source so that it implements the Avalon Contextualizable lifecycle stage interface. Merlin will build and supply a context object containing the following four context entries:

Merlin Standard Context Entries

Key Class Description
urn:avalon:home java.io.File The working directory.
urn:avalon:temp java.io.File The temporary directory that will be destroyed at the end of the session.
urn:avalon:name java.lang.String The name assigned to the component.
urn:avalon:partition java.lang.String The assigned partition name.

Adding Context Support to the Component

In order to receive a runtime context we need to update the HelloComponent source so that it implements the Avalon Contextualization stage interface. Merlin will build and supply a context object containing the standard four context entries.

HelloComponent.java 

package tutorial;

import java.io.File;

import org.apache.avalon.framework.logger.AbstractLogEnabled;
import org.apache.avalon.framework.context.Contextualizable;
import org.apache.avalon.framework.context.Context;
import org.apache.avalon.framework.context.ContextException;

/**
 * Component demonstrating access to standard context entries.
 * @avalon.component name="demo"
 */
public class HelloComponent extends AbstractLogEnabled 
  implements Contextualizable
{

    private File m_home = null;
    private File m_temp = null;
    private String m_name = "unknown";
    private String m_partition = "unknown";

   /**
    * Contextualization of the component by the container.
    * The context supplied by the container holds the 
    * Merlin standard context entries for the home and 
    * working directories, component name and partition.
    *
    * @avalon.context
    * @avalon.entry key="urn:avalon:name" 
    * @avalon.entry key="urn:avalon:partition" 
    * @avalon.entry key="urn:avalon:home" type="java.io.File"
    * @avalon.entry key="urn:avalon:temp" type="java.io.File"
    */
    public void contextualize( Context context )
      throws ContextException
    {
        m_home = (File) context.get( "urn:avalon:home" );
        m_temp = (File) context.get( "urn:avalon:temp" );
        m_name = (String) context.get( "urn:avalon:name" );
        m_partition = (String) context.get( "urn:avalon:partition" );

        StringBuffer buffer = new StringBuffer( "standard context entries" );
        buffer.append( "\n  name: " + m_name );
        buffer.append( "\n  home: " + m_home );
        buffer.append( "\n  temp: " + m_temp );
        buffer.append( "\n  partition: " + m_partition );

        getLogger().info( buffer.toString() );

    }
}

Build and run the tutorial. 

$ maven
$ merlin -execute target\classes

In the logging output we see the values provided by Merlin to the component. 

[INFO   ] (tutorial.hello): standard context entries
  name: hello
  home: F:\dev\avalon\merlin\platform\tutorials\context\avalon\home\tutorial\hello
  temp: C:\TEMP\tutorial\hello
  partition: /tutorial/

Custom Entries

The next tutorial shows how Merlin provides support for creating custom context entries.