Merlin Servlet

The Merlin Servlet package contains an abstract Servlet implementation that contains an embedded Merlin Kernel.

Development Notes

This is a minimal implementation of a Servlet that contains an embedded kernel. The behaviour is a function of the block.xml resource resolved via the initialisation parameters. No support is included for configuration of the kernel and as such the kernel is established relative to defaults. This will be enhanced in a later revision.

Example web.xml

web.xml 

<!DOCTYPE web-app 
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" 
    "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>

  <display-name>Merlin Servlet</display-name>

  <description>
    Merlin Servlet Test Page.
  </description>

  <servlet>
    <servlet-name>merlin</servlet-name>
    <servlet-class>org.apache.avalon.merlin.servlet.TestServlet</servlet-class>
    <init-param>
      <param-name>block</param-name>
      <param-value>/BLOCK-INF/block.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>
</web-app>

Servlet Implementation

MerlinServlet.java 

package org.apache.avalon.merlin.servlet;

import java.io.File;
import java.net.URL;
import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;

import org.apache.avalon.assembly.locator.DefaultLocator;
import org.apache.avalon.merlin.kernel.Kernel;
import org.apache.avalon.merlin.kernel.impl.DefaultKernel;

/**
 * Servlet that handles the establishment of a Merlin Kernel
 * and registration of the kernel base URL under the servlet 
 * context using the key {@link Kernel.BASE_URL_KEY}.
 *
 * @author Stephen McConnell
 */
public class MerlinServlet extends HttpServlet
{
    private DefaultKernel m_kernel;

    /**
     * Initializes Servlet by the web server container.
     *
     * @exception ServletException if an error occurs
     */
    public void init()
        throws ServletException
    {
        try
        {
            ClassLoader loader = Thread.currentThread().getContextClassLoader();

            String homePath = getServletContext().getRealPath( "." );
            File home = new File( homePath );

            String blockPath = getInitParameter( "block", "BLOCK-INF/block.xml" );
            URL block = new File( home, blockPath ).toURL();

            DefaultLocator context = new DefaultLocator();
            context.put( "urn:merlin:home", home );
            context.put( "urn:merlin:system", home );
            context.put( "urn:merlin:classloader.common", loader );
            context.put( "urn:merlin:classloader.system", loader );
            context.put( "urn:merlin:debug", "WARN" );
            context.put( "urn:merlin:logging.priority", "INFO" );
            context.put( "urn:merlin:block.url", block );
            context.makeReadOnly();

            m_kernel = new DefaultKernel();
            m_kernel.contextualize( context );
            m_kernel.initialize();

            getServletContext().setAttribute( Kernel.BASE_URL_KEY, m_kernel.getURL() );

            log( "kernel established" );
        }
        catch( Exception e )
        {
            throw new ServletException( "Initialization error.", e );
        }
    }

    /**
     * Disposes of container manager and container instance.
     */
    public void destroy()
    {
        if( m_kernel != null )
        { 
            m_kernel.shutdown();
            m_kernel = null;
        }
    }

    private String getInitParameter( final String name, final String defaultValue )
    {
        final String value = getInitParameter( name );
        if ( null == value )
        {
            return defaultValue;
        }
        else
        {
            return value;
        }
    }
}