Here is the base skeleton of the content portlet. The FSContentDrivenPortlet shows the files which are in the war file in which the portlet is deployed. The arbitrary name filesystem will be the content type interpreted by the portlet.

public class FSContentDrivenPortlet extends GenericPortlet
{

   /** The edit_content mode. */
   public static final PortletMode EDIT_CONTENT_MODE = new PortletMode("edit_content");

   /** The select_content mode. */
   public static final PortletMode SELECT_CONTENT_MODE = new PortletMode("select_content");

   ...

}

First the doDispatch(RenderRequest req, RenderResponse resp) is overridden in order to branch the request flow to a method that will take care of displaying the editor.

protected void doDispatch(RenderRequest req, RenderResponse resp)
                  throws PortletException, PortletSecurityException, IOException
{
   if (EDIT_CONTENT_MODE.equals(req.getPortletMode()) || 
       SELECT_CONTENT_MODE.equals(req.getPortletMode()))
   {
      doEditContent(req, resp);
   }
   else
   {
      super.doDispatch(req, resp);
   }
}

The portlet also needs a few utilities methods which take care of converting content URI to a file back and forth. There is also an implementation of a file filter that keep only text files and avoid the WEB-INF directory of the war file for security reasons.

protected File getFile(String contentURI) throws IOException
{
   String realPath = getPortletContext().getRealPath(contentURI);
   if (realPath == null)
   {
      throw new IOException("Cannot access war file content");
   }
   File file = new File(realPath);
   if (!file.exists())
   {
      throw new IOException("File " + contentURI + " does not exist");
   }
   return file;
}

   protected String getContentURI(File file) throws IOException
   {
      String rootPath = getPortletContext().getRealPath("/");
      if (rootPath == null)
      {
         throw new IOException("Cannot access war file content");
      }

      // Make it canonical
      rootPath = new File(rootPath).getCanonicalPath();

      // Get the portion of the path that is significant for us
      String filePath = file.getCanonicalPath();
      return filePath.length() >=
        rootPath.length() ? filePath.substring(rootPath.length()) : null;
   }

   private final FileFilter filter = new FileFilter()
   {
      public boolean accept(File file)
      {
         String name = file.getName();
         if (file.isDirectory())
         {
            return !"WEB-INF".equals(name);
         }
         else if (file.isFile())
         {
            return name.endsWith(".txt");
         }
         else
         {
            return false;
         }
      }
   };

The editor is probably the longest part of the portlet. It tries to stay simple though and goes directly to the point.

protected void doEditContent(RenderRequest req, RenderResponse resp)
                  throws PortletException, PortletSecurityException, IOException
{
   // Get the uri value optionally provided by the portal
   String uri = req.getParameter("content.uri");

   // Get the working directory directory
   File workingDir;
   if (uri != null)
   {
      workingDir = getFile(uri).getParentFile();
   }
   else
   {
      // Otherwise try to get the current directory we are browsing,
      // if no current dir exist we use the root
      String currentDir = req.getParameter("current_dir");
      if (currentDir == null)
      {
         currentDir = "/";
      }
      workingDir = getFile(currentDir);
   }

   // Get the parent path
   String parentPath = getContentURI(workingDir.getParentFile());

   // Get the children of the selected file, we use a filter
   // to retain only text files and avoid WEB-INF dir
   File[] children = workingDir.listFiles(filter);

   // Configure the response
   resp.setContentType("text/html");
   PrintWriter writer = resp.getWriter();

   //
   writer.print("Directories:<br/>");
   writer.print("<ul>");
   PortletURL choseDirURL = resp.createRenderURL();
   if (parentPath != null)
   {
      choseDirURL.setParameter("current_dir", parentPath);
      writer.print("<li><a href=\"" + choseDirURL + "\">..</a></li>");
   }
   for (int i = 0;i < children.length;i++)
   {
      File child = children[i];
      if (child.isDirectory())
      {
         choseDirURL.setParameter("current_dir", getContentURI(child));
         writer.print("<li><a href=\"" + choseDirURL + "\">" + child.getName() +
                      "</a></li>");
      }
   }
   writer.print("</ul><br/>");

   //
   writer.print("Files:<br/>");
   writer.print("<ul>");
   PortletURL selectFileURL = resp.createActionURL();
   selectFileURL.setParameter("content.action.select", "select");
   for (int i = 0;i < children.length;i++)
   {
      File child = children[i];
      if (child.isFile())
      {
         selectFileURL.setParameter("content.uri", getContentURI(child));
         writer.print("<li><a href=\"" + selectFileURL + "\">" + child.getName() +
                      "</a></li>");
      }
   }
   writer.print("</ul><br/>");

   //
   writer.close();
}

Last but not least the portlet needs to implement the doView(RenderRequest req, RenderResponse resp) method in order to display the file that the portal window wants to show.

protected void doView(RenderRequest req, RenderResponse resp)
                  throws PortletException, PortletSecurityException, IOException
{
   // Get the URI provided by the portal
   String uri = req.getParameter("uri");

   // Configure the response
   resp.setContentType("text/html");
   PrintWriter writer = resp.getWriter();

   //
   if (uri == null)
   {
      writer.print("No selected file");
   }
   else
   {
      File file = getFile(uri);
      FileInputStream in = null;
      try
      {
         in = new FileInputStream(file);
         FileChannel channel = in.getChannel();
         byte[] bytes = new byte[(int)channel.size()];
         ByteBuffer buffer = ByteBuffer.wrap(bytes);
         channel.read(buffer);
         writer.write(new String(bytes, 0, bytes.length, "UTF8"));
      }
      catch (FileNotFoundException e)
      {
         writer.print("No such file " + uri);
         getPortletContext().log("Cannot find file " + uri, e);
      }
      finally
      {
         if (in != null)
         {
            in.close();
         }
      }
   }

   //
   writer.close();
}

Finally we need to make the portal aware of the fact that the portlet can edit and interpret content. For that we need a few descriptors. The portlet.xml descriptor will define our portlet, the portlet-instances.xml will create a single instance of our portlet. The web.xml descriptor will contain a servlet context listener that will hook the content type in the portal content type registry.

<?xml version="1.0" encoding="UTF-8"?>
<portlet-app
   xmlns="http://java.sun.com/xml/ns/portlet/portlet-app_1_0.xsd"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://java.sun.com/xml/ns/portlet/portlet-app_1_0.xsd
                       http://java.sun.com/xml/ns/portlet/portlet-app_1_0.xsd"
   version="1.0">
   ...
   <portlet>
      <description>File System Content Driven Portlet</description>
      <portlet-name>FSContentDrivenPortlet</portlet-name>
      <display-name>File System Content Driven Portlet</display-name>
      <portlet-class>org.jboss.portal.core.portlet.test.FSContentDrivenPortlet</portlet-class>
      <supports>
         <mime-type>text/html</mime-type>
      </supports>
      <portlet-info>
         <title>File Portlet</title>
         <keywords>sample,test</keywords>
      </portlet-info>
   </portlet>
   ...
</portlet-app>

The portlet.xml descriptor

<deployments>
   ...
   <deployment>
      <instance>
         <instance-id>FSContentDrivenPortletInstance</instance-id>
         <portlet-ref>FSContentDrivenPortlet</portlet-ref>
      </instance>
   </deployment>
   ...
</deployments

The portlet-instances.xml descriptor

<web-app>
   ...
   <context-param>
      <param-name>org.jboss.portal.content_type</param-name>
      <param-value>filesystem</param-value>
   </context-param>
   <context-param>
      <param-name>org.jboss.portal.portlet_instance</param-name>
      <param-value>FSContentDrivenPortletInstance</param-value>
   </context-param>
   <listener>
      <listener-class>org.jboss.content.ContentTypeRegistration</listener-class>
   </listener>
   ...
</web-app>

The web.xml descriptor