Using the NetBeans Platform on the Server with Wicket on the Client

package it.tidalwave.netbeans.servlet;

import java.io.IOException;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import org.openide.util.Lookup;

public class NetBeansPlatformDecoratorServlet implements Servlet
  {
    private ServletConfig config;
    private Servlet delegate;

    public void init (final ServletConfig config)
      throws ServletException
      {
        try
          {
            NetBeansPlatformUtils.boot(config.getServletContext());
          }
        catch (Exception e)
          {
            throw new ServletException(e);
          }

        final ClassLoader classLoaderSave = NetBeansPlatformUtils.installPlatformClassLoader();

        try
          {
            this.config = config;
            final String className = config.getInitParameter("netbeans.servlet.delegate");
            final Class delegateClass = Thread.currentThread().getContextClassLoader().loadClass(className);
            delegate = (Servlet)Lookup.getDefault().lookup(delegateClass);

            if (delegate == null)
              {
                throw new RuntimeException("Can't lookup " + className);
              }

            delegate.init(config);
          }
        catch (Exception e)
          {
            throw new ServletException(e);
          }
        finally
          {
            Thread.currentThread().setContextClassLoader(classLoaderSave);
          }
      }

    public ServletConfig getServletConfig()
      {
        return delegate.getServletConfig();
      }

    public void service (final ServletRequest req, final ServletResponse res)
      throws ServletException, IOException
      {
        final ClassLoader classLoaderSave = NetBeansPlatformUtils.installPlatformClassLoader();

        try
          {
            delegate.service(req, res);
          }
        catch (Exception e)
          {
            throw new ServletException(e);
          }
        finally
          {
            Thread.currentThread().setContextClassLoader(classLoaderSave);
          }
      }

    public String getServletInfo()
      {
        return delegate.getServletInfo();
      }

    public void destroy()
      {
        final ClassLoader classLoaderSave = NetBeansPlatformUtils.installPlatformClassLoader();

        try
          {
            delegate.destroy();
          }
        finally
          {
            Thread.currentThread().setContextClassLoader(classLoaderSave);
          }
      }
  }

These are the main concepts:

1. As usual for decorators, the behaviour is forwarded to a delegate.
2. The decoration consists in setting the context class loader (Thread.currentThread().getClassLoader()) from the NetBeans Platform and restoring it at the end of the method call (taking care of using a finally block for safety). The details are implemented in the NetBeansPlatformUtils class, discussed below.
3. The delegate instance is retrieved by asking it to the Lookup class, as usual for NetBeans Platform applications (this is why you had to declare all the relevant Servlets and Filters in META-INF/services as previously described). While the common idiom is to pass the class literal as argument (e.g. MyService s = Lookup.getDefault().lookup(MyService.class)), here we can't do that for two reasons: first, because we want the code to be universal, so the class name must be passed in a parameter ("netbeans.servlet.delegate"); and second, because we couldn't get the literal in any way, since it would be never available in the static compilation classpath (there are no good ways to have a .war statically include the Platform stuff; if you tried to extract the .jar files from the .nbm modules - which per se is a cumbersome approach - you would face possible ClassCastExceptions since you would have the same class available in both Tomcat and the Platform class loaders - remember that the same bytecode in two different classloaders is treated as two different classes).
4. Last but not least, at the first invocation of a Servlet or Filter delegate you must initialize the Platform: that's the task of NetBeansPlatformUtils.boot().