An example how to register `ProxyServlet` instances during runtime.

MetaProxyServlet.java

package pl.copernik.servlet;

import java.io.IOException;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Map.Entry;
import java.util.Objects;
import java.util.TreeMap;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
import javax.servlet.http.HttpServletResponse;
import org.mitre.dsmiley.httpproxy.ProxyServlet;

public class MetaProxyServlet extends HttpServlet {

   private static final long serialVersionUID = 1L;

   /**
    * To configure a {@link ProxyServlet} without modifying its code.
    */
   private static class ProxyServletConfig implements ServletConfig {

      private static final String  P_TARGET_URI = "targetUri";
      private final String         servletName;
      private final ServletContext servletContext;
      private final String         targetUri;

      public ProxyServletConfig(String servletName, ServletContext servletContext, String targetUri) {
         this.servletName = servletName;
         this.servletContext = servletContext;
         this.targetUri = targetUri;
      }

      @Override
      public String getServletName() {
         return servletName;
      }

      @Override
      public ServletContext getServletContext() {
         return servletContext;
      }

      @Override
      public String getInitParameter(String name) {
         return P_TARGET_URI.equals(name) ? targetUri : null;
      }

      @Override
      public Enumeration<String> getInitParameterNames() {
         return Collections.enumeration(Collections.singleton(P_TARGET_URI));
      }
   }

   /**
    * Serves to modify the pathInfo of the request.
    */
   private static class RewrittenServletRequest extends HttpServletRequestWrapper {

      private final String pathInfo;

      public RewrittenServletRequest(HttpServletRequest request, String pathInfo) {
         super(request);
         this.pathInfo = pathInfo;
      }

      @Override
      public String getPathInfo() {
         return pathInfo;
      }
   }

   private TreeMap<String, ProxyServlet> proxyServlets = new TreeMap<>();

   /**
    * Adds a reverse proxy between {@code pathPrefix} of this servlets namespace
    * and {@code targetUri}.
    * 
    * @param pathPrefix
    *        must start with '/'
    * @param targetUri
    *        URI to be proxied
    * @throws ServletException
    */
   public void addRedirect(String pathPrefix, String targetUri) throws ServletException {
      Objects.requireNonNull(pathPrefix);
      Objects.requireNonNull(targetUri);
      final ProxyServlet servlet = new ProxyServlet();
      servlet.init(new ProxyServletConfig(pathPrefix, getServletContext(), targetUri));
      proxyServlets.put(pathPrefix, servlet);
   }

   @Override
   protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
      String pathInfo = req.getPathInfo();
      Entry<String, ProxyServlet> entry = proxyServlets.floorEntry(pathInfo);
      // entry.getKey() precedes, but not necessarily is a prefix for pathInfo
      if (entry != null && pathInfo.startsWith(entry.getKey())) {
         String rewrittenPathInfo = pathInfo.substring(entry.getKey().length());
         entry.getValue()//
              .service(new RewrittenServletRequest(req, rewrittenPathInfo), resp);
      } else {
         resp.sendError(HttpServletResponse.SC_NOT_FOUND);
      }
   }

}