实例4(搜索引擎）
package coreservlets;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.net.*;


public class SearchEngines extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    String searchString = request.getParameter("searchString");
    if ((searchString == null) ||
        (searchString.length() == 0)) {
      reportProblem(response, "Missing search string.");
      return;
    }
    // The URLEncoder changes spaces to "+" signs and other
    // non-alphanumeric characters to "%XY", where XY is the
    // hex value of the ASCII (or ISO Latin-1) character.
    // Browsers always URL-encode form values, so the
    // getParameter method decodes automatically. But since
    // were just passing this on to another server, we need to
    // re-encode it.
    searchString = URLEncoder.encode(searchString);
    String numResults = request.getParameter("numResults");
    if ((numResults == null) ||
        (numResults.equals("0")) ||
        (numResults.length() == 0)) {
      numResults = "10";
    }
    String searchEngine =
      request.getParameter("searchEngine");
    if (searchEngine == null) {
      reportProblem(response, "Missing search engine name.");
      return;
    }
    SearchSpec[] commonSpecs = SearchSpec.getCommonSpecs();
    for(int i=0; i<commonSpecs.length; i++) {
      SearchSpec searchSpec = commonSpecs[i];
      if (searchSpec.getName().equals(searchEngine)) {
        String url =
          searchSpec.makeURL(searchString, numResults);
        response.sendRedirect(url);
        return;
      }
    }
    reportProblem(response, "Unrecognized search engine.");
  }

  private void reportProblem(HttpServletResponse response,
                             String message)
      throws IOException {
    response.sendError(response.SC_NOT_FOUND,
                       "<H2>" + message + "</H2>");
  }
  
  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
    doGet(request, response);
  }
}