Core servlets chapter 6 Generating server response: http status codes

A request GET /…/someName HTTP/1.1 Host:…. Header2:… … HeaderN:…

A response HTTP/ OK Content-type: text/html Header2:… … HeaderN:… (blank line) …

(some) HTTP 1.1 status codes – information to the client to take some further action – request was successful – files have moved LOCATION header may provide new address – client error – server error In servlets, you send the code using response.SC_SOME_MESSCODE

Status codes and responses In response to 100 (Expect request header with value Continue) the server could send 100 SC_Continue – client should go ahead or 417 SC_EXPECTATION_FAILED – it won’t accept the document 200 OK – the default for servlets. 202 Accepted, send SC_ACCEPTED to indicate request is still being processed. 204 No Content – send SC_NO_CONTENT to indicate browser should continue to display previous document. Useful if the client is reloading but you can tell that the doc is up to date. 205 reset is used to clear form fields. Send SC_RESET_CONTENT, new in HTTP Moved Permanently and 302 Found: SC_MOVED_PERMANENTLY typically sends a Location header with the new URL. 302 may indicate a temporary move. Servlet uses SC_MOVED_TEMPORARILY

More codes 303 (See Other)

Redirect vs refresh In 302 the browser automatically loads the URL from the location header. Track user behavior by routing them to a link on your own site and then recording what they select Side effects – you can return both a Set-Cookie response header via response.addCookie and a 302 status code by means of a response.sendRedirect(url). This latter is shorter and easier than using both response.setStatus(response.SC_MOVED_TEMPORAR ILY) and respnse.setHeader(“Location”,url)

Wrongdestination servlet sends you to…

The servlet that presents a form

After submitting query

I modified this to count hits to various engines using a file (see Hunter chapt 3 notes) Yahoo 3 MSN 1 Google 1 AllTheWeb 1 AltaVista 0 HotBot 0 Lycos 1

First create a file – this servlet could be run on startup… or file could be built manually import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class WriteToFile extends GenericServlet { public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { res.setContentType("text/html"); PrintWriter out = res.getWriter(); out.println("starting to write"); FileWriter fileWriter = null; PrintWriter printWriter = null; try { fileWriter = new FileWriter("EngineCounter.initial"); printWriter = new PrintWriter(fileWriter); for(int j=0;j<7;j++) printWriter.println("0"); return; } catch (IOException e) { // problem during write // Log the exception. See Chapter 5. out.println("exception"+e.toString()); } finally { // Make sure to close the file if (printWriter != null) { printWriter.close();} out.println("writing complete.."); //and don’t forget to close the file //fout.close();}} public String getServletInfo() { return "A servlet that writes to a file in work/ dir"; }}

logic Next, count “hits” to engine by searching a string array for the name of the engine selected. Read file contents, increment one value Re-write file contents My array: String names[]={"Yahoo","MSN","Google","AllTheWeb","AltaVista","HotBot", "Lycos"}; I didn’t write it, but another servlet could be run at the end of the day to view engine-hit distribution

My update file function in the redirect servlet public void update_file(String name)throws IOException{ int pos=-1; for(int k=0;k<names.length;k++)if(name.equals(names[k])){pos=k; System.out.println("name is "+name+" pos is"+pos);} int ct[]=new int[names.length]; int i=0; FileInputStream fin=new FileInputStream("C:/apache-tomcat /apache-tomcat /bin/enginecounter.initial"); Scanner s=new Scanner(fin); while(s.hasNext()){ ct[i]=s.nextInt(); i++; } fin.close(); if(pos!=-1)ct[pos]++; FileWriter fileWriter = new FileWriter("EngineCounter.initial"); PrintWriter printWriter = new PrintWriter(fileWriter); for(int j=0;j<7;j++) printWriter.println(ct[j]); if (printWriter != null) { printWriter.close(); }//if }//update