Simple Proxy Servlet Example for Slow Requests to Prevent Timeout
In java if you are dealing with slow requests to a server but you need a fast response but you can't modify the code that produces that response, one solution is to create a proxy that will call that final URL for you and tell you when it's finished.
Here you have one Servlet example that does that. Mind the following features:
- It needs to be in the same application.
- It allows GET and POST data.
- It keeps cookie headers (tomcat session is not lost).
- Additional headers will be lost.
- It will only accept one request per session.
- It will only spawn a threat per request.
- If more than one different requests are sent before the final call has finished it will response the most recent one.
- If the user goes to a different page while the proxy hasn't finished the request, the response will be kept in session.
- Once the final URL request is finished it can be called again.
- It will send and extra parameter to the final URL.
In this example a JSP called slow.jsp will be called. This JSP is meant to last 15 seconds to complete. If called with the SimpleProxyServlet a processing.jsp page will be shown every 3 seconds until the slow.jsp is finished.
Example working
Download war file with sources here
SimpleProxyServlet.java
package com.test;
import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.Enumeration;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.net.HttpURLConnection;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
public class SimpleProxyServlet extends HttpServlet {
private static final long serialVersionUID = 2110292828406248147L;
//this servlet proxy should always receive this parameter
public static final String PARAM_PROXY= "proxy_final_url";
//will wait the first time
private static final long WAIT_THE_FIRST_TIME = 500l;
//process that will keep calling itself until the final url is loaded
private static final String JSP_PROCESS = "/processing.jsp";
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response, null);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
@SuppressWarnings("unchecked")
//this will get all post parameter data
Enumeration<String> parameterNames = request.getParameterNames();
StringBuilder sb = new StringBuilder();
while (parameterNames.hasMoreElements()) {
String paramName = parameterNames.nextElement();
sb.append(paramName + "=");
sb.append(request.getParameter(paramName) );
if(parameterNames.hasMoreElements()){
sb.append("&");
}
}
processRequest(request, response, sb.toString());
}
private void processRequest(HttpServletRequest request, HttpServletResponse response, String postData)
throws ServletException, IOException {
PrintWriter out = response.getWriter();
String urlParam = (String)request.getParameter(PARAM_PROXY);
HttpSession session = request.getSession(true);
AsynchornousReq prevAsReq = null;
if(session.getAttribute("userId")!=null ){
prevAsReq = (AsynchornousReq) session.getAttribute("userId");
}
response.setContentType("text/html");
// this will create the final URL inside your proyect
String final_url = request.getScheme() + "://" +
request.getServerName() + ":" +
request.getServerPort() +
request.getContextPath() + "/" + urlParam ;
// there is previous request, then it should be the first
if(prevAsReq==null){
// if the final URL parameter is empty it should return error
if(urlParam==null){
response.setStatus(HttpServletResponse.SC_BAD_REQUEST);
out.print("Error " + PARAM_PROXY + " = null");
return;
}
// creates an Asynchronous URL load request
AsynchornousReq asReq = new AsynchornousReq(final_url, request.getQueryString(), postData);
// the first time waits in case the response is not long enough
// fell free to remove this..
try {
Thread.sleep(WAIT_THE_FIRST_TIME);
} catch (InterruptedException e) { e.printStackTrace(); }
if(asReq.isFinished()){
out.print(asReq.getResponse());
response.setStatus(asReq.getResponseCode());
return;
// if hasn't finished therefore is an slow request
} else {
session.setAttribute("userId", asReq);
}
// this request is already in session
} else {
if(prevAsReq.isFinished()){
session.setAttribute("userId", null);
out.print(prevAsReq.getResponse());
response.setStatus(prevAsReq.getResponseCode());
return;
// in case another request different from the original has been requested
} else if(urlParam!=null && !prevAsReq.getUrl().equals(final_url)) {
AsynchornousReq as = new AsynchornousReq(final_url, request.getQueryString(), postData);
session.setAttribute("userId", as);
}
}
request.getRequestDispatcher(JSP_PROCESS).forward(request,response);
}
/**
* This class will create an asynchronous call to an URL
* It will get the response code from the final URL
* boolean finished will be set to true when done
* @author jcgonzalez.com
*/
class AsynchornousReq {
AsynchornousReq(String final_url, String urlGetParams, String postData){
this.url = final_url;
this.urlGetParams = urlGetParams;
this.postData = postData;
loadURL();
}
private String url;
private boolean finished = false;
private Integer responseCode = HttpServletResponse.SC_OK;
private String response = null;
private String urlGetParams = null;
private String postData = null;
private void loadURL(){
ExecutorService executorService = Executors.newSingleThreadExecutor();
executorService.execute(new Runnable() {
public void run() {
response= getResponse(url);
finished = true;
}
});
executorService.shutdown();
}
private String getResponse(String url) {
StringBuilder response = new StringBuilder();
try {
if(urlGetParams!=null){
url=url+"?"+urlGetParams;
}
URL obj = new URL(url);
HttpURLConnection con = (HttpURLConnection) obj.openConnection();
// Send post request
con.setDoOutput(true);
if(postData!=null){
DataOutputStream wr = new DataOutputStream(con.getOutputStream());
wr.writeBytes(postData);
wr.flush();
wr.close();
}
responseCode = con.getResponseCode();
BufferedReader in = new BufferedReader(
new InputStreamReader(
con.getInputStream()));
String inputLine;
while ((inputLine = in.readLine()) != null)
response.append(inputLine);
in.close();
} catch (MalformedURLException e) {
StringWriter sw = new StringWriter();
e.printStackTrace(new PrintWriter(sw));
response.append(sw);
} catch (IOException e) {
StringWriter sw = new StringWriter();
e.printStackTrace(new PrintWriter(sw));
response.append(sw);
}
return ""+response.toString();
}
public boolean isFinished() {
return finished;
}
public String getResponse() {
return response;
}
public String getUrl() {
return url;
}
public void setUrlGetParams(String urlGetParams) {
this.urlGetParams = urlGetParams;
}
public void setPostData(String postData) {
this.postData = postData;
}
public Integer getResponseCode() {
return responseCode;
}
}
}
processing.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<%@ page import="java.util.Set"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<meta http-equiv="refresh" content="4">
<title>Página Procesando</title>
</head>
<body>
<br/><br/>
<span style="font-family : Verdana, Arial;font-size:10px; font-weight : bold;color: #052878;">
Processing slow request, please wait...
</span>
<br/>
<div id="container" style="width:99%; height:5px; border:1px solid black;">
<div id="progress-bar" style="width:10%;background-color: #E5E6F0; height:5px;"></div>
</div>
<script>
var width = 0;
window.onload = function(e){
setInterval(function () {
width = width >= 100 ? 0 : width+5;
document.getElementById('progress-bar').style.width = width + '%'; }, 200);
};
</script>
</body>
Notes
- For a much more developed proxy Serlvet (you would need to adapt it for the slow response feature) go to https://github.com/mitre/HTTP-Proxy-Servlet
