HTTP轮询模型
长短轮询
http协议是一种client-server模型的应用层协议，这种c-s的模式虽然大多数情况都能满足需求，但是某些场景也需要服务端能够将一些信息实时的推送到客户端，即实现服务器向客户端推消息的功能。
比如：
- 配置管理中心服务端需要将更新的配置推送到client端
- 消息推送服务器需要将一些消息推送到Android、iOS客户端
利用Http协议实现服务器推送有两种常见的思路：
短轮询拉
客户端不停的去向服务器发送轮询请求，如果有数据更新，客户端能也能尽快（取决于轮询间隔）获取最新的数据，这种方式被称为Http短轮询。
这种方式有如下缺点：
- 因为是短轮询，因此一定时间t内需要进行轮询的次数就更多，而Http的连接是需要tcp三次握手等资源开销的。
- 由图可以看出，但是服务端数据发生更新时，客户端并不是立刻收到更新的数据（除去网络传输仍然还需要时间），而只能是在下一次轮询的时候才能感知到数据的变更。
长轮询推
长轮询的思路是这样的：尽量减少轮询的次数，从而减少资源开销。为了减少轮询次数，那么每次轮询的时间跨度就需要比较长，因此成为长轮询，同时也希望长轮询模型的每一次轮询效率要高于短轮询。
长轮询模型有这么几个特征：
- 每次轮询的间隔不固定
- 服务器对每次轮询做出响应的条件是：超时或者数据更新
- 长轮询模型中，客户端能实时感知到服务器端数据更新
由于很多服务器都具有异步处理连接的能力，因此图中的阻塞消耗的资源比较小。
异步Servlet
下面是利用Servlet规范中提供的异步Servlet作为服务端的Http长轮询模型，实现了客户端能实时获取服务端某个配置文件内容。
异步Servlet是Servlet3.0出来的新特性，对于需要异步处理的连接，Servlet引擎会将处理该请求的工作线程回收进工作线程池，而不是阻塞在该请求上。
package httplongconnection;
import java.io.File;
import java.io.FileFilter;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import javax.servlet.AsyncContext;
import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.commons.codec.digest.DigestUtils;
import org.apache.commons.io.monitor.FileAlterationListener;
import org.apache.commons.io.monitor.FileAlterationMonitor;
import org.apache.commons.io.monitor.FileAlterationObserver;
@WebServlet(urlPatterns = "/long", asyncSupported = true)
public class HttpLongConnectionServlet extends HttpServlet {
/**
*
*/
private static final long serialVersionUID = 1L;
static FileAlterationObserver observer;
static {
FileAlterationMonitor monitor = new FileAlterationMonitor(1000L);// 每隔1000毫秒扫描一次
// 需要监听的文件目录
observer = new FileAlterationObserver(new File("E:/J2EE_workspace/httplongconnection/src/main/resources"), new FileFilter() {
public boolean accept(File pathname) {
// TODO Auto-generated method stub
return true;
}
});
System.out.println("observer");
monitor.addObserver(observer);
try {
monitor.start();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
// TODO Auto-generated method stub
final AsyncContext ctx = req.startAsync();
ctx.addListener(new AsyncListener() {
public void onTimeout(AsyncEvent event) throws IOException {
// TODO Auto-generated method stub
ctx.complete();
observer.removeListener((FileAlterationListener)ctx.getRequest().getAttribute("fileListener"));
}
public void onStartAsync(AsyncEvent event) throws IOException {
// TODO Auto-generated method stub
}
public void onError(AsyncEvent event) throws IOException {
// TODO Auto-generated method stub
}
public void onComplete(AsyncEvent event) throws IOException {
// TODO Auto-generated method stub
observer.removeListener((FileAlterationListener)ctx.getRequest().getAttribute("fuck"));
}
});
ctx.setTimeout(50 * 1000);
new Thread(new BizProcessor(ctx)).start();
}
class BizProcessor implements Runnable {
private String checkSum;
private AsyncContext asyncContext;
private boolean checkSumEqual(InputStream is, String originalCheckSum) {
try {
String digest = DigestUtils.md5Hex(is);
if (digest.equals(originalCheckSum)) {
return true;
}
this.checkSum = digest;
System.out.println(
"File has changed. new md5 is " + this.checkSum + ", old checsum is " + originalCheckSum);
return false;
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return false;
}
public BizProcessor(AsyncContext asyncContext) {
super();
this.asyncContext = asyncContext;
}
public void run() {
// TODO Auto-generated method stub
// sleep
HttpServletRequest req = (HttpServletRequest) asyncContext.getRequest();
String cSum = String.valueOf(req.getParameter("checkSum"));
// 文件update
InputStream is = null;
try {
is = new FileInputStream(new File("E:/J2EE_workspace/httplongconnection/src/main/resources/config.txt"));
} catch (FileNotFoundException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
if (is != null && !checkSumEqual(is, cSum)) {
try {
is.close();
is = this.getClass().getClassLoader().getResourceAsStream("config.txt");
PrintWriter out = asyncContext.getResponse().getWriter();
String content = org.apache.commons.io.IOUtils.toString(is, "UTF-8");
System.out.println(content);
out.write(checkSum + "\002" + content);
out.flush();
out.close();
is.close();
} catch (IOException e) {
e.printStackTrace();
}
asyncContext.complete();
}
System.out.println("register");
register(asyncContext);
// 没有发生文件更新，则等待超时发生
}
private void register(AsyncContext ctx) {
// TODO Auto-generated method stub
FileListerAdapter listner = new FileListerAdapter(ctx);
ctx.getRequest().setAttribute("fileListener", listner);
observer.addListener(listner);
}
}
}
<html>
<script src="http://code.jquery.com/jquery-latest.js">script>
<script src="md5.js">script>
<body>
<h2>Hello World!h2>
<textarea id="show" rows="40" cols="80">
textarea>
body>
<script type="text/javascript">
var cSum = "0";
poll();
function poll() {
$.ajax({
url: "/httplongconnection/long",
data : {"checkSum" : cSum},
success: function(response) {
if (response == null || response.length == 0) {
poll();
return;
}
var msg = response.split("\002");
var checkSum = md5(msg[1]), content = msg[1];
if (checkSum != cSum) {
cSum = checkSum;
$("#show").val(content);
}
poll();
}});
}
script>
html>
package httplongconnection;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import javax.servlet.AsyncContext;
import org.apache.commons.codec.digest.DigestUtils;
import org.apache.commons.io.monitor.FileAlterationListenerAdaptor;
public class FileListerAdapter extends FileAlterationListenerAdaptor {
AsyncContext ctx;
public FileListerAdapter(AsyncContext ctx) {
super();
this.ctx = ctx;
}
@Override
public void onFileChange(File file) {
if (!file.exists() || !file.canRead()) {
System.out.println("The file " + file + " is not exists or is not readable!");
return;
}
try {
InputStream is = new FileInputStream(file);
if (ctx.getResponse().isCommitted()) {
return;
}
PrintWriter out = ctx.getResponse().getWriter();
String content = org.apache.commons.io.IOUtils.toString(is, "UTF-8");
System.out.println(content);
String digest = DigestUtils.md5Hex(is);
out.write(digest + "\002" + content);
System.out.println("yy");
System.out.println(content);
is.close();
out.flush();
out.close();
} catch (IOException e) {
e.printStackTrace();
}
ctx.complete();
//TODO 读取操作
super.onFileChange(file);
}
@Override
public void onFileCreate(File file) {
//TODO 读取操作
super.onFileCreate(file);
}
@Override
public void onFileDelete(File file) {
super.onFileDelete(file);
}
@Override
public void onDirectoryChange(File directory) {
System.out.println("----The directory " + directory + " has changed.");
super.onDirectoryChange(directory);
}
@Override
public void onDirectoryCreate(File directory) {
super.onDirectoryCreate(directory);
}
@Override
public void onDirectoryDelete(File directory) {
super.onDirectoryDelete(directory);
}
}