Long Polling长轮询实现进阶

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介绍

由于Long Polling长轮询详解 这篇文章中的code实现较为简单，尤其是服务端处理较为粗暴，有一些同学反馈希望服务端处理阻塞这块内容进行更深入讨论等等，所以这里专门补一篇实现进阶，让大家对长轮询有更加深刻的理解。

疑问

对上篇文章，同学反馈有两个疑问。

服务端实现使用的是同步servlet，性能比较差，能支撑的连接数比较少？

同步servlet来hold请求，确实会导致后续请求得不到及时处理，servlet3.0开始支持异步处理，可以更高效的处理请求。

服务端如何去hold住请求，sleep好吗？

同步servlet hold住请求的处理逻辑必须在servlet的doGet方法中，一般先fetch数据，准备好了，就返回，没准备好，就sleep片刻，再来重复。

异步servlet hold住请求比较简单，只要开启异步，执行完doGet方法后，不会自动返回此次请求，需要等到请求的context被complete，这样很巧妙的请求就自动hold住了。

实现

客户端实现

客户端实现基本和上篇差不多，没什么改变。

package com.andy.example.longpolling.client;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.net.HttpURLConnection;

import java.net.URL;

import java.util.concurrent.atomic.AtomicLong;

/**

* Created by andy on 17/7/8.

*/

public class AbstractBootstrap {

//同步URL

protected static final String URL = "http://localhost:8080/long-polling";

//异步URL

protected static final String ASYNC_URL = "http://localhost:8080/long-polling-async";

private final AtomicLong sequence = new AtomicLong();

protected void poll() {

//循环执行，保证每次longpolling结束，再次发起longpolling

while (!Thread.interrupted()) {

doPoll();

}

}

protected void doPoll() {

System.out.println("第" + (sequence.incrementAndGet()) + "次 longpolling");

long startMillis = System.currentTimeMillis();

HttpURLConnection connection = null;

try {

URL getUrl = new URL(URL);

connection = (HttpURLConnection) getUrl.openConnection();

//50s作为长轮询超时时间

connection.setReadTimeout(50000);

connection.setConnectTimeout(3000);

connection.setRequestMethod("GET");

connection.setUseCaches(false);

connection.setDoOutput(true);

connection.setDoInput(true);

connection.setRequestProperty("Content-Type", "application/json;charset=UTF-8");

connection.setRequestProperty("Accept-Charset", "application/json;charset=UTF-8");

connection.connect();

if (200 == connection.getResponseCode()) {

BufferedReader reader = null;

try {

reader = new BufferedReader(new InputStreamReader(connection.getInputStream(), "UTF-8"));

StringBuilder result = new StringBuilder(256);

String line = null;

while ((line = reader.readLine()) != null) {

result.append(line);

}

System.out.println("结果 " + result);

} finally {

if (reader != null) {

reader.close();

}

}

}

} catch (IOException e) {

System.out.println("request failed");

} finally {

long elapsed = (System.currentTimeMillis() - startMillis) / 1000;

System.out.println("connection close" + " " + "elapse " + elapsed + "s");

if (connection != null) {

connection.disconnect();

}

System.out.println();

}

}

}

package com.andy.example.longpolling.client;

import java.io.IOException;

/**

* Created by andy on 17/7/6.

*/

public class ClientBootstrap extends AbstractBootstrap {

public static void main(String[] args) throws IOException {

ClientBootstrap bootstrap = new ClientBootstrap();

//发起longpolling

bootstrap.poll();

System.in.read();

}

}

服务端实现

长轮询服务端同步servlet处理

服务端同步servlet和上篇差不多，没什么改动，增加了相关注释。

package com.andy.example.longpolling.server;

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 java.io.IOException;

import java.io.PrintWriter;

import java.util.Random;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.atomic.AtomicLong;

/**

* Created by andy on 17/7/6.

*/

@WebServlet(urlPatterns = "/long-polling")

public class LongPollingServlet extends HttpServlet {

private final Random random = new Random();

private final AtomicLong sequence = new AtomicLong();

private final AtomicLong value = new AtomicLong();

@Override

protected void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

System.out.println();

final long currentSequence = sequence.incrementAndGet();

System.out.println("第" + (currentSequence) + "次 longpolling");

//由于客户端设置的超时时间是50s，

//为了更好的展示长轮询，这边random 100，模拟服务端hold住大于50和小于50的情况。

//再具体场景中，这块在具体实现上，

//对于同步servlet，首先这里必须阻塞，因为一旦doGet方法走完，容器就认为可以结束这次请求，返回结果给客户端。

//所以一般实现如下：

// while(结束){ //结束条件，超时或者拿到数据

// data = fetchData();

// if(data == null){

// sleep();

// }

// }

int sleepSecends = random.nextInt(100);

System.out.println(currentSequence + " wait " + sleepSecends + " second");

try {

TimeUnit.SECONDS.sleep(sleepSecends);

} catch (InterruptedException e) {

}

PrintWriter out = response.getWriter();

long result = value.getAndIncrement();

out.write(Long.toString(result));

out.flush();

}

}

长轮询服务端异步servlet处理

由于同步servlet，性能较差，所有的请求操作必须在doGet方法中完成，包括等待数据，占用了容器的处理线程，会导致后续的请求阻塞，来不及处理。servlet 3.0支持异步处理，使用异步处理doGet方法执行完成后，结果也不会返回到客户端，会等到请求的context被complete才会写回客户端，这样一来，容器的处理线程不会受阻，请求结果可由另外的业务线程进行写回，也就轻松实现了hold操作。

package com.andy.example.longpolling.server;

import javax.servlet.*;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Random;

import java.util.concurrent.*;

import java.util.concurrent.atomic.AtomicLong;

/**

* Created by andy on 17/7/7.

*/

/**

* 开启异步servlet，asyncSupported = true

*/

@WebServlet(urlPatterns = "/long-polling-async", asyncSupported = true)

public class LongPollingAsyncServlet extends HttpServlet {

private Random random = new Random();

private final AtomicLong sequence = new AtomicLong();

private final AtomicLong value = new AtomicLong();

private static ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L,

TimeUnit.MILLISECONDS, new ArrayBlockingQueue(100));

@Override

protected void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

System.out.println();

final long currentSequence = sequence.incrementAndGet();

System.out.println("第" + (currentSequence) + "次 longpolling async");

//设置request异步处理

AsyncContext asyncContext = request.startAsync();

//异步处理超时时间，这里需要注意，jetty容器默认的这个值设置的是30s，

//如果超时，异步处理没有完成(通过是否asyncContext.complete()来进行判断)，将会重试(会再次调用doGet方法)。

//这里由于客户端long polling设置的是50s，所以这里如果小于50，会导致重试。

asyncContext.setTimeout(51000);

asyncContext.addListener(new AsyncListener() {

@Override

public void onComplete(AsyncEvent event) throws IOException {

}

//超时处理，注意asyncContext.complete();，表示请求处理完成

@Override

public void onTimeout(AsyncEvent event) throws IOException {

AsyncContext asyncContext = event.getAsyncContext();

asyncContext.complete();

}

@Override

public void onError(AsyncEvent event) throws IOException {

}

@Override

public void onStartAsync(AsyncEvent event) throws IOException {

}

});

//提交线程池异步写会结果

//具体场景中可以有具体的策略进行操作

executor.submit(new HandlePollingTask(currentSequence, asyncContext));

}

class HandlePollingTask implements Runnable {

private AsyncContext asyncContext;

private long sequense;

public HandlePollingTask(long sequense, AsyncContext asyncContext) {

this.sequense = sequense;

this.asyncContext = asyncContext;

}

@Override

public void run() {

try {

//通过asyncContext拿到response

PrintWriter out = asyncContext.getResponse().getWriter();

int sleepSecends = random.nextInt(100);

System.out.println(sequense + " wait " + sleepSecends + " second");

try {

TimeUnit.SECONDS.sleep(sleepSecends);

} catch (InterruptedException e) {

}

long result = value.getAndIncrement();

out.write(Long.toString(result));

} catch (Exception e) {

System.out.println(sequense + "handle polling failed");

} finally {

//数据写回客户端

asyncContext.complete();

}

}

}

}

结果

同步servlet实现结果

同步servlet客户端结果

同步servlet服务端结果

异步servlet实现结果

异步servlet客户端结果

异步servlet服务端结果

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