轮询(Polling)：是指不管服务器端有没有更新，客户端(通常是指浏览器)都定时的发送请求进行查询，轮询的结果可能是服务器端有新的更新过来，也可能什么也没有，只是返回个空的信息。不管结果如何，客户端处理完后到下一个定时时间点将继续下一轮的轮询。

长轮询(Long Polling)：长轮询的服务其客户端是不做轮询的，客户端在发起一次请求后立即挂起，一直到服务器端有更新的时候，服务器才会主动推送信息到客户端。 在服务器端有更新并推送信息过来之前这个周期内，客户端不会有新的多余的请求发生，服务器端对此客户端也啥都不用干，只保留最基本的连接信息，一旦服务器有更新将推送给客户端，客户端将相应的做出处理，处理完后再重新发起下一轮请求。

可见，长轮询的特点：

服务器端会阻塞请求直到有数据传递或超时才返回.

客户端响应处理函数会在处理完服务器返回的信息后，再次发出请求，重新建立连接.

当客户端处理接收的数据、重新建立连接时，服务器端可能有新的数据到达；这些信息会被服务器端保存直到客户端重新建立连接，客户端会一次把当前服务器端所有的信息取回。

Java-长轮询(Long polling)实现

服务端

package _20200418.example;

import com.sun.net.httpserver.HttpServer;

import java.io.IOException;

import java.io.OutputStream;

import java.net.InetSocketAddress;

import java.nio.charset.StandardCharsets;

import java.util.Random;

import java.util.concurrent.Executors;

import java.util.concurrent.TimeUnit;

import java.util.concurrent.atomic.AtomicLong;

/**

* 长轮询服务端

*

* Created by zfh on 2020/04/18

*/

public class server {

private final AtomicLong value = new AtomicLong();

private void start() throws IOException {

HttpServer httpServer = HttpServer.create(new InetSocketAddress(8080), 0);

httpServer.setExecutor(Executors.newCachedThreadPool());

httpServer.createContext("/long-polling", httpExchange -> {

byte[] data = fetchData();

httpExchange.sendResponseHeaders(200, data.length);

OutputStream outputStream = httpExchange.getResponseBody();

outputStream.write(data);

outputStream.close();

httpExchange.close();

});

httpServer.start();

}

private byte[] fetchData() {

try {

// 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。

Random random = new Random();

TimeUnit.SECONDS.sleep(random.nextInt(100));

} catch (InterruptedException ignored) {

}

return Long.toString(value.getAndIncrement()).getBytes(StandardCharsets.UTF_8);

}

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

server server = new server();

server.start();

}

}

客户端

package _20200418.example;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

import java.net.HttpURLConnection;

import java.net.URL;

import java.nio.charset.StandardCharsets;

import java.util.concurrent.atomic.AtomicLong;

/**

* 长轮询-客户端

*

* Created by zfh on 2020/04/18

*/

public class client {

private static final String SYNC_URL = "http://localhost:8080/long-polling";

private final AtomicLong sequence = new AtomicLong();

void poll() {

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

// 结束条件，超时或者拿到数据

while (!Thread.interrupted()) {

doPoll();

}

}

private void doPoll() {

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

long startMillis = System.currentTimeMillis();

HttpURLConnection connection = null;

try {

URL getUrl = new URL(SYNC_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(), StandardCharsets.UTF_8));

StringBuilder result = new StringBuilder(256);

String line;

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();

}

}

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

client bootstrap = new client();

bootstrap.poll();

Thread.sleep(Integer.MAX_VALUE);

}

}