网络编程-轮询和长轮询
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
* <p>
* 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
* <p>
* 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);
}
}
