Java实现一个简单的长轮询的示例代码

 @Slf4j
 public class ConfigClientWorker {
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     private final CloseableHttpClient httpClient;
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     private final ScheduledExecutorService executorService;
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     public ConfigClientWorker(String url, String dataId) {
         this.executorService = Executors.newSingleThreadScheduledExecutor(runnable -> {
             Thread thread = new Thread(runnable);
             thread.setName("client.worker.executor-%d");
             thread.setDaemon(true);
             return thread;
         });
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         // ① httpClient 客户端超时时间要大于长轮询约定的超时时间
         RequestConfig requestConfig = RequestConfig.custom().setSocketTimeout(40000).build();
         this.httpClient = HttpClientBuilder.create().setDefaultRequestConfig(requestConfig).build();
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         executorService.execute(new LongPollingRunnable(url, dataId));
     }
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     class LongPollingRunnable implements Runnable {
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         private final String url;
         private final String dataId;
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         public LongPollingRunnable(String url, String dataId) {
             this.url = url;
             this.dataId = dataId;
         }
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         @SneakyThrows
         @Override
         public void run() {
             String endpoint = url + "?dataId=" + dataId;
             log.info("endpoint: {}", endpoint);
             HttpGet request = new HttpGet(endpoint);
             CloseableHttpResponse response = httpClient.execute(request);
             switch (response.getStatusLine().getStatusCode()) {
                 case 200: {
                     BufferedReader rd = new BufferedReader(new InputStreamReader(response.getEntity()
                             .getContent()));
                     StringBuilder result = new StringBuilder();
                     String line;
                     while ((line = rd.readLine()) != null) {
                         result.append(line);
                     }
                     response.close();
                     String configInfo = result.toString();
                     log.info("dataId: [{}] changed, receive configInfo: {}", dataId, configInfo);
                     break;
                 }
                 // ② 304 响应码标记配置未变更
                 case 304: {
                     log.info("longPolling dataId: [{}] once finished, configInfo is unchanged, longPolling again", dataId);
                     break;
                 }
                 default: {
                     throw new RuntimeException("unExcepted HTTP status code");
                 }
             }
             executorService.execute(this);
         }
     }
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     public static void main(String[] args) throws IOException {
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         new ConfigClientWorker("http://127.0.0.1:8080/listener", "user");
         System.in.read();
     }
 }

 @RestController
 @Slf4j
 @SpringBootApplication
 public class ConfigServer {
 ​
     @Data
     private static class AsyncTask {
         // 长轮询请求的上下文，包含请求和响应体
         private AsyncContext asyncContext;
         // 超时标记
         private boolean timeout;
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         public AsyncTask(AsyncContext asyncContext, boolean timeout) {
             this.asyncContext = asyncContext;
             this.timeout = timeout;
         }
     }
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     // guava 提供的多值 Map，一个 key 可以对应多个 value
     private Multimap<String, AsyncTask> dataIdContext = Multimaps.synchronizedSetMultimap(HashMultimap.create());
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     private ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("longPolling-timeout-checker-%d")
             .build();
     private ScheduledExecutorService timeoutChecker = new ScheduledThreadPoolExecutor(1, threadFactory);
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     // 配置监听接入点
     @RequestMapping("/listener")
     public void addListener(HttpServletRequest request, HttpServletResponse response) {
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         String dataId = request.getParameter("dataId");
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         // 开启异步！！！
         AsyncContext asyncContext = request.startAsync(request, response);
         AsyncTask asyncTask = new AsyncTask(asyncContext, true);
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         // 维护 dataId 和异步请求上下文的关联
         dataIdContext.put(dataId, asyncTask);
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         // 启动定时器，30s 后写入 304 响应
         timeoutChecker.schedule(() -> {
             if (asyncTask.isTimeout()) {
                 dataIdContext.remove(dataId, asyncTask);
                 response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
               // 标志此次异步线程完成结束！！！
                 asyncContext.complete();
             }
         }, 30000, TimeUnit.MILLISECONDS);
     }
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     // 配置发布接入点
     @RequestMapping("/publishConfig")
     @SneakyThrows
     public String publishConfig(String dataId, String configInfo) {
         log.info("publish configInfo dataId: [{}], configInfo: {}", dataId, configInfo);
         Collection<AsyncTask> asyncTasks = dataIdContext.removeAll(dataId);
         for (AsyncTask asyncTask : asyncTasks) {
             asyncTask.setTimeout(false);
             HttpServletResponse response = (HttpServletResponse)asyncTask.getAsyncContext().getResponse();
             response.setStatus(HttpServletResponse.SC_OK);
             response.getWriter().println(configInfo);
             asyncTask.getAsyncContext().complete();
         }
         return "success";
     }
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     public static void main(String[] args) {
         SpringApplication.run(ConfigServer.class, args);
     }
 }

 16:41:14.824 [client.worker.executor-%d] INFO cn.haoxiaoyong.poll.ConfigClientWorker - longPolling dataId: [user] once finished, configInfo is unchanged, longPolling again

 2022-08-25 16:45:56.663  INFO 90650 --- [nio-8080-exec-2] cn.haoxiaoyong.poll.ConfigServer         : publish configInfo dataId: [user], configInfo: helloworld