public NacosConfigService(Properties properties) throws NacosException {
    ValidatorUtils.checkInitParam(properties);
    String encodeTmp = properties.getProperty(PropertyKeyConst.ENCODE);
    if (StringUtils.isBlank(encodeTmp)) {
        this.encode = Constants.ENCODE;
    } else {
        this.encode = encodeTmp.trim();
    }
    initNamespace(properties); //
    this.configFilterChainManager = new ConfigFilterChainManager(properties);
    //初始化網(wǎng)絡(luò)通信組件
    this.agent = new MetricsHttpAgent(new ServerHttpAgent(properties));
    this.agent.start(); 
    //初始化ClientWorker
    this.worker = new ClientWorker(this.agent, this.configFilterChainManager, properties);
}

public ClientWorker(final HttpAgent agent, final ConfigFilterChainManager configFilterChainManager,
        final Properties properties) {
    this.agent = agent;
    this.configFilterChainManager = configFilterChainManager; //初始化配置過濾管理器
    // Initialize the timeout parameter
    init(properties); //初始化配置
    //初始化一個定時調(diào)度的線程池，重寫了threadfactory方法
    this.executor = Executors.newScheduledThreadPool(1, new ThreadFactory() {
        @Override
        public Thread newThread(Runnable r) {
            Thread t = new Thread(r);
            t.setName("com.alibaba.nacos.client.Worker." + agent.getName());
            t.setDaemon(true);
            return t;
        }
    });
     //初始化一個定時調(diào)度的線程池，從里面的name名字來看，似乎和長輪訓(xùn)有關(guān)系。而這個長輪訓(xùn)應(yīng)該是和nacos服務(wù)端的長輪訓(xùn)
    this.executorService = Executors
            .newScheduledThreadPool(Runtime.getRuntime().availableProcessors(), new ThreadFactory() {
                @Override
                public Thread newThread(Runnable r) {
                    Thread t = new Thread(r);
                    t.setName("com.alibaba.nacos.client.Worker.longPolling." + agent.getName());
                    t.setDaemon(true);
                    return t;
                }
            });
    //設(shè)置定時任務(wù)的執(zhí)行頻率，并且調(diào)用checkConfigInfo這個方法，猜測是定時去檢測配置是否發(fā)生了變化
        //首次執(zhí)行延遲時間為1毫秒、延遲時間為10毫秒
    this.executor.scheduleWithFixedDelay(new Runnable() {
        @Override
        public void run() {
            try {
                checkConfigInfo();
            } catch (Throwable e) {
                LOGGER.error("[" + agent.getName() + "] [sub-check] rotate check error", e);
            }
        }
    }, 1L, 10L, TimeUnit.MILLISECONDS);
}

public void checkConfigInfo() {
    // Dispatch tasks.
    int listenerSize = cacheMap.size(); //
    // Round up the longingTaskCount.
     // 向上取整為批數(shù)，監(jiān)聽的配置數(shù)量除以3000，得到一個整數(shù)，代表長輪訓(xùn)任務(wù)的數(shù)量
    int longingTaskCount = (int) Math.ceil(listenerSize / ParamUtil.getPerTaskConfigSize());
     //currentLongingTaskCount表示當前的長輪訓(xùn)任務(wù)數(shù)量，如果小于計算的結(jié)果，則可以繼續(xù)創(chuàng)建
    if (longingTaskCount > currentLongingTaskCount) {
        for (int i = (int) currentLongingTaskCount; i < longingTaskCount; i++) {
            // The task list is no order.So it maybe has issues when changing.
            executorService.execute(new LongPollingRunnable(i));
        }
        currentLongingTaskCount = longingTaskCount;
    }
}

class LongPollingRunnable implements Runnable {
    private final int taskId; //表示當前任務(wù)批次id
    public LongPollingRunnable(int taskId) {
        this.taskId = taskId;
    }
    @Override
    public void run() {
        List<CacheData> cacheDatas = new ArrayList<CacheData>();
        List<String> inInitializingCacheList = new ArrayList<String>();
        try {
            // 遍歷CacheMap，把CacheMap中和當前任務(wù)id相同的緩存，保存到cacheDatas
            // 通過checkLocalConfig方法
            for (CacheData cacheData : cacheMap.values()) {
                if (cacheData.getTaskId() == taskId) {
                    cacheDatas.add(cacheData);
                    try {
                        checkLocalConfig(cacheData);
                        if (cacheData.isUseLocalConfigInfo()) { //這里表示數(shù)據(jù)有變化，需要通知監(jiān)聽器
                            cacheData.checkListenerMd5(); //通知所有針對當前配置設(shè)置了監(jiān)聽的監(jiān)聽器
                        }
                    } catch (Exception e) {
                        LOGGER.error("get local config info error", e);
                    }
                }
            }
           //省略部分
        } catch (Throwable e) {
            // If the rotation training task is abnormal, the next execution time of the task will be punished
            LOGGER.error("longPolling error : ", e);
            executorService.schedule(this, taskPenaltyTime, TimeUnit.MILLISECONDS); //出現(xiàn)異常，到下一次taskPenaltyTime后重新執(zhí)行任務(wù)
        }
    }
}

private void checkLocalConfig(CacheData cacheData) {
    final String dataId = cacheData.dataId;
    final String group = cacheData.group;
    final String tenant = cacheData.tenant;
    File path = LocalConfigInfoProcessor.getFailoverFile(agent.getName(), dataId, group, tenant);
    // 沒有 -> 有
    if (!cacheData.isUseLocalConfigInfo() && path.exists()) {
        String content = LocalConfigInfoProcessor.getFailover(agent.getName(), dataId, group, tenant);
        final String md5 = MD5Utils.md5Hex(content, Constants.ENCODE);
        cacheData.setUseLocalConfigInfo(true);
        cacheData.setLocalConfigInfoVersion(path.lastModified());
        cacheData.setContent(content);
        String encryptedDataKey = LocalEncryptedDataKeyProcessor
                .getEncryptDataKeyFailover(agent.getName(), dataId, group, tenant);
        cacheData.setEncryptedDataKey(encryptedDataKey);
        
        LOGGER.warn(
                "[{}] [failover-change] failover file created. dataId={}, group={}, tenant={}, md5={}, content={}",
                agent.getName(), dataId, group, tenant, md5, ContentUtils.truncateContent(content));
        return;
    }
     // 有 -> 沒有。不通知業(yè)務(wù)監(jiān)聽器，從server拿到配置后通知。
    // If use local config info, then it doesn't notify business listener and notify after getting from server.
    if (cacheData.isUseLocalConfigInfo() && !path.exists()) {
        cacheData.setUseLocalConfigInfo(false);
        LOGGER.warn("[{}] [failover-change] failover file deleted. dataId={}, group={}, tenant={}", agent.getName(),
                dataId, group, tenant);
        return;
    }
     // 有變更
    if (cacheData.isUseLocalConfigInfo() && path.exists() && cacheData.getLocalConfigInfoVersion() != path
            .lastModified()) {
        String content = LocalConfigInfoProcessor.getFailover(agent.getName(), dataId, group, tenant);
        final String md5 = MD5Utils.md5Hex(content, Constants.ENCODE);
        cacheData.setUseLocalConfigInfo(true);
        cacheData.setLocalConfigInfoVersion(path.lastModified());
        cacheData.setContent(content);
        String encryptedDataKey = LocalEncryptedDataKeyProcessor
                .getEncryptDataKeyFailover(agent.getName(), dataId, group, tenant);
        cacheData.setEncryptedDataKey(encryptedDataKey);
        LOGGER.warn(
                "[{}] [failover-change] failover file changed. dataId={}, group={}, tenant={}, md5={}, content={}",
                agent.getName(), dataId, group, tenant, md5, ContentUtils.truncateContent(content));
    }
}

void checkListenerMd5() {
    for (ManagerListenerWrap wrap : listeners) {
        if (!md5.equals(wrap.lastCallMd5)) {
            safeNotifyListener(dataId, group, content, type, md5, wrap);
        }
    }
}

// check server config
//從服務(wù)端獲取發(fā)生變化的數(shù)據(jù)的DataID列表，保存在List<String>集合中
List<String> changedGroupKeys = checkUpdateDataIds(cacheDatas, inInitializingCacheList);
if (!CollectionUtils.isEmpty(changedGroupKeys)) {
    LOGGER.info("get changedGroupKeys:" + changedGroupKeys);
}
//遍歷發(fā)生了變更的配置項
for (String groupKey : changedGroupKeys) {
    String[] key = GroupKey.parseKey(groupKey);
    String dataId = key[0];
    String group = key[1];
    String tenant = null;
    if (key.length == 3) {
        tenant = key[2];
    }
    try {
        //逐項根據(jù)這些配置項獲取配置信息
        ConfigResponse response = getServerConfig(dataId, group, tenant, 3000L);
        //把配置信息保存到CacheData中
        CacheData cache = cacheMap.get(GroupKey.getKeyTenant(dataId, group, tenant));
        cache.setContent(response.getContent());
        cache.setEncryptedDataKey(response.getEncryptedDataKey());
        if (null != response.getConfigType()) {
            cache.setType(response.getConfigType());
        }
        LOGGER.info("[{}] [data-received] dataId={}, group={}, tenant={}, md5={}, content={}, type={}",
                    agent.getName(), dataId, group, tenant, cache.getMd5(),
                    ContentUtils.truncateContent(response.getContent()), response.getConfigType());
    } catch (NacosException ioe) {
        String message = String
            .format("[%s] [get-update] get changed config exception. dataId=%s, group=%s, tenant=%s",
                    agent.getName(), dataId, group, tenant);
        LOGGER.error(message, ioe);
    }
}
//再遍歷CacheData這個集合，找到發(fā)生變化的數(shù)據(jù)進行通知
for (CacheData cacheData : cacheDatas) {
    if (!cacheData.isInitializing() || inInitializingCacheList
        .contains(GroupKey.getKeyTenant(cacheData.dataId, cacheData.group, cacheData.tenant))) {
        cacheData.checkListenerMd5();
        cacheData.setInitializing(false);
    }
}
inInitializingCacheList.clear();
 //繼續(xù)傳遞當前線程進行輪詢
executorService.execute(this);

/**
 * 從Server獲取值變化了的DataID列表。返回的對象里只有dataId和group是有效的。 保證不返回NULL。
 */
List<String> checkUpdateDataIds(List<CacheData> cacheDatas, List<String> inInitializingCacheList) throws IOException {
    StringBuilder sb = new StringBuilder();
    for (CacheData cacheData : cacheDatas) { //把需要檢查的配置項，拼接成一個字符串
        if (!cacheData.isUseLocalConfigInfo()) { //找到isUseLocalConfigInfo=false的緩存
            sb.append(cacheData.dataId).append(WORD_SEPARATOR);
            sb.append(cacheData.group).append(WORD_SEPARATOR);
            if (StringUtils.isBlank(cacheData.tenant)) {
                sb.append(cacheData.getMd5()).append(LINE_SEPARATOR);
            } else {
                sb.append(cacheData.getMd5()).append(WORD_SEPARATOR);
                sb.append(cacheData.getTenant()).append(LINE_SEPARATOR);
            }
            if (cacheData.isInitializing()) {//
                // cacheData 首次出現(xiàn)在cacheMap中&首次check更新
                inInitializingCacheList
                    .add(GroupKey.getKeyTenant(cacheData.dataId, cacheData.group, cacheData.tenant));
            }
        }
    }
    boolean isInitializingCacheList = !inInitializingCacheList.isEmpty();
    return checkUpdateConfigStr(sb.toString(), isInitializingCacheList);
}

List<String> checkUpdateConfigStr(String probeUpdateString, boolean isInitializingCacheList) throws Exception {
    //拼接參數(shù)和header
    Map<String, String> params = new HashMap<String, String>(2);
    params.put(Constants.PROBE_MODIFY_REQUEST, probeUpdateString);
    Map<String, String> headers = new HashMap<String, String>(2);
    headers.put("Long-Pulling-Timeout", "" + timeout);
    // told server do not hang me up if new initializing cacheData added in
    if (isInitializingCacheList) {
        headers.put("Long-Pulling-Timeout-No-Hangup", "true");
    }
    if (StringUtils.isBlank(probeUpdateString)) {//判斷可能發(fā)生變更的字符串是否為空，如果是，則直接返回。
        return Collections.emptyList();
    }
    try {
        // In order to prevent the server from handling the delay of the client's long task,
        // increase the client's read timeout to avoid this problem.
        // 設(shè)置readTimeoutMs，也就是本次請求等待響應(yīng)的超時時間，默認是30s
        long readTimeoutMs = timeout + (long) Math.round(timeout >> 1);
        //發(fā)起遠程調(diào)用
        HttpRestResult<String> result = agent
                .httpPost(Constants.CONFIG_CONTROLLER_PATH + "/listener", headers, params, agent.getEncode(),
                        readTimeoutMs);
        if (result.ok()) { //如果響應(yīng)成功
            setHealthServer(true);
            return parseUpdateDataIdResponse(result.getData()); //解析并更新數(shù)據(jù)，返回的是確實發(fā)生了數(shù)據(jù)變更的字符串：tenant/group/dataid。
        } else {//如果響應(yīng)失敗
            setHealthServer(false);
            LOGGER.error("[{}] [check-update] get changed dataId error, code: {}", agent.getName(),
                    result.getCode());
        }
    } catch (Exception e) {
        setHealthServer(false);
        LOGGER.error("[" + agent.getName() + "] [check-update] get changed dataId exception", e);
        throw e;
    }
    return Collections.emptyList();
}

//# ConfigController.java
@PostMapping("/listener")
@Secured(action = ActionTypes.READ, parser = ConfigResourceParser.class)
public void listener(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED", true);
    String probeModify = request.getParameter("Listening-Configs");
    if (StringUtils.isBlank(probeModify)) {
        throw new IllegalArgumentException("invalid probeModify");
    }
    probeModify = URLDecoder.decode(probeModify, Constants.ENCODE);
    Map<String, String> clientMd5Map;
    try {
        //解析客戶端傳遞過來的可能發(fā)生變化的配置項目，轉(zhuǎn)化為Map集合(key=dataId,value=md5)
        clientMd5Map = MD5Util.getClientMd5Map(probeModify);
    } catch (Throwable e) {
        throw new IllegalArgumentException("invalid probeModify");
    }
    // 開始執(zhí)行長輪訓(xùn)。
    inner.doPollingConfig(request, response, clientMd5Map, probeModify.length());
}

public String doPollingConfig(HttpServletRequest request, HttpServletResponse response,
        Map<String, String> clientMd5Map, int probeRequestSize) throws IOException {
    // 判斷當前請求是否支持長輪訓(xùn)。（）
    if (LongPollingService.isSupportLongPolling(request)) {
        longPollingService.addLongPollingClient(request, response, clientMd5Map, probeRequestSize);
        return HttpServletResponse.SC_OK + "";
    }
    //如果是短輪詢，走下面的請求，下面的請求就是把客戶端傳過來的數(shù)據(jù)和服務(wù)端的數(shù)據(jù)逐項進行比較，保存到changeGroups中。
    // Compatible with short polling logic.
    List<String> changedGroups = MD5Util.compareMd5(request, response, clientMd5Map);
    // Compatible with short polling result.
    String oldResult = MD5Util.compareMd5OldResult(changedGroups);
    String newResult = MD5Util.compareMd5ResultString(changedGroups);
    
    String version = request.getHeader(Constants.CLIENT_VERSION_HEADER);
    if (version == null) {
        version = "2.0.0";
    }
    int versionNum = Protocol.getVersionNumber(version);
    // Before 2.0.4 version, return value is put into header.
    if (versionNum < START_LONG_POLLING_VERSION_NUM) {
        response.addHeader(Constants.PROBE_MODIFY_RESPONSE, oldResult);
        response.addHeader(Constants.PROBE_MODIFY_RESPONSE_NEW, newResult);
    } else {
        request.setAttribute("content", newResult);
    }
    Loggers.AUTH.info("new content:" + newResult);
    // Disable cache.
    response.setHeader("Pragma", "no-cache");
    response.setDateHeader("Expires", 0);
    response.setHeader("Cache-Control", "no-cache,no-store");
    response.setStatus(HttpServletResponse.SC_OK);
    return HttpServletResponse.SC_OK + "";
}

public void addLongPollingClient(HttpServletRequest req, HttpServletResponse rsp, Map<String, String> clientMd5Map,
        int probeRequestSize) {
    //獲取客戶端長輪訓(xùn)的超時時間
    String str = req.getHeader(LongPollingService.LONG_POLLING_HEADER); 
    //不允許斷開的標記
    String noHangUpFlag = req.getHeader(LongPollingService.LONG_POLLING_NO_HANG_UP_HEADER);
    //應(yīng)用名稱
    String appName = req.getHeader(RequestUtil.CLIENT_APPNAME_HEADER);
    //
    String tag = req.getHeader("Vipserver-Tag");
    //延期時間，默認為500ms
    int delayTime = SwitchService.getSwitchInteger(SwitchService.FIXED_DELAY_TIME, 500);

    // Add delay time for LoadBalance, and one response is returned 500 ms in advance to avoid client timeout.
    // 提前500ms返回一個響應(yīng)，避免客戶端出現(xiàn)超時
    long timeout = Math.max(10000, Long.parseLong(str) - delayTime);
    if (isFixedPolling()) {
        timeout = Math.max(10000, getFixedPollingInterval());
        // Do nothing but set fix polling timeout.
    } else {
        long start = System.currentTimeMillis();
        //通過md5判斷客戶端請求過來的key是否有和服務(wù)器端有不一致的，如果有，則保存到changedGroups中。
        List<String> changedGroups = MD5Util.compareMd5(req, rsp, clientMd5Map);
        if (changedGroups.size() > 0) { //如果發(fā)現(xiàn)有變更，則直接把請求返回給客戶端
            generateResponse(req, rsp, changedGroups);
            LogUtil.CLIENT_LOG.info("{}|{}|{}|{}|{}|{}|{}", System.currentTimeMillis() - start, "instant",
                    RequestUtil.getRemoteIp(req), "polling", clientMd5Map.size(), probeRequestSize,
                    changedGroups.size());
            return;
        } else if (noHangUpFlag != null && noHangUpFlag.equalsIgnoreCase(TRUE_STR)) { //如果noHangUpFlag為true，說明不需要掛起客戶端，所以直接返回。
            LogUtil.CLIENT_LOG.info("{}|{}|{}|{}|{}|{}|{}", System.currentTimeMillis() - start, "nohangup",
                    RequestUtil.getRemoteIp(req), "polling", clientMd5Map.size(), probeRequestSize,
                    changedGroups.size());
            return;
        }
    }
    //獲取請求端的ip
    String ip = RequestUtil.getRemoteIp(req);
    // Must be called by http thread, or send response.
    //把當前請求轉(zhuǎn)化為一個異步請求（意味著此時tomcat線程被釋放，也就是客戶端的請求，需要通過asyncContext來手動觸發(fā)返回，否則一直掛起）
    final AsyncContext asyncContext = req.startAsync();
    // AsyncContext.setTimeout() is incorrect, Control by oneself
    asyncContext.setTimeout(0L); //設(shè)置異步請求超時時間，
    //執(zhí)行長輪訓(xùn)請求
    ConfigExecutor.executeLongPolling(
            new ClientLongPolling(asyncContext, clientMd5Map, ip, probeRequestSize, timeout, appName, tag));
}

class ClientLongPolling implements Runnable {
    @Override
    public void run() {
        //構(gòu)建一個異步任務(wù)，延后29.5s執(zhí)行
        asyncTimeoutFuture = ConfigExecutor.scheduleLongPolling(new Runnable() {
            @Override
            public void run() { //如果達到29.5s，說明這個期間沒有做任何配置修改，則自動觸發(fā)執(zhí)行
                try {
                    getRetainIps().put(ClientLongPolling.this.ip, System.currentTimeMillis());
                    // Delete subsciber's relations.
                    allSubs.remove(ClientLongPolling.this); //移除訂閱關(guān)系
                    if (isFixedPolling()) { //如果是固定間隔的長輪訓(xùn)
                        LogUtil.CLIENT_LOG
                                .info("{}|{}|{}|{}|{}|{}", (System.currentTimeMillis() - createTime), "fix",
                                        RequestUtil.getRemoteIp((HttpServletRequest) asyncContext.getRequest()),
                                        "polling", clientMd5Map.size(), probeRequestSize);
                        //比較變更的key
                        List<String> changedGroups = MD5Util
                                .compareMd5((HttpServletRequest) asyncContext.getRequest(),
                                        (HttpServletResponse) asyncContext.getResponse(), clientMd5Map);
                        if (changedGroups.size() > 0) {//如果大于0，表示有變更，直接響應(yīng)
                            sendResponse(changedGroups);
                        } else {
                            sendResponse(null); //否則返回null
                        }
                    } else {
                        LogUtil.CLIENT_LOG
                                .info("{}|{}|{}|{}|{}|{}", (System.currentTimeMillis() - createTime), "timeout",
                                        RequestUtil.getRemoteIp((HttpServletRequest) asyncContext.getRequest()),
                                        "polling", clientMd5Map.size(), probeRequestSize);
                        sendResponse(null);
                    }
                } catch (Throwable t) {
                    LogUtil.DEFAULT_LOG.error("long polling error:" + t.getMessage(), t.getCause());
                }
            }
        }, timeoutTime, TimeUnit.MILLISECONDS);
        allSubs.add(this);  //把當前線程添加到訂閱事件隊列中
    }
}

/**
 * 長輪詢訂閱關(guān)系
 */
final Queue<ClientLongPolling> allSubs;
allSubs.add(this);

public LongPollingService() {
    allSubs = new ConcurrentLinkedQueue<ClientLongPolling>();
    ConfigExecutor.scheduleLongPolling(new StatTask(), 0L, 10L, TimeUnit.SECONDS);
    // Register LocalDataChangeEvent to NotifyCenter.
    NotifyCenter.registerToPublisher(LocalDataChangeEvent.class, NotifyCenter.ringBufferSize);
    //注冊LocalDataChangeEvent訂閱事件
    NotifyCenter.registerSubscriber(new Subscriber() {
        @Override
        public void onEvent(Event event) {
            if (isFixedPolling()) {
                // Ignore.
            } else {
                if (event instanceof LocalDataChangeEvent) { //如果觸發(fā)了LocalDataChangeEvent，則執(zhí)行下面的代碼
                    LocalDataChangeEvent evt = (LocalDataChangeEvent) event;
                    ConfigExecutor.executeLongPolling(new DataChangeTask(evt.groupKey, evt.isBeta, evt.betaIps));
                }
            }
        }
        @Override
        public Class<? extends Event> subscribeType() {
            return LocalDataChangeEvent.class;
        }
    });
}

class DataChangeTask implements Runnable {
    @Override
    public void run() {
        try {
            ConfigCacheService.getContentBetaMd5(groupKey); //
            //遍歷所有訂閱事件表
            for (Iterator<ClientLongPolling> iter = allSubs.iterator(); iter.hasNext(); ) {
                ClientLongPolling clientSub = iter.next(); //得到ClientLongPolling
                //判斷當前的ClientLongPolling中，請求的key是否包含當前修改的groupKey
                if (clientSub.clientMd5Map.containsKey(groupKey)) {
                    // If published tag is not in the beta list, then it skipped.
                    if (isBeta && !CollectionUtils.contains(betaIps, clientSub.ip)) { //如果是beta方式且betaIps不包含當前客戶端ip，直接返回
                        continue;
                    }
                    // If published tag is not in the tag list, then it skipped.
                    if (StringUtils.isNotBlank(tag) && !tag.equals(clientSub.tag)) {//如果配置了tag標簽且不包含當前客戶端的tag，直接返回
                        continue;
                    }
					//
                    getRetainIps().put(clientSub.ip, System.currentTimeMillis());
                    iter.remove(); // Delete subscribers' relationships. 移除當前客戶端的訂閱關(guān)系
                    LogUtil.CLIENT_LOG
                            .info("{}|{}|{}|{}|{}|{}|{}", (System.currentTimeMillis() - changeTime), "in-advance",
                                    RequestUtil
                                            .getRemoteIp((HttpServletRequest) clientSub.asyncContext.getRequest()),
                                    "polling", clientSub.clientMd5Map.size(), clientSub.probeRequestSize, groupKey);
                    clientSub.sendResponse(Arrays.asList(groupKey)); //響應(yīng)客戶端請求。
                }
            }
        } catch (Throwable t) {
            LogUtil.DEFAULT_LOG.error("data change error: {}", ExceptionUtil.getStackTrace(t));
        }
    }
}