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9.7 dubbo心跳机制

dubbo的心跳机制：

目的：检测provider与consumer之间的connection连接是不是还连接着，如果连接断了，需要作出相应的处理。
原理：
- provider：dubbo的心跳默认是在heartbeat（默认是60s）内如果没有接收到消息，就会发送心跳消息，如果连着3次（180s）没有收到心跳响应，provider会关闭channel。
- consumer：dubbo的心跳默认是在60s内如果没有接收到消息，就会发送心跳消息，如果连着3次（180s）没有收到心跳响应，consumer会进行重连。

来看源码调用链。先看provider端。

一、provider端心跳机制

              -->openServer(URL url)
                 url：dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=10.10.10.10&bind.port=20880&default.server=netty4&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=21999&qos.port=22222&side=provider&timestamp=1520660491836
                -->createServer(URL url)
                    -->HeaderExchanger.bind(URL url, ExchangeHandler handler)
                       url：dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=10.10.10.10&bind.port=20880&channel.readonly.sent=true&codec=dubbo&default.server=netty4&dubbo=2.0.0&generic=false&heartbeat=60000&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=21999&qos.port=22222&side=provider&timestamp=1520660491836 handler:DubboProtocol.requestHandler
                      -->new DecodeHandler(new HeaderExchangeHandler(handler)))
                        -->NettyTransporter.bind(URL url, ChannelHandler listener)
                           listener:上边的DecodeHandler实例
                          -->new NettyServer(URL url, ChannelHandler handler)
                            -->ChannelHandler.wrapInternal(ChannelHandler handler, URL url)
                               handler:上边的DecodeHandler实例
                            -->doOpen()//开启netty服务
                      -->new HeaderExchangeServer(Server server)
                         server:上述的NettyServer
                        -->startHeatbeatTimer()

服务端在开启netty服务时，在调用createServer时，会从url的parameters map中获取heartbeat配置，代码如下：

 1     private ExchangeServer createServer(URL url) {
 2 
 3         ...
 4 
 5         url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
 6        
 7         ...
 8 
 9         ExchangeServer server;
10         try {
11             server = Exchangers.bind(url, requestHandler);
12         } catch (RemotingException e) {
13             throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e);
14         }
15 
16         ...
17 
18         return server;
19     }

其中：int DEFAULT_HEARTBEAT = 60 * 1000，即当用户没有配置heartbeat（心跳时间）时，默认heartbeat=60s（即60s内没有接收到任何请求，就会发送心跳信息）。那么这个heartbeat到底该怎么配？

provider端：

1     <dubbo:service ...>
2         <dubbo:parameter key="heartbeat" value="3000"/>
3     </dubbo:service>

consumer端：

1     <dubbo:reference ...>
2         <dubbo:parameter key="heartbeat" value="3000"/>
3     </dubbo:reference>

再来看调用链，当执行到这一句。

1 ChannelHandler.wrapInternal(ChannelHandler handler, URL url)

会形成一个handler调用链，调用链如下：

1 MultiMessageHandler
2 -->handler: HeartbeatHandler
3    -->handler: AllChannelHandler
4          -->url: providerUrl
5          -->executor: FixedExecutor
6          -->handler: DecodeHandler
7             -->handler: HeaderExchangeHandler
8                -->handler: ExchangeHandlerAdapter（DubboProtocol.requestHandler）

这也是netty接收到请求后的处理链路，注意其中有一个HeartbeatHandler。

最后，执行new HeaderExchangeServer(Server server)，来看源码：

 1 public class HeaderExchangeServer implements ExchangeServer {
 2     /** 心跳定时器 */
 3     private final ScheduledExecutorService scheduled = Executors.newScheduledThreadPool(1,
 4             new NamedThreadFactory(
 5                     "dubbo-remoting-server-heartbeat",
 6                     true));
 7     /** NettyServer */
 8     private final Server server;
 9     // heartbeat timer
10     private ScheduledFuture<?> heatbeatTimer;
11     // heartbeat timeout (ms), default value is 0 , won't execute a heartbeat.
12     private int heartbeat;
13     private int heartbeatTimeout;
14     private AtomicBoolean closed = new AtomicBoolean(false);
15 
16     public HeaderExchangeServer(Server server) {
17         if (server == null) {
18             throw new IllegalArgumentException("server == null");
19         }
20         this.server = server;
21         this.heartbeat = server.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);
22         this.heartbeatTimeout = server.getUrl().getParameter(Constants.HEARTBEAT_TIMEOUT_KEY, heartbeat * 3);
23         if (heartbeatTimeout < heartbeat * 2) {
24             throw new IllegalStateException("heartbeatTimeout < heartbeatInterval * 2");
25         }
26         startHeatbeatTimer();
27     }
28 
29     private void startHeatbeatTimer() {
30         stopHeartbeatTimer();
31         if (heartbeat > 0) {
32             heatbeatTimer = scheduled.scheduleWithFixedDelay(
33                     new HeartBeatTask(new HeartBeatTask.ChannelProvider() {
34                         public Collection<Channel> getChannels() {
35                             return Collections.unmodifiableCollection(
36                                     HeaderExchangeServer.this.getChannels());
37                         }
38                     }, heartbeat, heartbeatTimeout),
39                     heartbeat, heartbeat, TimeUnit.MILLISECONDS);
40         }
41     }
42 
43     private void stopHeartbeatTimer() {
44         try {
45             ScheduledFuture<?> timer = heatbeatTimer;
46             if (timer != null && !timer.isCancelled()) {
47                 timer.cancel(true);
48             }
49         } catch (Throwable t) {
50             logger.warn(t.getMessage(), t);
51         } finally {
52             heatbeatTimer = null;
53         }
54     }
55 }

创建HeaderExchangeServer时，初始化了heartbeat（心跳间隔时间）和heartbeatTimeout（心跳响应超时时间：即如果最终发送的心跳在这个时间内都没有返回，则做出响应的处理）。

heartbeat默认是0（从startHeatbeatTimer()方法可以看出只有heartbeat>0的情况下，才会发心跳，这里heartbeat如果从url的parameter map中获取不到，就是0，但是我们在前边看到dubbo会默认设置heartbeat=60s到parameter map中，所以此处的heartbeat=60s）；
heartbeatTimeout：默认是heartbeat*3。（原因：假设一端发出一次heartbeatRequest，另一端在heartbeat内没有返回任何响应-包括正常请求响应和心跳响应，此时不能认为是连接断了，因为有可能还是网络抖动什么的导致了tcp包的重传超时等）
scheduled是一个含有一个线程的定时线程执行器（其中的线程名字为："dubbo-remoting-server-heartbeat-thread-*"）

之后启动心跳定时任务：

首先如果原来有心跳定时任务，关闭原来的定时任务
之后启动scheduled中的定时线程，从启动该线程开始，每隔heartbeat执行一次HeartBeatTask任务（第一次执行是在启动线程后heartbeat时）

来看一下HeartBeatTask的源码：

 1 final class HeartBeatTask implements Runnable {
 2     // channel获取器：用于获取所有需要进行心跳检测的channel
 3     private ChannelProvider channelProvider;
 4     private int heartbeat;
 5     private int heartbeatTimeout;
 6 
 7     HeartBeatTask(ChannelProvider provider, int heartbeat, int heartbeatTimeout) {
 8         this.channelProvider = provider;
 9         this.heartbeat = heartbeat;
10         this.heartbeatTimeout = heartbeatTimeout;
11     }
12 
13     public void run() {
14         try {
15             long now = System.currentTimeMillis();
16             for (Channel channel : channelProvider.getChannels()) {
17                 if (channel.isClosed()) {
18                     continue;
19                 }
20                 try {
21                     // 获取最后一次读操作的时间
22                     Long lastRead = (Long) channel.getAttribute(
23                             HeaderExchangeHandler.KEY_READ_TIMESTAMP);
24                     // 获取最后一次写操作的时间
25                     Long lastWrite = (Long) channel.getAttribute(
26                             HeaderExchangeHandler.KEY_WRITE_TIMESTAMP);
27                     // 如果在heartbeat内没有进行读操作或者写操作，则发送心跳请求
28                     if ((lastRead != null && now - lastRead > heartbeat)
29                             || (lastWrite != null && now - lastWrite > heartbeat)) {
30                         Request req = new Request();
31                         req.setVersion("2.0.0");
32                         req.setTwoWay(true);
33                         req.setEvent(Request.HEARTBEAT_EVENT);
34                         channel.send(req);
35                         if (logger.isDebugEnabled()) {
36                             logger.debug("Send heartbeat to remote channel " + channel.getRemoteAddress()
37                                     + ", cause: The channel has no data-transmission exceeds a heartbeat period: " + heartbeat + "ms");
38                         }
39                     }
40                     //正常消息和心跳在heartbeatTimeout都没接收到
41                     if (lastRead != null && now - lastRead > heartbeatTimeout) {
42                         logger.warn("Close channel " + channel
43                                 + ", because heartbeat read idle time out: " + heartbeatTimeout + "ms");
44                         // consumer端进行重连
45                         if (channel instanceof Client) {
46                             try {
47                                 ((Client) channel).reconnect();
48                             } catch (Exception e) {
49                                 //do nothing
50                             }
51                         } else {// provider端关闭连接
52                             channel.close();
53                         }
54                     }
55                 } catch (Throwable t) {
56                     logger.warn("Exception when heartbeat to remote channel " + channel.getRemoteAddress(), t);
57                 }
58             }
59         } catch (Throwable t) {
60             logger.warn("Unhandled exception when heartbeat, cause: " + t.getMessage(), t);
61         }
62     }
63 
64     interface ChannelProvider {
65         Collection<Channel> getChannels();
66     }
67 }

HeartBeatTask首先获取所有的channelProvider#getChannels获取所有需要心跳检测的channel，channelProvider实例是HeaderExchangeServer中在启动线程定时执行器的时候创建的内部类。

1                     new HeartBeatTask.ChannelProvider() {
2                         public Collection<Channel> getChannels() {
3                             return Collections.unmodifiableCollection(
4                                     HeaderExchangeServer.this.getChannels());
5                         }
6                     }

来看一下HeaderExchangeServer.this.getChannels()：

 1     public Collection<Channel> getChannels() {
 2         return (Collection) getExchangeChannels();
 3     }
 4 
 5     public Collection<ExchangeChannel> getExchangeChannels() {
 6         Collection<ExchangeChannel> exchangeChannels = new ArrayList<ExchangeChannel>();
 7         Collection<Channel> channels = server.getChannels();
 8         if (channels != null && channels.size() > 0) {
 9             for (Channel channel : channels) {
10                 exchangeChannels.add(HeaderExchangeChannel.getOrAddChannel(channel));
11             }
12         }
13         return exchangeChannels;
14     }

实际上就是获取NettyServer中的全部channel连接。

获取到需要心跳检测的channel后，对每一个channel进行如下判断：

如果在heartbeat内没有进行读操作或者写操作，则发送心跳请求
如果正常消息和心跳在heartbeatTimeout都没接收到，consumer端会进行重连，provider端会关闭channel

这里比较关键的是lastRead和lastWrite的设置。先来看一下获取：

1 Long lastRead = (Long) channel.getAttribute(HeaderExchangeHandler.KEY_READ_TIMESTAMP);
2 Long lastWrite = (Long) channel.getAttribute(HeaderExchangeHandler.KEY_WRITE_TIMESTAMP);

说明有地方在设置这两个值到channel中。

从请求和响应处理来看，无论是请求还是响应都会按照这个顺序处理一遍。

1 MultiMessageHandler
2 -->handler: HeartbeatHandler
3    -->handler: AllChannelHandler
4          -->url: providerUrl
5          -->executor: FixedExecutor
6          -->handler: DecodeHandler
7             -->handler: HeaderExchangeHandler
8                -->handler: ExchangeHandlerAdapter（DubboProtocol.requestHandler）

其中HeartbeatHandler源码如下：

 1 public class HeartbeatHandler extends AbstractChannelHandlerDelegate {
 2 
 3     private static final Logger logger = LoggerFactory.getLogger(HeartbeatHandler.class);
 4 
 5     public static String KEY_READ_TIMESTAMP = "READ_TIMESTAMP";
 6 
 7     public static String KEY_WRITE_TIMESTAMP = "WRITE_TIMESTAMP";
 8 
 9     public HeartbeatHandler(ChannelHandler handler) {
10         super(handler);
11     }
12 
13     public void connected(Channel channel) throws RemotingException {
14         setReadTimestamp(channel);
15         setWriteTimestamp(channel);
16         handler.connected(channel);
17     }
18 
19     public void disconnected(Channel channel) throws RemotingException {
20         clearReadTimestamp(channel);
21         clearWriteTimestamp(channel);
22         handler.disconnected(channel);
23     }
24 
25     public void sent(Channel channel, Object message) throws RemotingException {
26         setWriteTimestamp(channel);
27         handler.sent(channel, message);
28     }
29 
30     public void received(Channel channel, Object message) throws RemotingException {
31         setReadTimestamp(channel);
32         if (isHeartbeatRequest(message)) {
33             Request req = (Request) message;
34             if (req.isTwoWay()) {
35                 Response res = new Response(req.getId(), req.getVersion());
36                 res.setEvent(Response.HEARTBEAT_EVENT);
37                 channel.send(res);
38                 if (logger.isInfoEnabled()) {
39                     int heartbeat = channel.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);
40                     if (logger.isDebugEnabled()) {
41                         logger.debug("Received heartbeat from remote channel " + channel.getRemoteAddress()
42                                 + ", cause: The channel has no data-transmission exceeds a heartbeat period"
43                                 + (heartbeat > 0 ? ": " + heartbeat + "ms" : ""));
44                     }
45                 }
46             }
47             return;
48         }
49         if (isHeartbeatResponse(message)) {
50             if (logger.isDebugEnabled()) {
51                 logger.debug(
52                         new StringBuilder(32)
53                                 .append("Receive heartbeat response in thread ")
54                                 .append(Thread.currentThread().getName())
55                                 .toString());
56             }
57             return;
58         }
59         handler.received(channel, message);
60     }
61 
62     private void setReadTimestamp(Channel channel) {
63         channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
64     }
65 
66     private void setWriteTimestamp(Channel channel) {
67         channel.setAttribute(KEY_WRITE_TIMESTAMP, System.currentTimeMillis());
68     }
69 
70     private void clearReadTimestamp(Channel channel) {
71         channel.removeAttribute(KEY_READ_TIMESTAMP);
72     }
73 
74     private void clearWriteTimestamp(Channel channel) {
75         channel.removeAttribute(KEY_WRITE_TIMESTAMP);
76     }
77 
78     private boolean isHeartbeatRequest(Object message) {
79         return message instanceof Request && ((Request) message).isHeartbeat();
80     }
81 
82     private boolean isHeartbeatResponse(Object message) {
83         return message instanceof Response && ((Response) message).isHeartbeat();
84     }
85 }

连接完成时：设置lastRead和lastWrite
连接断开时：清空lastRead和lastWrite
发送消息时：设置lastWrite
接收消息时：设置lastRead

之后交由AllChannelHandler进行处理。之后会一直交由HeaderExchangeHandler进行处理。其对lastRead和lastWrite也做了设置和清理：

 1     public void connected(Channel channel) throws RemotingException {
 2         channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
 3         channel.setAttribute(KEY_WRITE_TIMESTAMP, System.currentTimeMillis());
 4         ...
 5     }
 6 
 7     public void disconnected(Channel channel) throws RemotingException {
 8         channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
 9         channel.setAttribute(KEY_WRITE_TIMESTAMP, System.currentTimeMillis());
10         ...
11     }
12 
13     public void sent(Channel channel, Object message) throws RemotingException {
14         Throwable exception = null;
15         try {
16             channel.setAttribute(KEY_WRITE_TIMESTAMP, System.currentTimeMillis());
17             ...
18         } catch (Throwable t) {
19             exception = t;
20         }
21     }
22 
23     public void received(Channel channel, Object message) throws RemotingException {
24         channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
25         ...
26     }

连接完成时：设置lastRead和lastWrite
连接断开时：也设置lastRead和lastWrite（为什么？）
发送消息时：设置lastWrite
接收消息时：设置lastRead

这里里有个疑问，从handler链来看，无论是请求还是响应都会按照handler链来处理一遍。那么在HeartbeatHandler中已经进行了lastWrite和lastRead的设置，为什么还要在HeaderExchangeHandler中再处理一遍？

最后，provider端认为连接断了，则会关闭channel。来看一下NettyChannel的close方法：

 1     public void close() {
 2         // 1 将close属性设为true
 3         try {
 4             super.close();
 5         } catch (Exception e) {
 6             logger.warn(e.getMessage(), e);
 7         }
 8         // 2 从全局NettyChannel缓存器中将当前的NettyChannel删掉
 9         try {
10             removeChannelIfDisconnected(channel);
11         } catch (Exception e) {
12             logger.warn(e.getMessage(), e);
13         }
14         // 3 清空当前的NettyChannel中的attributes属性
15         try {
16             attributes.clear();
17         } catch (Exception e) {
18             logger.warn(e.getMessage(), e);
19         }
20         // 4 关闭netty的channel，执行netty的channel的优雅关闭
21         try {
22             if (logger.isInfoEnabled()) {
23                 logger.info("Close netty channel " + channel);
24             }
25             channel.close();
26         } catch (Exception e) {
27             logger.warn(e.getMessage(), e);
28         }
29     }

从上边代码来看，假设consumer端挂了，provider端的心跳检测机制可以进行相关的资源回收，所以provider端的心跳检测机制是有必要的。

二、consumer端心跳机制

                      //创建ExchangeClient，对第一次服务发现providers路径下的相关url建立长连接
                      -->getClients(URL url)
                        -->getSharedClient(URL url)
                          -->ExchangeClient exchangeClient = initClient(url)
                            -->Exchangers.connect(url, requestHandler)
                              -->HeaderExchanger.connect(URL url, ExchangeHandler handler)
                                -->new DecodeHandler(new HeaderExchangeHandler(handler)))
                                  -->Transporters.connect(URL url, ChannelHandler... handlers)
                                    -->NettyTransporter.connect(URL url, ChannelHandler listener)
                                      -->new NettyClient(url, listener)
                                        -->new MultiMessageHandler(HeartbeatHandler(AllChannelHandler(handler)))
                                        -->getChannelCodec(url)//获取Codec2，这里是DubboCountCodec实例
                                        -->doOpen()//开启netty客户端
                                        -->doConnect()//连接服务端，建立长连接
                                -->new HeaderExchangeClient(Client client, boolean needHeartbeat)//上述的NettyClient实例，needHeartbeat：true
                                  -->startHeatbeatTimer()//启动心跳计数器

客户端在initClient(url)中设置了heartbeat参数（默认为60s，用户自己设置的方式见“一”中所讲），如下：

 1     /**
 2      * Create new connection
 3      */
 4     private ExchangeClient initClient(URL url) {
 5         ...
 6         // enable heartbeat by default
 7         url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
 8 
 9         ...
10 
11         ExchangeClient client;
12         try {
13             // connection should be lazy
14             if (url.getParameter(Constants.LAZY_CONNECT_KEY, false)) {
15                 client = new LazyConnectExchangeClient(url, requestHandler);
16             } else {
17                 client = Exchangers.connect(url, requestHandler);
18             }
19         } catch (RemotingException e) {
20             throw new RpcException("Fail to create remoting client for service(" + url + "): " + e.getMessage(), e);
21         }
22         return client;
23     }

与provider类似，来看一下最后开启心跳检测的地方。

 1 public class HeaderExchangeClient implements ExchangeClient {
 2     private static final ScheduledThreadPoolExecutor scheduled = new ScheduledThreadPoolExecutor(2, new NamedThreadFactory("dubbo-remoting-client-heartbeat", true));
 3     private final Client client;
 4     private final ExchangeChannel channel;
 5     // heartbeat timer
 6     private ScheduledFuture<?> heartbeatTimer;
 7     // heartbeat(ms), default value is 0 , won't execute a heartbeat.
 8     private int heartbeat;
 9     private int heartbeatTimeout;
10 
11     public HeaderExchangeClient(Client client, boolean needHeartbeat) {
12         if (client == null) {
13             throw new IllegalArgumentException("client == null");
14         }
15         this.client = client;
16         this.channel = new HeaderExchangeChannel(client);
17         String dubbo = client.getUrl().getParameter(Constants.DUBBO_VERSION_KEY);
18         this.heartbeat = client.getUrl().getParameter(Constants.HEARTBEAT_KEY, dubbo != null && dubbo.startsWith("1.0.") ? Constants.DEFAULT_HEARTBEAT : 0);
19         this.heartbeatTimeout = client.getUrl().getParameter(Constants.HEARTBEAT_TIMEOUT_KEY, heartbeat * 3);
20         if (heartbeatTimeout < heartbeat * 2) {
21             throw new IllegalStateException("heartbeatTimeout < heartbeatInterval * 2");
22         }
23         if (needHeartbeat) {
24             startHeatbeatTimer();
25         }
26     }
27 
28     private void startHeatbeatTimer() {
29         stopHeartbeatTimer();
30         if (heartbeat > 0) {
31             heartbeatTimer = scheduled.scheduleWithFixedDelay(
32                     new HeartBeatTask(new HeartBeatTask.ChannelProvider() {
33                         public Collection<Channel> getChannels() {
34                             return Collections.<Channel>singletonList(HeaderExchangeClient.this);
35                         }
36                     }, heartbeat, heartbeatTimeout),
37                     heartbeat, heartbeat, TimeUnit.MILLISECONDS);
38         }
39     }
40 
41     private void stopHeartbeatTimer() {
42         if (heartbeatTimer != null && !heartbeatTimer.isCancelled()) {
43             try {
44                 heartbeatTimer.cancel(true);
45                 scheduled.purge();
46             } catch (Throwable e) {
47                 if (logger.isWarnEnabled()) {
48                     logger.warn(e.getMessage(), e);
49                 }
50             }
51         }
52         heartbeatTimer = null;
53     }
54 }

主要看一下startHeartbeatTimer()方法，与provider相同，只是provider是获取NettyServer的所有的NettyChannel，而consumer只是获取当前的对象。

consumer的handler处理链与provider完全相同。

最后来看一下consumer的重连机制：AbstractClient#reconnect

 1     public void reconnect() throws RemotingException {
 2         disconnect();
 3         connect();
 4     }
 5 
 6     public void disconnect() {
 7         connectLock.lock();
 8         try {
 9             destroyConnectStatusCheckCommand();
10             try {
11                 Channel channel = getChannel();
12                 if (channel != null) {
13                     channel.close();
14                 }
15             } catch (Throwable e) {
16                 logger.warn(e.getMessage(), e);
17             }
18             try {
19                 doDisConnect();
20             } catch (Throwable e) {
21                 logger.warn(e.getMessage(), e);
22             }
23         } finally {
24             connectLock.unlock();
25         }
26     }
27 
28     protected void connect() throws RemotingException {
29         connectLock.lock();
30         try {
31             if (isConnected()) {
32                 return;
33             }
34             initConnectStatusCheckCommand();
35             doConnect();
36             if (!isConnected()) {
37                 throw new RemotingException(this, "Failed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " "
38                         + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion()
39                         + ", cause: Connect wait timeout: " + getTimeout() + "ms.");
40             } else {
41                 if (logger.isInfoEnabled()) {
42                     logger.info("Successed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " "
43                             + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion()
44                             + ", channel is " + this.getChannel());
45                 }
46             }
47             reconnect_count.set(0);
48             reconnect_error_log_flag.set(false);
49         } catch (RemotingException e) {
50             throw e;
51         } catch (Throwable e) {
52             throw new RemotingException(this, "Failed connect to server " + getRemoteAddress() + " from " + getClass().getSimpleName() + " "
53                     + NetUtils.getLocalHost() + " using dubbo version " + Version.getVersion()
54                     + ", cause: " + e.getMessage(), e);
55         } finally {
56             connectLock.unlock();
57         }
58     }

代码比较简单，先断连，再连接。

对于心跳机制，netty本身提供了空闲检测：IdleStateHandler。也可以直接基于此实现心跳机制。

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原文地址：https://www.cnblogs.com/java-zhao/p/8539046.html