ignore serialization ID for heartbeat message

[AlexStocks]

我构造了一个二进制心跳包对服务端进行测试。在代码中我循环增大serialID从0开始直到其最大值31。其中 serialID 取值 [1, 3, 4, 5, 6, 7] 时，服务端有以下异常日志：

2018-06-14 14:50:04,598 [New I/O server worker #1-1] WARN  alibaba.dubbo.rpc.protocol.dubbo.DubboCodec (DubboCodec.java:140) -  [DUBBO] Decode request failed: Flag error, expect OBJECT_NULL|OBJECT_DUMMY|OBJECT_DESC|OBJECT_DESC_ID, get 78, dubbo version: 2.5.4, current host: 127.0.0.1
java.io.IOException: Flag error, expect OBJECT_NULL|OBJECT_DUMMY|OBJECT_DESC|OBJECT_DESC_ID, get 78
  at com.alibaba.dubbo.common.serialize.support.dubbo.GenericObjectInput.readObject(GenericObjectInput.java:79)
  at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decodeHeartbeatData(ExchangeCodec.java:381)
  at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCodec.decodeBody(DubboCodec.java:121)
  at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decode(ExchangeCodec.java:118)
  at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decode(ExchangeCodec.java:79)
  at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCountCodec.decode(DubboCountCodec.java:46)
  at com.alibaba.dubbo.remoting.transport.netty.NettyCodecAdapter$InternalDecoder.messageReceived(NettyCodecAdapter.java:134)
  at org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:80)
  at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564)
  at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559)
  at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:274)
  at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:261)
  at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:349)
  at org.jboss.netty.channel.socket.nio.NioWorker.processSelectedKeys(NioWorker.java:280)
  at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:200)
  at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108)
  at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:44)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
  at java.lang.Thread.run(Thread.java:748)
2018-06-14 14:50:05,599 [New I/O server worker #1-1] WARN  com.alibaba.dubbo.remoting.transport.AbstractServer (AbstractServer.java:199) -  [DUBBO] All clients has discontected from /127.0.0.1:20000. You can graceful shutdown now., dubbo version: 2.5.4, current host: 127.0.0.1

经过与 相关同行 们讨论，说是这几个 ID 是内部保留的实例ID，如 https://github.com/apache/incubator-dubbo/blob/2.5.x/dubbo-common/src/main/java/com/alibaba/dubbo/common/serialize/support/json/FastJsonSerialization.java

针对这个例子，这个解释是 okay 的，可以解释这个错误。不过个人建议：服务端收到心跳包的时候不要检查 header 里面的 serialID，直接放过，可否？

因为心跳包不设计逻辑处理，不需要获取serialID对应的实例，这里检查这个字段有什么意义呢？因为这个原因就导致服务端抛出异常，个人觉得这个处理欠妥。

[ralf0131]

I just did some twist to the style.

[chickenlj]

I don't know why you want to try different serialID, which you mentioned from 0-31.

因为心跳包不设计逻辑处理，不需要获取serialID对应的实例，这里检查这个字段有什么意义呢？因为这个原因就导致服务端抛出异常，个人觉得这个处理欠妥。

But this part you mentioned is quite reasonable, if a heartbeat event doesn't have any payload, we don't have to deserialize anymore.

[chickenlj]

Be aware that Dubbo itself mixed read-only and heartbeat event handling internally.

[diecui1202]

Any suggestion for this issue ?

[zonghaishang]

心跳包不是没有body，只不过对应java的body是null, 但是null在不同序列化值不一样