Decode rpc invocation failed: expected map/object at java.lang.String (Ljava/lang/String;)

dubbo是阿里开源的一款分布式服务治理框架，在国内使用率比较高，今天在跟同事联调服务时报了下面的错误信息：

WARN  c.a.d.r.p.d.DecodeableRpcInvocation -  [DUBBO] Decode rpc invocation failed: expected map/object at java.lang.String (Ljava/lang/String;), dubbo version: 2.8.4, current host: xx.xx.xx.xx
com.alibaba.com.caucho.hessian.io.HessianProtocolException: expected map/object at java.lang.String (Ljava/lang/String;)
at com.alibaba.com.caucho.hessian.io.AbstractDeserializer.error(AbstractDeserializer.java:108) ~[dubbo-2.8.4.jar:2.8.4]
at com.alibaba.com.caucho.hessian.io.AbstractMapDeserializer.readObject(AbstractMapDeserializer.java:70) ~[dubbo-2.8.4.jar:2.8.4]
at com.alibaba.com.caucho.hessian.io.Hessian2Input.readObject(Hessian2Input.java:1696) ~[dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.common.serialize.support.hessian.Hessian2ObjectInput.readObject(Hessian2ObjectInput.java:94) ~[dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.rpc.protocol.dubbo.DecodeableRpcInvocation.decode(DecodeableRpcInvocation.java:150) ~[dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.rpc.protocol.dubbo.DecodeableRpcInvocation.decode(DecodeableRpcInvocation.java:74) ~[dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCodec.decodeBody(DubboCodec.java:138) [dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decode(ExchangeCodec.java:134) [dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.decode(ExchangeCodec.java:95) [dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCountCodec.decode(DubboCountCodec.java:46) [dubbo-2.8.4.jar:2.8.4]
at com.alibaba.dubbo.remoting.transport.netty.NettyCodecAdapter$InternalDecoder.messageReceived(NettyCodecAdapter.java:134) [dubbo-2.8.4.jar:2.8.4]
at org.jboss.netty.channel.SimpleChannelUpstreamHandler.handleUpstream(SimpleChannelUpstreamHandler.java:70) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:564) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.DefaultChannelPipeline.sendUpstream(DefaultChannelPipeline.java:559) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:268) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.Channels.fireMessageReceived(Channels.java:255) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:88) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.process(AbstractNioWorker.java:109) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.socket.nio.AbstractNioSelector.run(AbstractNioSelector.java:312) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.socket.nio.AbstractNioWorker.run(AbstractNioWorker.java:90) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:178) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.util.ThreadRenamingRunnable.run(ThreadRenamingRunnable.java:108) [netty-3.7.0.Final.jar:na]
at org.jboss.netty.util.internal.DeadLockProofWorker$1.run(DeadLockProofWorker.java:42) [netty-3.7.0.Final.jar:na]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_92]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_92]
at java.lang.Thread.run(Thread.java:745) [na:1.8.0_92]

最终查到问题出在我们两人使用的dubbo不一样，虽然都是2.8.4版本，但hessian反序列化的类在实现上有所不同，导致解析报文出错。

注：其实通信双方dubbo版本不兼容，出现的问题不只上面一种，笔者将调用方与服务提供方的dubbo版本互换则得到另外一个错误，但这两种错误最后出问题的地方都是Hessian反序列化的地方！