dubbo调用服务出现如下异常

log4j:WARN No appenders could be found for logger (org.springframework.context.support.ClassPathXmlApplicationContext).
log4j:WARN Please initialize the log4j system properly.
log4j:WARN See http://logging.apache.org/log4j/1.2/faq.html#noconfig for more info.
Exception in thread "main" com.alibaba.dubbo.rpc.RpcException: Failed to invoke the method IFtpHdfsFileTransfer in the service com.cetc.di.io.manager.IFTPtoHDFSDubboService. Tried 3 times of the providers [172.28.3.40:21000] (1/1) from the registry 172.28.3.50:2181 on the consumer 172.28.0.74 using the dubbo version 2.4.8. Last error is: Failed to invoke remote method: IFtpHdfsFileTransfer, provider: dubbo://172.28.3.40:21000/com.cetc.di.io.manager.IFTPtoHDFSDubboService?anyhost=true&application=consumer-of-ftp2HDFS-app&check=false&dubbo=2.0.0&interface=com.cetc.di.io.manager.IFTPtoHDFSDubboService&methods=IFtpHdfsFileTransfer&pid=7280&side=consumer&timestamp=1446533377245, cause: Failed to send response: Response [id=2, version=2.0.0, status=20, event=false, error=null, result=RpcResult [result=com.cetc.di.io.manager.FTPtoHDFSReturn@46c79a0c, exception=null]], cause: java.lang.IllegalStateException: Serialized class com.cetc.di.io.manager.FTPtoHDFSReturn must implement java.io.Serializable
java.lang.IllegalStateException: Serialized class com.cetc.di.io.manager.FTPtoHDFSReturn must implement java.io.Serializable
    at com.alibaba.com.caucho.hessian.io.SerializerFactory.getDefaultSerializer(SerializerFactory.java:261)
    at com.alibaba.com.caucho.hessian.io.SerializerFactory.getSerializer(SerializerFactory.java:233)
    at com.alibaba.com.caucho.hessian.io.Hessian2Output.writeObject(Hessian2Output.java:406)
    at com.alibaba.dubbo.common.serialize.support.hessian.Hessian2ObjectOutput.writeObject(Hessian2ObjectOutput.java:92)
    at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCodec.encodeResponseData(DubboCodec.java:200)
    at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.encodeResponse(ExchangeCodec.java:273)
    at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.encode(ExchangeCodec.java:77)
    at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCountCodec.encode(DubboCountCodec.java:41)
    at com.alibaba.dubbo.remoting.transport.netty.NettyCodecAdapter$InternalEncoder.encode(NettyCodecAdapter.java:82)
    at org.jboss.netty.handler.codec.oneone.OneToOneEncoder.doEncode(OneToOneEncoder.java:66)
    at org.jboss.netty.handler.codec.oneone.OneToOneEncoder.handleDownstream(OneToOneEncoder.java:59)
    at org.jboss.netty.channel.DefaultChannelPipeline.sendDownstream(DefaultChannelPipeline.java:591)
    at org.jboss.netty.channel.DefaultChannelPipeline$DefaultChannelHandlerContext.sendDownstream(DefaultChannelPipeline.java:784)
    at org.jboss.netty.channel.SimpleChannelHandler.writeRequested(SimpleChannelHandler.java:292)
    at com.alibaba.dubbo.remoting.transport.netty.NettyHandler.writeRequested(NettyHandler.java:99)
    at org.jboss.netty.channel.SimpleChannelHandler.handleDownstream(SimpleChannelHandler.java:254)
    at org.jboss.netty.channel.DefaultChannelPipeline.sendDownstream(DefaultChannelPipeline.java:591)
    at org.jboss.netty.channel.DefaultChannelPipeline.sendDownstream(DefaultChannelPipeline.java:582)
    at org.jboss.netty.channel.Channels.write(Channels.java:704)
    at org.jboss.netty.channel.Channels.write(Channels.java:671)
    at org.jboss.netty.channel.AbstractChannel.write(AbstractChannel.java:248)
    at com.alibaba.dubbo.remoting.transport.netty.NettyChannel.send(NettyChannel.java:98)
    at com.alibaba.dubbo.remoting.transport.AbstractPeer.send(AbstractPeer.java:51)
    at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.received(HeaderExchangeHandler.java:171)
    at com.alibaba.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:52)
    at com.alibaba.dubbo.remoting.transport.dispather.ChannelEventRunnable.run(ChannelEventRunnable.java:82)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)

    at com.alibaba.dubbo.rpc.cluster.support.FailoverClusterInvoker.doInvoke(FailoverClusterInvoker.java:101)
    at com.alibaba.dubbo.rpc.cluster.support.AbstractClusterInvoker.invoke(AbstractClusterInvoker.java:226)
    at com.alibaba.dubbo.rpc.cluster.support.wrapper.MockClusterInvoker.invoke(MockClusterInvoker.java:72)
    at com.alibaba.dubbo.rpc.proxy.InvokerInvocationHandler.invoke(InvokerInvocationHandler.java:52)
    at com.alibaba.dubbo.common.bytecode.proxy0.IFtpHdfsFileTransfer(proxy0.java)
    at com.cetc.di.io.manager.Consumer.main(Consumer.java:20)
Caused by: com.alibaba.dubbo.remoting.RemotingException: Failed to send response: Response [id=2, version=2.0.0, status=20, event=false, error=null, result=RpcResult [result=com.cetc.di.io.manager.FTPtoHDFSReturn@46c79a0c, exception=null]], cause: java.lang.IllegalStateException: Serialized class com.cetc.di.io.manager.FTPtoHDFSReturn must implement java.io.Serializable
java.lang.IllegalStateException: Serialized class com.cetc.di.io.manager.FTPtoHDFSReturn must implement java.io.Serializable
    at com.alibaba.com.caucho.hessian.io.SerializerFactory.getDefaultSerializer(SerializerFactory.java:261)
    at com.alibaba.com.caucho.hessian.io.SerializerFactory.getSerializer(SerializerFactory.java:233)
    at com.alibaba.com.caucho.hessian.io.Hessian2Output.writeObject(Hessian2Output.java:406)
    at com.alibaba.dubbo.common.serialize.support.hessian.Hessian2ObjectOutput.writeObject(Hessian2ObjectOutput.java:92)
    at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCodec.encodeResponseData(DubboCodec.java:200)
    at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.encodeResponse(ExchangeCodec.java:273)
    at com.alibaba.dubbo.remoting.exchange.codec.ExchangeCodec.encode(ExchangeCodec.java:77)
    at com.alibaba.dubbo.rpc.protocol.dubbo.DubboCountCodec.encode(DubboCountCodec.java:41)
    at com.alibaba.dubbo.remoting.transport.netty.NettyCodecAdapter$InternalEncoder.encode(NettyCodecAdapter.java:82)
    at org.jboss.netty.handler.codec.oneone.OneToOneEncoder.doEncode(OneToOneEncoder.java:66)
    at org.jboss.netty.handler.codec.oneone.OneToOneEncoder.handleDownstream(OneToOneEncoder.java:59)
    at org.jboss.netty.channel.DefaultChannelPipeline.sendDownstream(DefaultChannelPipeline.java:591)
    at org.jboss.netty.channel.DefaultChannelPipeline$DefaultChannelHandlerContext.sendDownstream(DefaultChannelPipeline.java:784)
    at org.jboss.netty.channel.SimpleChannelHandler.writeRequested(SimpleChannelHandler.java:292)
    at com.alibaba.dubbo.remoting.transport.netty.NettyHandler.writeRequested(NettyHandler.java:99)
    at org.jboss.netty.channel.SimpleChannelHandler.handleDownstream(SimpleChannelHandler.java:254)
    at org.jboss.netty.channel.DefaultChannelPipeline.sendDownstream(DefaultChannelPipeline.java:591)
    at org.jboss.netty.channel.DefaultChannelPipeline.sendDownstream(DefaultChannelPipeline.java:582)
    at org.jboss.netty.channel.Channels.write(Channels.java:704)
    at org.jboss.netty.channel.Channels.write(Channels.java:671)
    at org.jboss.netty.channel.AbstractChannel.write(AbstractChannel.java:248)
    at com.alibaba.dubbo.remoting.transport.netty.NettyChannel.send(NettyChannel.java:98)
    at com.alibaba.dubbo.remoting.transport.AbstractPeer.send(AbstractPeer.java:51)
    at com.alibaba.dubbo.remoting.exchange.support.header.HeaderExchangeHandler.received(HeaderExchangeHandler.java:171)
    at com.alibaba.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:52)
    at com.alibaba.dubbo.remoting.transport.dispather.ChannelEventRunnable.run(ChannelEventRunnable.java:82)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:745)

    at com.alibaba.dubbo.remoting.exchange.support.DefaultFuture.returnFromResponse(DefaultFuture.java:190)
    at com.alibaba.dubbo.remoting.exchange.support.DefaultFuture.get(DefaultFuture.java:110)
    at com.alibaba.dubbo.remoting.exchange.support.DefaultFuture.get(DefaultFuture.java:84)
    at com.alibaba.dubbo.rpc.protocol.dubbo.DubboInvoker.doInvoke(DubboInvoker.java:96)
    at com.alibaba.dubbo.rpc.protocol.AbstractInvoker.invoke(AbstractInvoker.java:144)
    at com.alibaba.dubbo.rpc.listener.ListenerInvokerWrapper.invoke(ListenerInvokerWrapper.java:74)
    at com.alibaba.dubbo.rpc.protocol.dubbo.filter.FutureFilter.invoke(FutureFilter.java:53)
    at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
    at com.alibaba.dubbo.monitor.support.MonitorFilter.invoke(MonitorFilter.java:75)
    at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
    at com.alibaba.dubbo.rpc.filter.ConsumerContextFilter.invoke(ConsumerContextFilter.java:48)
    at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper$1.invoke(ProtocolFilterWrapper.java:91)
    at com.alibaba.dubbo.rpc.protocol.InvokerWrapper.invoke(InvokerWrapper.java:53)
    at com.alibaba.dubbo.rpc.cluster.support.FailoverClusterInvoker.doInvoke(FailoverClusterInvoker.java:77)
    ... 5 more

解决方案：