Strange Netty error whilst deserializing

Question

I am using Netty to send objects over a Socket. It works fine except when I try to send this one type of Packet, I get this error on the receiving side

[16:57:14] [nioEventLoopGroup-3-1/INFO]: [Castle_Defense_Lobby] Connected to proxy
[16:57:14] [nioEventLoopGroup-3-1/ERROR]: Unexpected exception from downstream, disconnecting...
io.netty.handler.codec.DecoderException: java.io.InvalidClassException: failed to read class descriptor
    at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:259) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:141) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.DefaultChannelHandlerContext.invokeChannelRead(DefaultChannelHandlerContext.java:340) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.DefaultChannelHandlerContext.fireChannelRead(DefaultChannelHandlerContext.java:326) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:785) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:116) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:494) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:461) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:378) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:350) [CastleDefense-Lobby-1.0.jar:?]
    at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:101) [CastleDefense-Lobby-1.0.jar:?]
    at java.lang.Thread.run(Unknown Source) [?:1.8.0-ea]
Caused by: java.io.InvalidClassException: failed to read class descriptor
    at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readClassDesc(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readObject0(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readObject(Unknown Source) ~[?:1.8.0-ea]
    at io.netty.handler.codec.serialization.ObjectDecoder.decode(ObjectDecoder.java:73) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.handler.codec.LengthFieldBasedFrameDecoder.decode(LengthFieldBasedFrameDecoder.java:343) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:228) ~[CastleDefense-Lobby-1.0.jar:?]
    ... 11 more
Caused by: java.lang.ClassNotFoundException: com.iKeirNez.CastleDefense.common.packets.PacketServers
    at java.net.URLClassLoader$1.run(Unknown Source) ~[?:1.8.0-ea]
    at java.net.URLClassLoader$1.run(Unknown Source) ~[?:1.8.0-ea]
    at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0-ea]
    at java.net.URLClassLoader.findClass(Unknown Source) ~[?:1.8.0-ea]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0-ea]
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source) ~[?:1.8.0-ea]
    at java.lang.ClassLoader.loadClass(Unknown Source) ~[?:1.8.0-ea]
    at java.lang.Class.forName0(Native Method) ~[?:1.8.0-ea]
    at java.lang.Class.forName(Unknown Source) ~[?:1.8.0-ea]
    at io.netty.handler.codec.serialization.ClassLoaderClassResolver.resolve(ClassLoaderClassResolver.java:31) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.handler.codec.serialization.CompactObjectInputStream.readClassDescriptor(CompactObjectInputStream.java:55) ~[CastleDefense-Lobby-1.0.jar:?]
    at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readClassDesc(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readObject0(Unknown Source) ~[?:1.8.0-ea]
    at java.io.ObjectInputStream.readObject(Unknown Source) ~[?:1.8.0-ea]
    at io.netty.handler.codec.serialization.ObjectDecoder.decode(ObjectDecoder.java:73) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.handler.codec.LengthFieldBasedFrameDecoder.decode(LengthFieldBasedFrameDecoder.java:343) ~[CastleDefense-Lobby-1.0.jar:?]
    at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:228) ~[CastleDefense-Lobby-1.0.jar:?]
    ... 11 more
[16:57:14] [nioEventLoopGroup-3-1/WARN]: Lost connection, attempting reconnect...
[16:57:14] [nioEventLoopGroup-3-2/INFO]: Successfully reconnected

I can confirm that the class exists in the same package on both sides (as I use Maven to shade it in). Here is the class I am sending

package com.iKeirNez.CastleDefense.common.packets;

import com.iKeirNez.packetapi.api.packets.Packet;

import java.net.InetSocketAddress;
import java.util.Map;

/**
 * Sent from Proxy to Lobby, notifying of what servers to listen for
 * Created by iKeirNez on 11/04/2014.
 */
public class PacketServers implements Packet {

    private static final long serialVersionUID = 524383226296782276L;

    public Map<String, InetSocketAddress> servers;

    public PacketServers(Map<String, InetSocketAddress> servers){
        this.servers = servers;
    }
}

I am really confused as to why this error is being thrown, I also checked that InetSocketAddress is serialializable and that seems to be the case.

Edit: The class "Packet" is a very simple class which extends Serializable

package com.iKeirNez.packetapi.api.packets;

import java.io.Serializable;

/**
 * Created by iKeirNez on 06/04/2014.
 */
public interface Packet extends Serializable {
}

Answer 1

When setting up pipeline pass correct ClassLoader to ObjectDecoder:

new ObjectDecoder(ClassResolvers.weakCachingConcurrentResolver(PacketServers.class.getClassLoader()))