I am trying to attach a capability to the player that controls a custom health state. When the

AttachCapabilitiesEvent.Entity

or

AttachCapabilitiesEvent<Entity>

is fired I check for an instance of EntityPlayer and call

event.addCapability(new ResourceLocation(Reference.MOD_ID, "dbno"), new DBNOHandlerProvider((EntityPlayer) event.getObject()));

The constructor then sets a player variable in the capability to the value passed to it in the above statement. The setter for this player variable then sends an update packet to the client for good measure. The player is serialized by writing his UUID to NBT. This causes a NullPointerException:

[19:58:33] [Server thread/ERROR]: SimpleChannelHandlerWrapper exception
java.lang.NullPointerException
	at net.minecraftforge.fml.common.network.FMLOutboundHandler$OutboundTarget$4.selectNetworks(FMLOutboundHandler.java:132) ~[FMLOutboundHandler$OutboundTarget$4.class:?]
	at net.minecraftforge.fml.common.network.FMLOutboundHandler.write(FMLOutboundHandler.java:293) ~[FMLOutboundHandler.class:?]
	at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:658) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:716) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:651) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.handler.codec.MessageToMessageEncoder.write(MessageToMessageEncoder.java:112) ~[MessageToMessageEncoder.class:4.0.23.Final]
	at io.netty.handler.codec.MessageToMessageCodec.write(MessageToMessageCodec.java:116) ~[MessageToMessageCodec.class:4.0.23.Final]
	at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:658) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:716) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:706) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.channel.AbstractChannelHandlerContext.writeAndFlush(AbstractChannelHandlerContext.java:741) ~[AbstractChannelHandlerContext.class:4.0.23.Final]
	at io.netty.channel.DefaultChannelPipeline.writeAndFlush(DefaultChannelPipeline.java:895) ~[DefaultChannelPipeline.class:4.0.23.Final]
	at io.netty.channel.AbstractChannel.writeAndFlush(AbstractChannel.java:240) ~[AbstractChannel.class:4.0.23.Final]
	at net.minecraftforge.fml.common.network.simpleimpl.SimpleNetworkWrapper.sendTo(SimpleNetworkWrapper.java:254) [SimpleNetworkWrapper.class:?]
	at XFactHD.rssmc.RainbowSixSiegeMC$NET.sendMessageToClient(RainbowSixSiegeMC.java:320) [RainbowSixSiegeMC$NET.class:?]
	at XFactHD.rssmc.common.capability.dbnoHandler.DBNOHandler.sendUpdatePacket(DBNOHandler.java:196) [DBNOHandler.class:?]
	at XFactHD.rssmc.common.capability.dbnoHandler.DBNOHandler.setPlayer(DBNOHandler.java:36) [DBNOHandler.class:?]
	at XFactHD.rssmc.common.capability.dbnoHandler.DBNOHandlerProvider.<init>(DBNOHandlerProvider.java:18) [DBNOHandlerProvider.class:?]
	at XFactHD.rssmc.common.capability.dbnoHandler.DBNOEventHandler.attachCapability(DBNOEventHandler.java:141) [DBNOEventHandler.class:?]
	at net.minecraftforge.fml.common.eventhandler.ASMEventHandler_18_DBNOEventHandler_attachCapability_Entity.invoke(.dynamic) [?:?]
	at net.minecraftforge.fml.common.eventhandler.ASMEventHandler.invoke(ASMEventHandler.java:90) [ASMEventHandler.class:?]
	at net.minecraftforge.fml.common.eventhandler.EventBus.post(EventBus.java:185) [EventBus.class:?]
	at net.minecraftforge.event.ForgeEventFactory.gatherCapabilities(ForgeEventFactory.java:565) [ForgeEventFactory.class:?]
	at net.minecraftforge.event.ForgeEventFactory.gatherCapabilities(ForgeEventFactory.java:550) [ForgeEventFactory.class:?]
	at net.minecraft.entity.Entity.<init>(Entity.java:253) [Entity.class:?]
	at net.minecraft.entity.EntityLivingBase.<init>(EntityLivingBase.java:192) [EntityLivingBase.class:?]
	at net.minecraft.entity.player.EntityPlayer.<init>(EntityPlayer.java:182) [EntityPlayer.class:?]
	at net.minecraft.entity.player.EntityPlayerMP.<init>(EntityPlayerMP.java:166) [EntityPlayerMP.class:?]
	at net.minecraft.server.management.PlayerList.createPlayerForUser(PlayerList.java:527) [PlayerList.class:?]
	at net.minecraft.server.network.NetHandlerLoginServer.tryAcceptPlayer(NetHandlerLoginServer.java:137) [NetHandlerLoginServer.class:?]
	at net.minecraft.server.network.NetHandlerLoginServer.update(NetHandlerLoginServer.java:64) [NetHandlerLoginServer.class:?]
	at net.minecraft.network.NetworkManager.processReceivedPackets(NetworkManager.java:309) [NetworkManager.class:?]
	at net.minecraft.network.NetworkSystem.networkTick(NetworkSystem.java:197) [NetworkSystem.class:?]
	at net.minecraft.server.MinecraftServer.updateTimeLightAndEntities(MinecraftServer.java:807) [MinecraftServer.class:?]
	at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:688) [MinecraftServer.class:?]
	at net.minecraft.server.integrated.IntegratedServer.tick(IntegratedServer.java:156) [IntegratedServer.class:?]
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:537) [MinecraftServer.class:?]
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_131]

This stacktrace is logged and seems to be ignored. I don't know if this causes the capability to be attached. When I then left click one of my items that is completely unrelated to the capability, it causes a crash with the exact same stacktrace. This crash happens even though I am checking on the client if the client player entity already got the capability attached and if not, I queue the NBT data for deserialization until the capability has been attached on the client.

If the item class, the capability class or the event handler class for attaching the capability is needed, I will happily provide them.

Edited July 12, 20178 yr by XFactHD

You can't send a packet to the player as that player is being constructed(this is the stage the capabilities are attached at) as at that stage the player's connection field is null and that is used by fml to send the packet. And even if it wouldn't the player's uuid is not properly set from their GameProfile at that stage either. You need to send the packet later.

Yeah, that makes sense, I should have thought a bit more about the fact that the event is fired in the entity's constructor . Thanks for your help.