My teams mod requires a lot of testing with multiple players, and in the past I've just run two copies from Eclipse and connected using the "Open to LAN" features.

 

The first problem I had with 1.7.10 was that everyone was called "ForgeDevName", and so could not play LAN together. So I set up some custom launch configurations that use the program argument "--username jamioflan" or "--username Notch" and ran two different copies. Now when I connect I get the following error on the connecting client

 

A detailed walkthrough of the error, its code path and all known details is as follows:

---------------------------------------------------------------------------------------

 

-- Head --

Stacktrace:

at com.mojang.util.UUIDTypeAdapter.fromUUID(UUIDTypeAdapter.java:22)

at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdapter.java:13)

at com.mojang.util.UUIDTypeAdapter.write(UUIDTypeAdapter.java:10)

at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)

at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)

at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)

at com.google.gson.Gson.toJson(Gson.java:593)

at com.google.gson.Gson.toJson(Gson.java:572)

at com.google.gson.Gson.toJson(Gson.java:527)

at com.google.gson.Gson.toJson(Gson.java:507)

at com.mojang.authlib.yggdrasil.YggdrasilAuthenticationService.makeRequest(YggdrasilAuthenticationService.java:54)

at com.mojang.authlib.yggdrasil.YggdrasilMinecraftSessionService.joinServer(YggdrasilMinecraftSessionService.java:76)

at net.minecraft.client.network.NetHandlerLoginClient.handleEncryptionRequest(NetHandlerLoginClient.java:62)

at net.minecraft.network.login.server.S01PacketEncryptionRequest.processPacket(S01PacketEncryptionRequest.java:54)

at net.minecraft.network.login.server.S01PacketEncryptionRequest.processPacket(S01PacketEncryptionRequest.java:62)

at net.minecraft.network.NetworkManager.processReceivedPackets(NetworkManager.java:247)

at net.minecraft.client.multiplayer.GuiConnecting.updateScreen(GuiConnecting.java:116)

 

-- Affected screen --

Details:

Screen name: net.minecraft.client.multiplayer.GuiConnecting

Stacktrace:

at net.minecraft.client.Minecraft.runTick(Minecraft.java:1751)

at net.minecraft.client.Minecraft.runGameLoop(Minecraft.java:1038)

at net.minecraft.client.Minecraft.run(Minecraft.java:961)

at net.minecraft.client.main.Main.main(Main.java:164)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)

at java.lang.reflect.Method.invoke(Unknown Source)

at net.minecraft.launchwrapper.Launch.launch(Launch.java:134)

at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)

at java.lang.reflect.Method.invoke(Unknown Source)

at GradleStart.bounce(GradleStart.java:108)

at GradleStart.startClient(GradleStart.java:101)

at GradleStart.main(GradleStart.java:66)

 

It appears to be a problem with the new UUIDs. Is there perhaps a parameter like --username that sets a valid UUID? Or is there any other workaround / fix?

--username takes the actual Mojang login username (more than likely your email), not the display name, and needs to be paired with --password.

Setting these didn't work. I got the same problem with lack of UUID

You have to login with two original account if you want to test out the lan stuff.

 

Just to know if you open up a dedicated server and two client you can test stuff out there. If its working there it will work on LAN too. Mostly....