I'm getting 

[00:59:39] [Server thread/FATAL] [ne.mi.fm.lo.RuntimeDistCleaner/DISTXFORM]: Attempted to load class net/minecraft/client/gui/screen/Screen for invalid dist DEDICATED_SERVER
[00:59:39] [Server thread/FATAL] [minecraft/ThreadTaskExecutor]: Error executing task on Server
java.lang.RuntimeException: Attempted to load class net/minecraft/client/gui/screen/Screen for invalid dist DEDICATED_SERVER
	at net.minecraftforge.fml.loading.RuntimeDistCleaner.processClassWithFlags(RuntimeDistCleaner.java:71) ~[forge-1.16.5-36.2.20_mapped_snapshot_20210309-1.16.5-launcher.jar:36.2] {}
	at cpw.mods.modlauncher.LaunchPluginHandler.offerClassNodeToPlugins(LaunchPluginHandler.java:85) ~[modlauncher-8.0.9.jar:?] {}
	at cpw.mods.modlauncher.ClassTransformer.transform(ClassTransformer.java:120) ~[modlauncher-8.0.9.jar:?] {}
	at cpw.mods.modlauncher.TransformingClassLoader$DelegatedClassLoader.findClass(TransformingClassLoader.java:265) ~[modlauncher-8.0.9.jar:?] {}
	at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:136) ~[modlauncher-8.0.9.jar:?] {re:classloading}
	at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:98) ~[modlauncher-8.0.9.jar:?] {re:classloading}
	at java.lang.ClassLoader.loadClass(ClassLoader.java:351) ~[?:1.8.0_302] {}
	at io.github.davidqf555.minecraft.towerofgod.common.blocks.FloorTeleportationTerminalBlock.onBlockActivated(FloorTeleportationTerminalBlock.java:56) ~[?:?] {re:classloading}
	at net.minecraft.block.AbstractBlock$AbstractBlockState.onBlockActivated(AbstractBlock.java:732) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
	at net.minecraft.server.management.PlayerInteractionManager.func_219441_a(PlayerInteractionManager.java:347) ~[forge:?] {re:classloading}
	at net.minecraft.network.play.ServerPlayNetHandler.processTryUseItemOnBlock(ServerPlayNetHandler.java:986) ~[forge:?] {re:classloading}
	at net.minecraft.network.play.client.CPlayerTryUseItemOnBlockPacket.processPacket(CPlayerTryUseItemOnBlockPacket.java:45) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
	at net.minecraft.network.play.client.CPlayerTryUseItemOnBlockPacket.processPacket(CPlayerTryUseItemOnBlockPacket.java:12) ~[forge:?] {re:classloading,pl:runtimedistcleaner:A}
	at net.minecraft.network.PacketThreadUtil.lambda$checkThreadAndEnqueue$0(PacketThreadUtil.java:19) ~[forge:?] {re:classloading}
	at net.minecraft.util.concurrent.TickDelayedTask.run(TickDelayedTask.java:20) ~[forge:?] {re:classloading}
	at net.minecraft.util.concurrent.ThreadTaskExecutor.run(ThreadTaskExecutor.java:139) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.util.concurrent.RecursiveEventLoop.run(RecursiveEventLoop.java:22) ~[forge:?] {re:classloading}
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:759) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:159) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.util.concurrent.ThreadTaskExecutor.driveOne(ThreadTaskExecutor.java:109) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.server.MinecraftServer.driveOneInternal(MinecraftServer.java:742) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.server.MinecraftServer.driveOne(MinecraftServer.java:736) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.util.concurrent.ThreadTaskExecutor.driveUntil(ThreadTaskExecutor.java:122) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.server.MinecraftServer.runScheduledTasks(MinecraftServer.java:722) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:668) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at net.minecraft.server.MinecraftServer.lambda$startServer$0(MinecraftServer.java:233) ~[forge:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
	at java.lang.Thread.run(Thread.java:748) [?:1.8.0_302] {}

for this mod when playing it on dedicated servers. I understand that Screens should not be used on dedicated servers, but I was using packets to make sure that the screen only opened on client. I'm wondering if I need to add checks or something elsewhere to make sure that the packet is only handled on clients. 

Edited March 3, 20223 yr by DavidQF555
Solved

2 minutes ago, diesieben07 said:

The packet class is still loaded on both sides. You have to use DistExecutor when accessing dist-only classes from common code.

I updated my handle method to: 

private void handle(NetworkEvent.Context context) {
        NetworkDirection dir = context.getDirection();
        if (dir == NetworkDirection.PLAY_TO_CLIENT) {
            context.enqueueWork(() -> DistExecutor.safeRunWhenOn(Dist.CLIENT, () -> () -> Minecraft.getInstance().displayGuiScreen(new FloorTeleportationTerminalScreen(level, teleporter, direction))));
            context.setPacketHandled(true);
        }
    }

Just to check, is the way I'm using DistExecutor correct in terms of convention?

12 minutes ago, diesieben07 said:

No (and safeRunWhenOn should yell at you). The actual code must be in a separate class.

Ok I think I got it. Basically, I can't import any client only code (marked with OnlyIn) in any of my common code?

3 minutes ago, diesieben07 said:

Imports are entirely irrelevant.

What I mean is using OnlyIn client methods/classes. For classes though, I can usually tell whether I'm using client only classes by just looking at imports. 

Is it necessary to use DistExecutor? It seems like the main issue is that I was using client only classes in my packets but moving them to another class solved that. Is using DistExecutor in my packet handling methods that already check directions just for safety?

8 minutes ago, diesieben07 said:

You should not check the network direction, instead specify it when registering your packet, then FML will disconnect when the packet is sent in an invalid manner.

Ok, I'll do that. Is there any point in using DistExecutor for handling then?