How to debug dedicated server problems.

[Jontom Xire]

I have finally got around to setting up a dedicated Minecraft server and my mod crashes.

 

The first crash was easy. It complained about trying to instantiate a Screen. A quick search of my code showed that my client event handler class was "static final" initialising my screen. I changed it to dynamic initialisation, moved client tick event registration into the FMLClientSetupEvent handler and all was well...or so I hoped.

I now get this error:

[17:03:22] [main/ERROR] [minecraft/Main]: Failed to start the minecraft server
net.minecraftforge.fml.LoadingFailedException: Loading errors encountered: [
	Stats and Skills (stats_and_skills) has failed to load correctly
§7java.lang.RuntimeException: Attempted to load class net/minecraft/client/player/LocalPlayer for invalid dist DEDICATED_SERVER

So, I did a search of my code for LocalPlayer...and found nothing. I am not importing that class, not instantiating it, not (apparently) referencing it, anywhere.

 

I am not asking you to solve this problem. There will, I am sure, be others after this. I am asking for tips on how to find the root cause of such problems and would also appreciate tips on avoiding them in the first place.

 

[Jontom Xire]

I've tracked down this specific issue to this code, and now I am confused because I am sure I am handling this as per the documentation:

 

    public static void handler(SyncMsg msg, Supplier<NetworkEvent.Context> ctx)
    {
        ctx.get().enqueueWork(() -> DistExecutor.unsafeRunWhenOn(Dist.CLIENT, () -> () -> safe_handler(msg)));
        ctx.get().setPacketHandled(true);
    }

    public static void safe_handler(SyncMsg msg)
    {
        Player      player = (Player)(Minecraft.getInstance().player);
        PlayerData  data   = PlayerData.get(player);

        ...

 

The crash occurs when loading the mod, long before any of this code is executed. Minecraft.getInstance() is a static function, but surely this won't get executed until a message is received by the client.

[Jontom Xire]

And to answer my own question...the documentation, if you read the "Common mistakes" section REALLY closely, holds all the answers, including a hint about race conditions when using static members in a normal client installation. The bit I really needed to read was about: ...@OnlyIn(Dist) annotation....

 

Now all I need to do is figure out which import that is in.

 

HOWEVER:

The documentation also says:

 

    There is NO reason for using this annotation directly. Use DistExecutor or a check on FMLEnvironment#dist instead.

 

I am. It doesn't work! Yes my supplied method is in the same class, but I tried putting it in a different class and it didn't work either.

 

Edited February 19 by Jontom Xire

[Jontom Xire]

I changed "DistExecutor.unsafeRunWhenOn" to "DistExecutor.safeRunWhenOn". No change. However, I also at the same time made "DistExecutor.safeRunWhenOn" call an empty function in another class. Completely empty. However I did not remove/comment out my original safe_handler() function which is no longer being called AT ALL from ANYWHERE! Still got a crash. Code that is simply not being called at all.

 

I renamed the function to NOT_CALLED() to be really really sure it isn't being called at all somehow. Still crashed. Commented out the body of the NOT_CALLED() function and it works just fine.

Looks like @OnlyIn is my only option, as long as I can find the import...

[Alpvax]

On 2/19/2023 at 5:14 PM, Jontom Xire said:

Attempted to load class net/minecraft/client/player/LocalPlayer for invalid dist DEDICATED_SERVER

Classes in the client package don't exist on the server.

On 2/19/2023 at 5:56 PM, Jontom Xire said:

public static void handler(SyncMsg msg, Supplier<NetworkEvent.Context> ctx)
    {
        ctx.get().enqueueWork(() -> DistExecutor.unsafeRunWhenOn(Dist.CLIENT, () -> () -> safe_handler(msg)));
        ctx.get().setPacketHandled(true);
    }

    public static void safe_handler(SyncMsg msg)
    {
        Player      player = (Player)(Minecraft.getInstance().player);
        PlayerData  data   = PlayerData.get(player);

        ...

Are those methods in the same class? Any code that interacts with client only code should be in a seperate class.

Minecraft#player is an instance of LocalPlayer, so the game tries to load that class in order to cast it.

Please provide a complete stacktrace and git repo.

Edited March 10 by Alpvax