How do I make a single-player mod able to run on a server?

[Will Bradley]

Specifically, what do I need to do to make my code which works on single-player run on a server? I'm familiar with sidedness, I'm just lost on what boilerplate code/pitfalls I should be aware of.

 

Thanks!

[diesieben07]

Single player is like playing on a server with just one player.

If you wrote your code correctly, you do not need to do anything else.

If your mod does not work correctly on a server, chances are it is already buggy in single player.

[Will Bradley]

It works great in single player! But don't some operations like Minecraft.getInstance() crash servers, even though they work fine on single player worlds?

[diesieben07]

Minecraft.getInstance will only work on the logical client. Accessing it from the integrated server (as in singleplayer) will "work" in that it doesn't immediately crash, but it will not achieve useful results usually.

If your code is set up correctly, this code will never be loaded or executed on servers so you are fine. You can use @EventBusSubscriber with the Dist parameter to create distribution-specific code.

[Will Bradley]

According to the docs, EventBusSubscriber only works for classes with static event handlers— do I use level.isClientSide otherwise?

[diesieben07]

The two things have nothing to do with each other...?

[Will Bradley]

Both are tools used to avoid running client-only code on servers, correct?

[diesieben07]

No.

Specifying Dist for @EventBusSubscriber prevents the registration of the event handler on some distributions. Level#isClientSide checks the logical side for a level.

[Will Bradley]

Could you elaborate more? Specifically, is "distributions" referring to physical client vs. physical server? And what are the use cases for either method (@EventBusSubscriber vs Level#isClientSide)?

[diesieben07]

1 hour ago, Will Bradley said:

Could you elaborate more? Specifically, is "distributions" referring to physical client vs. physical server?

Correct. Dedicated server (running only a logical server) and client (running a logical client and logical server when playing single player).

1 hour ago, Will Bradley said:

And what are the use cases for either method (@EventBusSubscriber vs Level#isClientSide)?

@EventBusSubscriber subscribes something to the event bus. When specifying a Dist to it, it will only register the event bus on that distribution. This can be used to safely reference code, that only exists in either distribution (99.9% of the time this is client-only code). For example: Rendering only exists on the client. Therefore your mod will crash with NoClassDefFoundErrors when installed on a server, should it directly references such classes (indicated by @OnlyIn on the class) from common code. The solution is usually to use a distribution-only event subscriber using @EventBusSubscriber with Dist.CLIENT. This will for example be used to register renderers for your entities.

Level#isClientSide checks the logical side. This is used so that you only execute logic on the correct side. For example: Transferring items from an inventory to another should only be done server side. But you still want to run it in single player, so checking the distribution is the wrong thing here. You need to check !Level#isClientSide to check that you are on the server thread (be it running in a dedicated server or the server thread of a single player game).

[Will Bradley]

Got it, thanks! And is it correct to assume most operations (say, Snowball#shoot) can be called without worrying about Level#isClientSide ?

[diesieben07]

8 hours ago, Will Bradley said:

Got it, thanks! And is it correct to assume most operations (say, Snowball#shoot) can be called without worrying about Level#isClientSide ?

Many things can be cancelled server side only, yes.