I've got a "wand" item that, on right-click, I wish to be able to inflict various things on mobs: set on fire or apply a potion effect to them. My two attempts at doing this are below.

 

Apply a poison potion effect:

    @Nonnull
    @Override
    public ActionResult<ItemStack> onItemRightClick(@Nonnull ItemStack itemstack, World world, EntityPlayer player, EnumHand hand) {
        RayTraceResult target = Minecraft.getMinecraft().objectMouseOver;
        if (target.typeOfHit == RayTraceResult.Type.ENTITY) {
            if (target.entityHit instanceof EntityLivingBase) {
                EntityLivingBase entitylivingbase = (EntityLivingBase) target.entityHit;
                Potion potion = Potion.REGISTRY.getObject(new ResourceLocation("poison"));
                PotionEffect potioneffect = new PotionEffect(potion, 50, 1);
                if (potion.isInstant()) {
                    potion.affectEntity(null, null, entitylivingbase, potioneffect.getAmplifier(), 0.5D);
                } else {
                    entitylivingbase.addPotionEffect(potioneffect);
                }
            }
        }
        return ActionResult.newResult(EnumActionResult.SUCCESS, itemstack);
    }

The result of the above is that the potion effect doesn't appear to be applied at all.

 

Changing the code to instead set the entity on fire:

    @Nonnull
    @Override
    public ActionResult<ItemStack> onItemRightClick(@Nonnull ItemStack itemstack, World world, EntityPlayer player, EnumHand hand) {
        RayTraceResult target = Minecraft.getMinecraft().objectMouseOver;
        if (target.typeOfHit == RayTraceResult.Type.ENTITY) {
            target.entityHit.setFire(4);
        }
        return ActionResult.newResult(EnumActionResult.SUCCESS, itemstack);
    }

This results in a brief flash of fire effect on the entity, but it does not remain on fire for the full 4 seconds that I specify. I know that I've invoked the setFire method correctly because if I do so in a hitEntity() override like so:

    public boolean hitEntity(ItemStack stack, EntityLivingBase target, EntityLivingBase attacker) {
        target.setFire(4);
        return true;
    }

The entity stays on fire!

Minecraft.getMinecraft().objectMouseOver;

Is client side only, meaning since the server doesn't know about this it can't do it. Meaning it is reset next tick for the client.

OK, thanks. I'm now looking into how to send packets from the client to the server!

 

Solution: I ended up implementing InvokeWandMessage which encoded the RayTraceResult generated by PythonWandItem and sent it to the server for actioning. Works!!