Save capability forge 1.12

Hello guys, i've starting to code in java for dev some mods and in my first mod i try to save player first connection on the server. Since now i did a system with capability which is working perfectly but i can't manage the capability saving problem when i restart the server. I did my capability register but it's looks deprecated, it could be the issue ? 

Cordially

public class CommonProxy {
    public void init()
    {
        CapabilityManager.INSTANCE.register(IFirstJoin.class, new FirstJoinStorage(), FirstJoin.class);

        MinecraftForge.EVENT_BUS.register(new CapabilityHandler());
        MinecraftForge.EVENT_BUS.register(new EventHandler());
    }
}

public class CapabilityHandler
{
    public static final ResourceLocation FIRST_JOIN_CAP = new ResourceLocation(tonation.MODID, "first_join");

    @SubscribeEvent
    public void attachCapability(AttachCapabilitiesEvent<Entity> event)
    {
        if (!(event.getObject() instanceof EntityPlayer)) return;

        event.addCapability(FIRST_JOIN_CAP, new FirstJoinProvider());
    }
}

public class EventHandler {
    @SubscribeEvent
    public void onPlayerLogsIn(PlayerEvent.PlayerLoggedInEvent event)
    {
        EntityPlayer player = event.player;
        IFirstJoin first_join = player.getCapability(FirstJoinProvider.FIRST_JOIN_CAP, null);

        if(first_join.isFirstConnection() == 1){
            String message = String.format("Here is your first connection!", (int) first_join.isFirstConnection());
            player.sendMessage(new TextComponentString(message));
        }
        else{
            player.sendMessage(new TextComponentString("Hello back bud!"));
        }
        player.getCapability(FirstJoinProvider.FIRST_JOIN_CAP, null).setFirstJoin();
    }
}