I've been trying all day to get this mod to work for a server (it works fine in SP) but during startup it crashed with this:

 

Exception in server tick loop

 

cpw.mods.fml.common.LoaderException: java.lang.NoClassDefFoundError: net/minecraft/client/entity/EntityClientPlayerMP

at cpw.mods.fml.common.LoadController.transition(LoadController.java:162)

at cpw.mods.fml.common.Loader.preinitializeMods(Loader.java:515)

at cpw.mods.fml.server.FMLServerHandler.beginServerLoading(FMLServerHandler.java:88)

at cpw.mods.fml.common.FMLCommonHandler.onServerStart(FMLCommonHandler.java:314)

at net.minecraft.server.dedicated.DedicatedServer.func_71197_b(DedicatedServer.java:117)

at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:387)

at net.minecraft.server.MinecraftServer$2.run(MinecraftServer.java:685)

Caused by: java.lang.NoClassDefFoundError: net/minecraft/client/entity/EntityClientPlayerMP

at apocalypse.gadgets.content.Jetpack.<init>(Jetpack.java:31)

at apocalypse.gadgets.main.Gadgets.preInit(Gadgets.java:62)

 

 

I've read that crash happens when trying to load a client mod on a server, but this is not a client mod since it adds items.

The line it points to isn't doing anything except loading the texture.

 

Class with the error:

public class Jetpack extends ItemArmor
{
int delay = 0;
public static Tab tab = Gadgets.tab;

public Jetpack(ArmorMaterial material, int renderIndex, int armourType)
{
	super(material, renderIndex, armourType);
	this.setCreativeTab(tab);
	this.canRepair = true;

	if(armourType == 1)
	{
		this.setTextureName(Tools.location("jetpack"));
	}
}
@Override
public String getArmorTexture(ItemStack stack, Entity entity, int slot, String layer)
{
	if(stack.getItem().equals(Gadgets.jetpack))
	{
		return Tools.location("textures/items/jetpackWorn.png");
	}
	else
	{
		return null;

	}
}

public boolean hasFuel(EntityPlayer player)
{
	return player.inventory.hasItem(Items.coal);
}

public void consumeFuel(EntityPlayer player, int rate)
{
    this.delay += 1;
    if (this.delay > rate)
    {
    	if (player.inventory.hasItem(Items.coal))
    	{
    		player.inventory.consumeInventoryItem(Items.coal);
    	}
    	this.delay = 0;
    }
}

@Override
    public void onArmorTick(World world, EntityPlayer player, ItemStack stack)
{
	if (player == null)
    {
      return;
    }
	ItemStack item = player.inventory.armorItemInSlot(2);
    if (item != null)
    {
        if ((item.getItem() instanceof Jetpack))
        {
          ((Jetpack)item.getItem()).action(player, (int)player.posX, (int)player.posY, (int)player.posZ, true);
        }
    }
}
public void action(EntityPlayer player, int x, int y, int z, boolean key)
{
	Random rand = new Random();

	double velx = rand.nextDouble() * 0.05D - 0.025D;
	double vely = -0.2D;
	double velz = rand.nextDouble() * 0.05D - 0.025D;

    if ((Keyboard.isKeyDown(Minecraft.getMinecraft().gameSettings.keyBindJump.getKeyCode())) && (hasFuel(player)))
    {
    	player.motionY = 0.1D;
    	player.jumpMovementFactor = 0.05F;
    	player.worldObj.spawnParticle("flame", player.posX, player.posY-1, player.posZ, velx, vely, velz);
    	player.worldObj.spawnParticle("largesmoke", player.posX, player.posY-1, player.posZ, velx, vely, velz);
    	consumeFuel(player, 10);
    }
    if(hasFuel(player))
    {
    	player.fallDistance = 0.0F;
    }
}
}

 

Main class:

@Mod(modid = Gadgets.MODID, version = Gadgets.VERSION)
public class Gadgets
{
    public static final String MODID = "gadgets";
    public static final String VERSION = "1.0";
    
    @Instance(MODID)
public static Gadgets instance;

    @SidedProxy(clientSide = "apocalypse.gadgets.network.Client", serverSide = "apocalypse.gadgets.network.Server")
public static Server server;

public static final Tab tab = new Tab("tab");

public static int backpackID = 1;

public static Item jetpack, backpack, handgun, bomb, bullet, bullet2, heart;
public static Item harbinger, halsSword, swordOfAeons;
public static Block bin;

@EventHandler
public void preInit(FMLPreInitializationEvent event)
{
    	server.registerEntityRenderers();
    	server.registerEvents();
    	server.registerTiles();

	jetpack = new Jetpack(ArmorMaterial.IRON, 4, 1).setUnlocalizedName("jetpack").setTextureName(Tools.location("jetpack")).setCreativeTab(tab);
	GameRegistry.registerItem(jetpack, "jetpack");

	backpack = new Backpack().setUnlocalizedName("backpack").setTextureName(Tools.location("backpack")).setCreativeTab(tab);
	GameRegistry.registerItem(backpack, "backpack");
	handgun = new Handgun().setUnlocalizedName("handgun").setTextureName(Tools.location("handgun")).setCreativeTab(tab);
	GameRegistry.registerItem(handgun, "handgun");
	bomb = new Bomb().setUnlocalizedName("bomb").setTextureName(Tools.location("bomb")).setCreativeTab(tab);
	GameRegistry.registerItem(bomb, "bomb");
	bullet = new Item().setUnlocalizedName("bullet").setTextureName(Tools.location("bullet")).setCreativeTab(tab);
	GameRegistry.registerItem(bullet, "bullet");
	bullet2 = new Item().setUnlocalizedName("bullet2").setTextureName(Tools.location("bullet2"));
	GameRegistry.registerItem(bullet2, "bullet2");

	heart = new Item().setUnlocalizedName("heart").setTextureName(Tools.location("heart")).setCreativeTab(tab);
	GameRegistry.registerItem(heart, "heart");

	harbinger = new Sword("", 600, 8, true, true, Items.emerald, false).setUnlocalizedName("harbinger").setTextureName(Tools.location("harbinger")).setCreativeTab(tab);
	GameRegistry.registerItem(harbinger, "harbinger");
	halsSword = new EnergySword("", 20, 30, true, false, null, true).setUnlocalizedName("halsSword").setTextureName(Tools.location("halsSword")).setCreativeTab(tab);
	GameRegistry.registerItem(halsSword, "halsSword");
	swordOfAeons = new Sword("", 2000, 10, true, true, Items.nether_star, false).setUnlocalizedName("swordOfAeons").setTextureName(Tools.location("swordOfAeons")).setCreativeTab(tab);
	GameRegistry.registerItem(swordOfAeons, "swordOfAeons");

	bin = new TrashBin().setBlockName("bin").setBlockTextureName(Tools.location("bin")).setCreativeTab(tab);
	GameRegistry.registerBlock(bin, "bin");

	GameRegistry.addShapedRecipe(new ItemStack(Gadgets.swordOfAeons), new Object[]{"A  ", " B ", "  B",'A', Gadgets.harbinger,'B', Items.nether_star,});
	GameRegistry.addShapedRecipe(new ItemStack(Gadgets.jetpack), new Object[]{"ABA", "ADA", "CAC",'A', Items.iron_ingot,'B', Items.redstone,'C', Items.blaze_rod,'D', Items.gunpowder});
	GameRegistry.addShapedRecipe(new ItemStack(Gadgets.backpack), new Object[]{"AAA", "A A", "AAA",'A', Items.leather});
	GameRegistry.addShapedRecipe(new ItemStack(Gadgets.bomb), new Object[]{"AAB", "ACA", "AAA",'A', Items.iron_ingot,'B', Items.string,'C', Items.gunpowder});
	GameRegistry.addShapedRecipe(new ItemStack(Gadgets.handgun), new Object[]{"AAB", " CA", "  A",'A', Items.iron_ingot,'B', Blocks.lever,'C', Blocks.tripwire_hook});
	GameRegistry.addShapedRecipe(new ItemStack(Gadgets.bin), new Object[]{"ABA", "A A", "AAA",'A', Items.iron_ingot,'B', Blocks.trapdoor});
	GameRegistry.addSmelting(Items.iron_ingot, new ItemStack(Gadgets.bullet, 3), 0F);

	int id = Config.getEntityIdStart();
    EntityRegistry.registerModEntity(EntityBomb.class, "Bomb", id++, Gadgets.instance, 64, 10, true);
    EntityRegistry.registerModEntity(ProjectileBullet.class, "Bullet", id++, Gadgets.instance, 64, 10, true);
    }
    @EventHandler
    public void init(FMLInitializationEvent event)
    {
    	PacketHandler.initPackets();
	NetworkRegistry.INSTANCE.registerGuiHandler(instance, server);
    	
    }
    @EventHandler
    public void postInit(FMLPostInitializationEvent event)
    {
	MinecraftForge.addGrassSeed(new ItemStack(Gadgets.heart), 1);

	ChestGenHooks.getInfo(ChestGenHooks.DUNGEON_CHEST).addItem(new WeightedRandomChestContent(new ItemStack(Gadgets.bomb), 1, 2, 3));
	ChestGenHooks.getInfo(ChestGenHooks.DUNGEON_CHEST).addItem(new WeightedRandomChestContent(new ItemStack(Gadgets.bullet), 1, 6, 6));
	ChestGenHooks.getInfo(ChestGenHooks.DUNGEON_CHEST).addItem(new WeightedRandomChestContent(new ItemStack(Gadgets.harbinger), 1, 1, 1));
	ChestGenHooks.getInfo(ChestGenHooks.DUNGEON_CHEST).addItem(new WeightedRandomChestContent(new ItemStack(Gadgets.halsSword), 1, 1, 1));
    }
}

 

Why is this happening?

It crashes because you're trying to access a client class, EntityClientPlayerMP. Make sure any references to any client-only classes go through your proxy.

 

Side note: Minecraft is a client only class and that will also crash it on a server. To use keybindings on a server you need to create a packet that is sent to the server from the client when the space bar is pressed on the client.