Config

worlds:
  world:
    x: 0
    y: 0
    z: 0
    yaw: 0
    pitch: 0
  world_nether:
    x: 0
    y: 0
    z: 0
    yaw: 0
    pitch: 0

Plugin YAML

name: SameWorld
main: main.java.net.bigbadcraft.sameworld.SameWorld
version: 1.1.0
commands:
  swset:
    description: Sets the spawn point for that world
    permission: sameworld.set

SameWorld

package main.java.net.bigbadcraft.sameworld;

import java.util.Set;

import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerRespawnEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class SameWorld extends JavaPlugin implements Listener {
	
	private final ChatColor GREEN = ChatColor.GREEN;
	
	private Set<String> allowedWorlds;
	
	public void onEnable() {
		getServer().getPluginManager().registerEvents(this, this);
		saveDefaultConfig();
		allowedWorlds = getConfig().getConfigurationSection("worlds").getKeys(false);
		getCommand("swset").setExecutor(this);
	}
	
	@EventHandler(priority = EventPriority.MONITOR, ignoreCancelled = true)
	public void onDeath(PlayerRespawnEvent event) {
		Player player = event.getPlayer();
		if (allowedWorlds.size() > 0 && allowedWorlds.contains(player.getWorld().getName())) {
			event.setRespawnLocation(getLocation(player.getWorld().getName()));
			player.sendMessage(GREEN + "You respawned in the same world.");
		}
	}
	
	@Override
	public boolean onCommand(CommandSender sender, Command cmd, String lbl, String[] args) {
		if (!(sender instanceof Player)) {
			return true;
		}
		Player player = (Player) sender;
		Location loc = player.getLocation();
		Location underneath = loc.subtract(0.0, 1.0, 0.0);
		if (cmd.getName().equalsIgnoreCase("swset")) {
			if (args.length == 0) {
				if (underneath.getBlock() == null || underneath.getBlock().getType() == Material.AIR) {
					player.sendMessage(GREEN + "Cannot save spawn point on nothing.");
					return true;
				}
				saveSpawn(player.getLocation());
				player.sendMessage(GREEN + "Successfully saved new spawn point.");
			}
		}
		return true;
	}
	
	private void saveSpawn(Location loc) {
		reloadConfig();
		getConfig().set("worlds."+loc.getWorld().getName()+".x", loc.getBlockX());
		getConfig().set("worlds."+loc.getWorld().getName()+".y", loc.getBlockY());
		getConfig().set("worlds."+loc.getWorld().getName()+".z", loc.getBlockZ());
		getConfig().set("worlds."+loc.getWorld().getName()+".yaw", loc.getYaw());
		getConfig().set("worlds."+loc.getWorld().getName()+".pitch", loc.getPitch());
		saveConfig();
	}
	
	private Location getLocation(String path) {
		int x = getConfig().getInt("worlds."+path+".x");
		int y = getConfig().getInt("worlds."+path+".y");
		int z = getConfig().getInt("worlds."+path+".z");
		float yaw = (float) getConfig().getDouble("worlds."+path+".yaw");
		float pitch = (float) getConfig().getDouble("worlds."+path+".pitch");
		return new Location(Bukkit.getWorld(path), x+0.5D, y, z+0.5D, yaw, pitch);
	}

}