http://www.bukkit.fr/index.php?/topic/9460-plugin-probl%C3%A8me-de-bukkitgetworld/

plugin.yml

name: Wanted
version: 1.0
author: MinusKube
main: fr.minuskube.wanted.Wanted
commands:
  wanted:

Wanted.java

package fr.minuskube.wanted;

import java.io.File;
import java.io.IOException;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import org.apache.commons.lang.StringUtils;
import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.Location;
import org.bukkit.block.Block;
import org.bukkit.block.Sign;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.configuration.file.YamlConfiguration;
import org.bukkit.configuration.serialization.ConfigurationSerializable;
import org.bukkit.configuration.serialization.ConfigurationSerialization;
import org.bukkit.configuration.serialization.SerializableAs;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.block.BlockBreakEvent;
import org.bukkit.event.entity.PlayerDeathEvent;
import org.bukkit.plugin.java.JavaPlugin;

public class Wanted extends JavaPlugin implements Listener {
    @SerializableAs("BlockLocation")
    public static class BlockLocation extends Location implements ConfigurationSerializable {

        public BlockLocation(final Location location) {
            super(location.getWorld(), location.getBlockX(), location.getBlockY(), location.getBlockZ());
        }

        public BlockLocation(final Map<String, Object> map) {
            super(Bukkit.getWorld((String) map.get("world")), (int) map.get("x"), (int) map.get("y"), (int) map.get("z"));
        }

        @Override
        public boolean equals(final Object obj) {
            if (!(obj instanceof Location))
                return false;
            final Location loc = (Location) obj;
            return loc.getWorld().getName().equals(getWorld().getName()) && loc.getBlockX() == getBlockX()
                    && loc.getBlockY() == getBlockY() && loc.getBlockZ() == getBlockZ();
        }

        @Override
        public Map<String, Object> serialize() {
            final Map<String, Object> map = new HashMap<String, Object>();
            map.put("world", getWorld().getName());
            map.put("x", getBlockX());
            map.put("y", getBlockY());
            map.put("z", getBlockZ());
            return map;
        }
    }

    private static File              INTERNAL = null;
    private final Set<BlockLocation> signs    = new HashSet<BlockLocation>();

    @EventHandler(priority = EventPriority.MONITOR)
    public void onBlockBreak(final BlockBreakEvent event) {
        final Block block = event.getBlock();
        if (block.getState() != null && block.getState() instanceof Sign) {
            signs.remove(new BlockLocation(block.getLocation()));
            saveSigns();
        }
    }

    @Override
    public boolean onCommand(final CommandSender sender, final Command command, final String label, final String[] args) {
        if (command.getName().equalsIgnoreCase("wanted")) {
            if (sender instanceof Player) {
                final Player player = (Player) sender;
                if (player.hasPermission("wanted.define")) {
                    @SuppressWarnings("deprecation")
                    final Block block = player.getTargetBlock(null, 100);
                    if (block.getState() != null && block.getState() instanceof Sign) {
                        signs.add(new BlockLocation(block.getLocation()));
                        saveSigns();
                        player.sendMessage(ChatColor.GREEN + "A new wanted sign has been defined.");
                    } else
                        player.sendMessage(ChatColor.RED + "The targetted block isn't a sign.");
                } else
                    player.sendMessage(ChatColor.RED + "You don't have the permission to do this.");
            } else
                sender.sendMessage(ChatColor.RED + "This command can only be used by players.");
            return true;
        }
        return false;
    }

    @SuppressWarnings("unchecked")
    @Override
    public void onEnable() {
        ConfigurationSerialization.registerClass(BlockLocation.class);
        INTERNAL = new File(getDataFolder().getPath(), "internal.yml");
        if (!INTERNAL.exists())
            try {
                INTERNAL.createNewFile();
            } catch (final IOException e) {}
        final YamlConfiguration config = YamlConfiguration.loadConfiguration(INTERNAL);
        signs.clear();
        signs.addAll((Set<BlockLocation>) config.get("signs", new HashSet<BlockLocation>()));
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler(priority = EventPriority.MONITOR)
    public void onPlayerDeath(final PlayerDeathEvent event) {
        if (event.getEntity().getKiller() != null)
            signsLoop: for (final BlockLocation location : signs) {
                final Block block = location.getBlock();
                if (block.getState() != null && block.getState() instanceof Sign) {
                    final Sign sign = (Sign) block.getState();
                    for (int i = 0; i < 4; i++)
                        if (StringUtils.isBlank(sign.getLine(i))) {
                            sign.setLine(i, event.getEntity().getKiller().getName());
                            sign.update(true);
                            break signsLoop;
                        }
                }
            }
    }

    private void saveSigns() {
        final YamlConfiguration config = YamlConfiguration.loadConfiguration(INTERNAL);
        config.set("signs", signs);
        try {
            config.save(INTERNAL);
        } catch (final IOException e) {}
    }
}