[BukkitPlugin] "Attack2Jump" 攻撃されるとジャンプしてしまう。

Main.java

package org.densyakun.bukkit.attack2jump;

import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.EntityDamageByEntityEvent;
import org.bukkit.plugin.java.JavaPlugin;
import org.bukkit.util.Vector;

public class Main extends JavaPlugin implements Listener {
	
	private double a = 0.1;
	
	@Override
	public void onEnable() {
		a = getConfig().getDouble("value");
		getConfig().set("value", a);
		saveConfig();
		getServer().getPluginManager().registerEvents(this, this);
	}
	
	@EventHandler
	public void EntityDamageByEntity(EntityDamageByEntityEvent e) {
		if (e.getEntity() instanceof Player && e.getDamager() instanceof Player) {
			Vector b = e.getEntity().getVelocity();
			b.setY(a);
			e.getEntity().setVelocity(b);
		}
	}
}