404错误

package servlet;

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public class MyServlet implements Servlet {

	@Override
	public void destroy() {
		// TODO Auto-generated method stub
		
	}

	@Override
	public ServletConfig getServletConfig() {
		// TODO Auto-generated method stub
		return null;
	}

	@Override
	public String getServletInfo() {
		// TODO Auto-generated method stub
		return null;
	}

	@Override
	public void init(ServletConfig arg0) throws ServletException {
		 System.out.println("执行init()...");
		
	}

	@Override
	public void service(ServletRequest arg0, ServletResponse arg1) throws ServletException, IOException {
		arg1.getWriter().write("fsafs");
		
	}

}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <servlet>
    <servlet-name>myFirstServlet</servlet-name>
    <servlet-class>servlet/MyServlet.java</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>myFirstServlet</servlet-name>
    <url-pattern>/myFirstServlet</url-pattern>
  </servlet-mapping>
</web-app>

老师这是什么原因？