创建servlet三种方式（韩顺平老师课程）

1.实现Servlet接口，实现接口中的五个方法

public class Hello implements Servlet{  
  
     //该函数用于初始化servlet，类似于类的构造函数，该函数只会被调用一次  
    public void init(ServletConfig parm1) throws ServletException {  
        System.out.println("init it!");  
    }  
  
    public ServletConfig getServletConfig() {  
        // TODO: Add your code here  
        return null;  
    }  
  
     //这个函数用于处理业务逻辑  
     //当用户每访问servlet时，都会调用  
     //req:用于获得客户端（浏览器）信息  
     //res:用于向客户端（浏览器）返回信息  
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {  
        //从res中得到PrintWriter  
        System.out.println("Service it");  
        PrintWriter pw = res.getWriter();  
        pw.println("Hello world!");  
    }  
  
    public String getServletInfo() {  
        return "";  
    }  
  
     //销毁，释放内存,三种方式会触发destroy  
     //1. reload该servlet(webApps) 2. 关闭tomcat 3.关机  
    public void destroy() {  
        System.out.println("destroy it!");  
    }  
}

2.继承GenericServlet，需要实现service方法

public class Helloservlet extends GenericServlet{  
    //重写一个方法service即可  
    public void service(ServletRequest req, ServletResponse res){  
        //返回Hello world, generic  
        try{  
            PrintWriter pw = res.getWriter();  
            pw.println("Hello generic");  
        }  
        catch(Exception e){  
            e.printStackTrace();  
        }  
    }  
}

3.继承HttpServlet,重写doGet和doPost及其他需要用的方法.（最为常用的方式）

public class HelloServlet extends HttpServlet{  
    //处理get请求  
    public void doGet(HttpServletRequest req, HttpServletResponse res){  
        //业务逻辑  
        try{  
            PrintWriter pw = res.getWriter();  
            pw.println("hello, Servlet");  
        }  
        catch (Exception e){  
            e.printStackTrace();  
        }  
    }  
  
    public void doPost(HttpServletRequest req, HttpServletResponse res){  
        this.doGet(req,res);      
    }  
}