Java Servlet实现的三种方式

1 实现Servlet接口，然后实现接口中的五个方法，代码为：

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!");
    }
} 

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2 继承GenericServlet，只需要实现一个方法：service

public class HelloGen 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();
        }
    }
}

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3 继承HttpServlet,复写doGet和doPost方法

public class HelloHttp 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, Http");
        }
        catch (Exception e){
            e.printStackTrace();
        }
    }

    public void doPost(HttpServletRequest req, HttpServletResponse res){
        this.doGet(req,res);    
    }
}

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当然，每次写好代码之后，需要在web.xml中注册新的servlet 
web.xml:

<?xml version="1.0" encoding="UTF-8"?>

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
  version="4.0"
  metadata-complete="true">

  <display-name>Welcome to Tomcat</display-name>
  <description>
     Welcome to Tomcat
  </description>
    //第一种方式需要加入的部分
  <servlet>
    <servlet-name>hello</servlet-name>
    <servlet-class>com.thorin.Hello</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/thorin</url-pattern>
  </servlet-mapping>

  //第二种方式需要加入的部分
  <servlet>
    <servlet-name>hellogen</servlet-name>
    <servlet-class>com.thorin.HelloGen</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>hellogen</servlet-name>
    <url-pattern>/hellogen</url-pattern>
  </servlet-mapping>

  //第三种方式需要加入的部分
  <servlet>
    <servlet-name>hellohttp</servlet-name>
    <servlet-class>com.thorin.HelloHttp</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>hellohttp</servlet-name>
    <url-pattern>/hellohttp</url-pattern>
  </servlet-mapping>

</web-app>