Servlet继承体系结构

Servlet如何只定义1个service方法，其它的方法按需求设置

Servlet——接口

　　　　↑继承

　　GenericServlet——抽象类

　　　　　　↑继承

　　　　　HttpServlet——抽象类：推荐使用

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GenericServlet抽象类：只需要实现service方法

只用重写一个方法，其他的方法都做了空实现（仅声明了方法，方法体内没有内容）

只把service方法做了抽象（abstract）

需要其他方法时重写就行了

虽然方便，但真正开发时不使用。真正使用HttpServlet

 //
 // Source code recreated from a .class file by IntelliJ IDEA
 // (powered by Fernflower decompiler)
 //

 package javax.servlet;

 import java.io.IOException;
 import java.io.Serializable;
 import java.util.Enumeration;

 public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {
     private static final long serialVersionUID = 1L;
     private transient ServletConfig config;

     public GenericServlet() {
     }

     public void destroy() {
     }

     public String getInitParameter(String name) {
         return this.getServletConfig().getInitParameter(name);
     }

     public Enumeration<String> getInitParameterNames() {
         return this.getServletConfig().getInitParameterNames();
     }

     public ServletConfig getServletConfig() {
         return this.config;
     }

     public ServletContext getServletContext() {
         return this.getServletConfig().getServletContext();
     }

     public String getServletInfo() {
         return "";
     }

     public void init(ServletConfig config) throws ServletException {
         this.config = config;
         this.init();
     }

     public void init() throws ServletException {
     }

     public void log(String msg) {
         this.getServletContext().log(this.getServletName() + ": " + msg);
     }

     public void log(String message, Throwable t) {
         this.getServletContext().log(this.getServletName() + ": " + message, t);
     }

     public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

     public String getServletName() {
         return this.config.getServletName();
     }
 }

package cn.itcast.web.servlet;

import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;
import java.io.IOException;

/**
 * @author 旗木五五开
 * @create 2020-02-18 20:20
 */
@WebServlet("/demo2")
public class ServletDemo2 extends GenericServlet {
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("Demo2来袭");
    }
}

HttpServlet抽象类：对HTTP协议的封装，简化操作

为什么会有这个类的出现？

为了屏蔽get和post请求方式处理逻辑（继承HttpServlet，复写方法doGet();dopost();）。

因为将来都是调用service方法，service做一个方法分发，是哪个方式就调用哪个方法

HttpServlet基本的定义原理

实现步骤：

1. 1. 定义类继承HttpServlet
   2. 复写doGet();、dopost();方法

protected void service(HttpServletRequest req,HttpServletResponse resp)throws ServletException,IOException{
//        1.判断请求方式（method对应的请求方式）
        String method=req.getMethod();
        long lastModified;
        if(method.equals("GET")){//常量放前面可以防止空指针异常
        lastModified=this.getLastModified(req);
        if(lastModified==-1L){
        this.doGet(req,resp);//调用doGet方法
        }else{
        long ifModifiedSince;
        try{
        ifModifiedSince=req.getDateHeader("If-Modified-Since");
        }catch(IllegalArgumentException var9){
        ifModifiedSince=-1L;
        }

        if(ifModifiedSince<lastModified /1000L*1000L){
        this.maybeSetLastModified(resp,lastModified);
        this.doGet(req,resp);
        }else{
        resp.setStatus(304);
        }
        }
        }else if(method.equals("HEAD")){//HEAD
        lastModified=this.getLastModified(req);
        this.maybeSetLastModified(resp,lastModified);
        this.doHead(req,resp);
        }else if(method.equals("POST")){//POST
        this.doPost(req,resp);
        }else if(method.equals("PUT")){//PUT
        this.doPut(req,resp);
        }else if(method.equals("DELETE")){//DELETE
        this.doDelete(req,resp);
        }else if(method.equals("OPTIONS")){//OPTIONS
        this.doOptions(req,resp);
        }else if(method.equals("TRACE")){//TRACE
                          //以上对应HTTP7个请求方式。只需要关心两个常用的就行get和post
        this.doTrace(req,resp);
        }else{
        String errMsg=lStrings.getString("http.method_not_implemented");
        Object[]errArgs=new Object[]{method};
        errMsg=MessageFormat.format(errMsg,errArgs);
        resp.sendError(501,errMsg);
        }
        }
            

练习

package cn.itcast.web.servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

/**
 * @author 旗木五五开
 * @create 2020-02-18 23:44
 */
@WebServlet("/demo3")
public class ServletDemo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("dogat");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("dopost");
    }
}

通过浏览器直接请求是get方式

通过表单完成post请求

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>login</title>
</head>
<body>
<form action="/demo3" method="post">
    <input type="text" name="username">
    <br>
    <input type="submit" value="提交">
</form>

</body>
</html>