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Servlet--HttpServlet实现doGet和doPost请求的原理

Servlet--HttpServlet实现doGet和doPost请求的原理

一、HttpServlet简介

1、HttpServlet是GenericServlet的子类，又是在GenericServlet的基础上做了增强。

2、HttpServlet方法

二、HTTP实现doGet或doPost请求项目介绍

1、通过实现doGet请求和doPost请求实例来了解内部的工作原理。

2、doGet请求和doPost请求实例代码介绍：

A：创建一个Servlet类继承HttpServlet类

B：在index.jsp页面创建一个超链接请求

3、doGet请求和doPost请求实例实施介绍：

A、创建一个webproject项目。

B、创建一个Servlet类的名称为HttpServ继承HttpServlet类同时覆写doGet方法和doPost方法。

1、

2、

3、配置web.xml文件

4、创建Servlet代码展示

[java] view plain copy

package httpserve;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class HttpServ extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
System.out.println("发送get请求。。。。。。。。。。。");
}
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
resp.setContentType("text/html;charset=UTF-8");
System.out.println("发送post方法。。。。。。。。。。");
}
}

5、web.xml配置文件代码展示

[html] view plain copy

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
xmlns="http://java.sun.com/xml/ns/javaee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>HttpServ</servlet-name>
<servlet-class>httpserve.HttpServ</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HttpServ</servlet-name>
<url-pattern>/http</url-pattern>
</servlet-mapping>
</web-app>

C、在index.jsp页面创建一个超链接请求

[html] view plain copy

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
This is my JSP page. <br>
<a href="http://localhost:8080/test06/http">get请求1</a><br/>
<!-- 对于一个html页面来说，如果没有以http开始，则默认的前面会加上
协议类型://目前这个页面所在的服务器:目前端口/目前项目/你给的这个名称 -->
<a href="http">get请求2</a><hr/>
<form method = "post" action="http">
<input type="submit" value="提交"/>
</form>
</body>
</html>

D、发送doGet请求和doPost请求

a、在浏览器中输入测试地址，http://127.0.0.1:8080/test06/

b、打开项目的首页后分别点击get请求1、get请求2、和 post请求（提交按钮）

c、在控制台可以观察到分别调用了doGet和doPost方法。

三、HTTP实现doGet或doPost请求原理介绍

1、浏览器发送请求到HttpSevr类调用HttpServ的service（servletRequest, servletReponse）方法

2、由于没有找到这个方法，去调用父类（HttpServlet）的同名方法。

3、父类的service方法将ServletRequest req请求转换成HttpServletRequest请求，再去调用service(request, response) 方法。

将ServletRequest req请求转换成HttpServletRequest请求再调用service(request, response) 方法源码如下：

[java] view plain copy

public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
} catch (ClassCastException e) {
throw new ServletException("non-HTTP request or response");
}
service(request, response);
}

4、调用的service(request, response) 方法功能是判断用户发出是什么请求，如果是get则调用子类（HttpSevr）的doGet方法，如果是post则调用子类（HttpSevr）的doPost方法。

service(request, response) 方法源码如下：

[java] view plain copy

protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
// Invalid date header - proceed as if none was set
ifModifiedSince = -1;
}
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}

5、调用关系图

四、注意事项：

1、如果在HttpServ中覆盖了service(ServletRequest,SerlvetResonse)方法则这个类的所实现的doGet/doPost都不会再执行了。

因为service(ServletRequest,SerlvetResonse)是最高接口Servlet定义规范。在tomcat调用时，一定会在最终的子类中去找这个方法且调用它。

如果最终的子类没有则会调用父的service(ServletRequest,SerlvetResonse)。

2、如果覆盖了serivce(HttpServletRequest,HtpServletResponse)则会执行httpServlet中的service(ServletRequest,SerlvetResonse),但是由于子类中已经覆盖了serivce(HttpServletRequest,HtpServletResponset)所以，httpServlet中的serivce(HttpServletRequest,HtpServletResponset)就不再执行了，而是直接执行子类中同名同参数方法，且doXxxx也不会执行了，因为子类的serivce(HttpServletRequest,HtpServletResponset)没有调用doXxxx.

3、如果继承了HttpServlet没有实现任何的doXxx方法则会抛出一个异常

posted @ 2018-03-09 13:24 从未被超越阅读(730) 评论(0) 编辑收藏举报

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