10.JavaWeb基础 Servlet

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Servlet（服务器端小程序）是使用Java语言编写的服务器端程序，可以像JSP一样，

生成动态的Web页面。Servlet主要运行在服务器端，并且由服务器调用执行，是一种按照

Servlet标准开发的类。

Servlet程序是对Java对CGI程序的实现，但是与传统CGI的多线程处理操作不同的是，

Servlet采用了多线程的处理方式，这样就使得Servlet程序的运行效率比传统的CGI更高；

而且Servlet还保留有Java的可移植性的特点，这样使得Servlet更易使用，功能也更加强大。

一."Hello World"

Servlet的类肯定要继承HttpServlet类，并且至少覆写doGet方法。

HelloServlet ：

package com.zyy.servlet;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 HelloServlet extends HttpServlet {/** *  */protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {PrintWriter print = resp.getWriter();print.println("<center><h2>Hello World</h2></center>");print.close();}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

web.xml：

<servlet><servlet-name>hello</servlet-name><servlet-class>com.zyy.servlet.HelloServlet</servlet-class></servlet><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/helloServlet</url-pattern></servlet-mapping>

注意：每次修改web.xml后都要重启服务器。

地址栏输入：http://127.0.0.1:8080/Servlet/helloServlet

二.Servlet与表单

表单提交的方式是post的话，那么就要覆写doPost()方法。

servlet_1.html：

<!DOCTYPE html><html>  <head>    <title>servlet_1.html</title>            <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">        <!--<link rel="stylesheet" type="text/css" href="./styles.css">-->  </head>    <body>        <form action="InputServlet" method="post">        <input type="text" name="info">    <input type="submit" value="提交">        </form>      </body></html>

InputServlet：

package com.zyy.servlet;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 InputServlet extends HttpServlet {/** *  */protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {PrintWriter print = resp.getWriter();String info = req.getParameter("info");print.println("<center>" + info + "</center>");print.close();}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

其实这里非常简单，我们依然可以使用单独的doGet()、单独的doPost()。但是这样用doPost

调用doGet的话，可以实现表单的通用处理，即使是get请求，服务器端这边也做成了doPost。

web.xml：

<servlet><servlet-name>input</servlet-name><servlet-class>com.zyy.servlet.InputServlet</servlet-class></servlet><servlet-mapping><servlet-name>input</servlet-name><url-pattern>/html/InputServlet</url-pattern></servlet-mapping>

为什么是/html/InputServlet呢？

我的servlet_1.html是放在了html文件夹里，原来进到servlet_1.html的时候已经是

http://127.0.0.1:8080/html/servlet_1.html了，如果url-pattern 设置为

/InputServlet的话，提交后变成http://127.0.0.1:8080/InputServlet，这样就提

示404找不到资源的错误，也就是找不到servlet_1.html了。如果我们把此url-pattern就

为启动的钥匙，顺便还带你访问了html文件夹,这样就变成了/html/InputServlet。最终结

果就是http://127.0.0.1:8080/Servlet/html/InputServlet 。总结出一个规律：

要跳转的的页面如果不在根目录下，那么url-pattern就要在前面添加/xxx/xxx/Serlvet，直到配到

跳转页面的文件夹目录位置。

三.Servlet生命周期

1.加载Servlet

Web容器负责加载Servlet，当Web容器启动的时候或者是在第一次使用这个Servlet的

时候，容器会负责加载Servlet实例，但是用户必须通过web.xml文件描述指定Servlet的

位置，成功加载之后，Web容器会通过反射机制对Servlet进行实例化。

2.初始化

当一个Servlet被实例化之后，容器将调用init()方法初始化这个对象，初始化的目的是为

了让Servlet对象在处理客户端请求前完成一些初始化的工作，如建立数据库连接、读取资源文

件信息等等。如果初始化失败了，那么Servlet会被直接卸载。

3.处理服务

当有请求提交时，Servlet将调用service()方法进行处理。在service()方法中，Servlet

可以通过ServletRequest接受客户的请求，也可以利用ServletResponse设置响应信息。

4.销毁

当Web容器关系或检测到一个Servlet要从容器中删除的时候，会自动调用destory()方法，以

便让该实例释放掉所占的资源。

5.卸载

当一个Servlet调用完destory方法()后，此实例将等待被垃圾收集器所回收，如果需要再次使

用此Servlet时，会重新调用init()方法初始化。

这里不做过多介绍，这里提供一个

LifeCycleServlet：

package com.zyy.servlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LifeCycleServlet extends HttpServlet {/** *  */public void init() throws ServletException {System.out.println("1.Servlet初始化 init()");}protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {System.out.println("2.Servlet服务 doGet() doPost()");}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}public void destroy() {try {System.out.println("3.Servlet销毁 destroy()");Thread.sleep(3000);} catch (InterruptedException e) {e.printStackTrace();}}}

web.xml：

<servlet><servlet-name>life</servlet-name><servlet-class>com.zyy.servlet.LifeCycleServlet</servlet-class></servlet><servlet-mapping><servlet-name>life</servlet-name><url-pattern>/LifeCycleServlet</url-pattern></servlet-mapping>

四.Servlet取得初始化信息

web.xml配置初始化信息：

<servlet><servlet-name>init</servlet-name><servlet-class>com.zyy.servlet.InitParamServlet</servlet-class><init-param><param-name>initParam</param-name><param-value>CaMnter</param-value></init-param></servlet><servlet-mapping><servlet-name>init</servlet-name><url-pattern>/InitParamServlet</url-pattern></servlet-mapping>

InitParamServlet：

package com.zyy.servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletConfig;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class InitParamServlet extends HttpServlet {/** *  */private String initParam = null;public void init(ServletConfig config) throws ServletException {this.initParam = config.getInitParameter("initParam");}protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {PrintWriter print = resp.getWriter();String info = req.getParameter("info");print.println("<center>" + initParam + "</center>");print.close();}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

五.取得HttpSession和ServletContext（application）

SessionServlet：

package com.zyy.servlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class SessionServlet extends HttpServlet {/** *  */protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {HttpSession session = req.getSession();session.setAttribute("CaMnter", "Save you from anything 07");System.out.println(session.getId());System.out.println(session.getAttribute("CaMnter"));}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

web.xml：

<servlet><servlet-name>session</servlet-name><servlet-class>com.zyy.servlet.SessionServlet</servlet-class></servlet><servlet-mapping><servlet-name>session</servlet-name><url-pattern>/SessionServlet</url-pattern></servlet-mapping>

Tomcat后台：

ServletContextServlet：

这个需要通过HttpServlet的父类GenericServlet来取得。即super.getServletContext() 。

package com.zyy.servlet;import java.io.IOException;import javax.servlet.ServletContext;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ServletContextServlet extends HttpServlet {/** *  */protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {ServletContext application = super.getServletContext();System.out.println(application.getRealPath("/"));}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

web.xml：

<servlet><servlet-name>application</servlet-name><servlet-class>com.zyy.servlet.ServletContextServlet</servlet-class></servlet><servlet-mapping><servlet-name>application</servlet-name><url-pattern>/ServletContextServlet</url-pattern></servlet-mapping>

Tomcat后台：

六.Servlet客户端跳转

可以使用HttpServletResponse中的sendRedirect()方法，但是只能保存session

和application范围的属性。

ClientRedirectServlet：

package com.zyy.servlet;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class ClientRedirectServlet extends HttpServlet {/** *  */protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {HttpSession session = req.getSession();session.setAttribute("CaMnter_sess", "sess_Save you from anything 07");req.setAttribute("CaMnter_req", "req_Save you from anything 07");resp.sendRedirect("servlet_2.jsp");}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

servlet_2.jsp：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>           <title>servlet_2.jsp</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>     <center>                <h2>session： <%=session.getAttribute("CaMnter_sess") %> </h2>    <h2>request： <%=request.getAttribute("CaMnter_req") %> </h2>         </center>  </body></html>

web.xml：

<servlet><servlet-name>client</servlet-name><servlet-class>com.zyy.servlet.ClientRedirectServlet</servlet-class></servlet><servlet-mapping><servlet-name>client</servlet-name><url-pattern>/html/ClientRedirectServlet</url-pattern></servlet-mapping>

关于url-pattern的配置，上文的总结规律可以得出，这里不做多余的解释。

request为空，这是理所当然，response的跳转属于客户端跳转，request只能在服务器端

跳转下能保存，客户端跳转的情况下保存不了。客户端跳转只能保存session和application。

地址也变了。

七.Servlet服务器端跳转

需要依赖RequestDispatcher接口去完成服务器端跳转。

ServerRedirectServlet：

package com.zyy.servlet;import java.io.IOException;import javax.servlet.RequestDispatcher;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;public class ServerRedirectServlet extends HttpServlet {/** *  */protected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {HttpSession session = req.getSession();session.setAttribute("CaMnter_sess", "sess_Save you from anything 07");req.setAttribute("CaMnter_req", "req_Save you from anything 07");RequestDispatcher requestDispatcher = req.getRequestDispatcher("servlet_2.jsp");requestDispatcher.forward(req, resp);}protected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {this.doGet(req, resp);}}

servlet_2.jsp：

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html>  <head>           <title>servlet_2.jsp</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>     <center>                <h2>session： <%=session.getAttribute("CaMnter_sess") %> </h2>    <h2>request： <%=request.getAttribute("CaMnter_req") %> </h2>         </center>  </body></html>

web.xml：

<servlet><servlet-name>server</servlet-name><servlet-class>com.zyy.servlet.ServerRedirectServlet</servlet-class></servlet><servlet-mapping><servlet-name>server</servlet-name><url-pattern>/html/ServerRedirectServlet</url-pattern></servlet-mapping>

能接收的到request，因为是服务器端跳转。地址没变。

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