【Spring in action】SpringMVC的准备工作：Servlet基础

前言：

基于Spring思想的web框架SpringMVC。需要了解SpringMVC需要知道Servlet的工作方式，很简单。两篇文章，一篇各种原理组件，一篇实战搞定。高手请忽略。

正文：

一、java的Servlet的使用：

1、写一个Servlet继承HttpServlet：

1》publicclass MyServletextends HttpServlet

package com.demo;import java.io.IOException;import java.io.PrintWriter;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class MyServlet extends HttpServlet {/** *  */private static final long serialVersionUID = 1L;public MyServlet() {super();// TODO Auto-generated constructor stub}@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {// TODO Auto-generated method stub//super.doGet(req, resp);//logic HttpServletRequest请求,HttpServletResponse 响应PrintWriter out = resp.getWriter();out.print("hello servlet!");out.close();////String str = this.getServletContext().getInitParameter("servlet_name");//System.out.println(str);//resp.sendRedirect("http://www.baidu.com");}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {// TODO Auto-generated method stub// super.doPost(req, resp);doGet(req, resp);}}

2》在WEB-INF下写一个web.xml:

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" id="WebApp_ID" version="3.1">  <display-name>Servlet1</display-name>  <welcome-file-list>    <welcome-file>index.html</welcome-file>    <welcome-file>index.htm</welcome-file>    <welcome-file>index.jsp</welcome-file>    <welcome-file>default.html</welcome-file>    <welcome-file>default.htm</welcome-file>    <welcome-file>default.jsp</welcome-file>  </welcome-file-list>  <servlet>    <servlet-name>MyServlet</servlet-name>    <servlet-class>com.demo.MyServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>MyServlet</servlet-name>    <url-pattern>/myServlet</url-pattern>  </servlet-mapping></web-app>

总结1：

在Servlet1中：新建一个servlet-mapping对象：外网访问地址http://localhost:8080/Servlet1/myServlet ,由此处约定。

关键代码新建一个servlet对象：MyServlet-->com.demo.MyServlet。外网根据MyServlet输出hello servlet!

PS：常见问题：

1、严重: Allocate exception for servlet MyServlet
java.lang.ClassNotFoundException: com.demo
原因：    web.xml需要完整路径
例如错误代码：
<servlet>
    <servlet-name>MyServlet</servlet-name>
    <servlet-class>com.demo</servlet-class>
</servlet>
我的路径为com.demo.MyServlet，而我只是写了包名。

总结2:

servlet生命周期：构造方法-init初始化-doget／dopost-destory

2、写一个生命周期看效果：LoginServletextends HttpServlet 

package com.demo;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class LoginServlet */public class LoginServlet extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public LoginServlet() {                // TODO Auto-generated constructor stub        System.out.println("Constructor...");    }        @Override    public void init() throws ServletException {    // TODO Auto-generated method stub    System.out.println("init...");    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubSystem.out.println("doGet...");}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stub//System.out.println("doPost...");PrintWriter out = response.getWriter();out.print("<h1>hello servlet</h1>");//request.getRequestDispatcher("hello.html").forward(request, response);request.getRequestDispatcher("hello.html").include(request, response);}@Overridepublic void destroy() {// TODO Auto-generated method stubSystem.out.println("destroy...");}}

修改web.xml，添加LoginServlet：

<servlet>    <servlet-name>LoginServlet</servlet-name>    <servlet-class>com.demo.LoginServlet</servlet-class>  </servlet>  <servlet-mapping>    <servlet-name>LoginServlet</servlet-name>    <url-pattern>/LoginServlet</url-pattern>  </servlet-mapping>

直接执行该Servlet，执行结果：

九月 07, 2017 2:37:08 下午 org.apache.catalina.startup.Catalina start

信息: Server startup in 4956 ms

Constructor...

init...

doFilter...

doGet...

3、过滤器：对特定请求进行过滤。

作用：客户端和服务端请求资源之间，起到过滤作用：session管理、权限管理、日志管理等。

1》在web.xml下配置：和上面的servlet平级处配置：

<filter>    <filter-name>MyFlter</filter-name>    <filter-class>com.filter.MyFilter</filter-class>  </filter>  <filter-mapping>    <filter-name>MyFlter</filter-name>    <url-pattern>/*</url-pattern>  </filter-mapping>

2》写一个MyFilter继承自Filter

package com.filter;import java.io.IOException;import javax.servlet.Filter;import javax.servlet.FilterChain;import javax.servlet.FilterConfig;import javax.servlet.ServletException;import javax.servlet.ServletRequest;import javax.servlet.ServletResponse;public class MyFilter implements Filter{@Overridepublic void destroy() {// TODO Auto-generated method stub}@Overridepublic void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)throws IOException, ServletException {// TODO Auto-generated method stubSystem.out.println("doFilter...");//过滤传递arg2.doFilter(arg0, arg1);}@Overridepublic void init(FilterConfig arg0) throws ServletException {// TODO Auto-generated method stub}}

执行结果，由于设置/*，所有请求均会执行：

doFilter...

结果：不管调用什么，例如jdbclogin.html也会调用fliter过滤器
如果指定包下过滤：则重新配置如下：（admin下面生效）
<!-- /admin/* admin文件夹下的才需要过滤 -->
<filter-mapping>
  <filter-name>MyFlter</filter-name>
  <url-pattern>/admin/*</url-pattern>
</filter-mapping>

例如：
http://localhost:8080/Servers1/login.html不会执行过滤器
http://localhost:8080/Servers1/admin/admin.html会执行过滤器

4、获得请求头header信息：

package com.demo;import java.io.IOException;import java.util.Enumeration;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class HeaderServlet */@WebServlet("/HeaderServlet")public class HeaderServlet extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public HeaderServlet() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubEnumeration<String> enum1 = request.getHeaderNames();while (enum1.hasMoreElements()) {String string = (String) enum1.nextElement();String value = request.getHeader(string);System.out.println("name:"+string +"  value:"+value);}}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

此处不需要修改web.xml配置，原因是此处通过注解进行了mapping映射：

@WebServlet("/HeaderServlet")

执行结果：

doFilter...

name:host  value:localhost:8080

name:connection  value:keep-alive

name:upgrade-insecure-requests  value:1

name:accept  value:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

name:user-agent  value:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_4) AppleWebKit/603.1.30 (KHTML, like Gecko) Safari/522.0

name:accept-language  value:zh-cn

name:cache-control  value:no-cache

name:accept-encoding  value:gzip, deflate

PS：其中如何设置返回每一秒刷新一次：例如显示不断变化的时间日期：在doGet中输出：

response.setHeader("refresh", "1");PrintWriter out = response.getWriter();out.println(new Date().toString());

5、持久化技术：Cookies和Session：

给客户端设置Cookies和从客户端取Cookies：

1》设置Cookies：

package com.demo;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.Cookie;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class CookieServlet */@WebServlet("/CookieServlet")public class CookieServlet extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public CookieServlet() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubCookie c = new Cookie("geek99", "www.geek99.com");c.setMaxAge(60*60*24*7);response.addCookie(c);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

执行结果：日志打印仅一个过滤器表示执行了。

doFilter...

2》从客户端获取Cookies：

package com.demo;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.Cookie;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;/** * Servlet implementation class GetCookieServlet */@WebServlet("/GetCookieServlet")public class GetCookieServlet extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public GetCookieServlet() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubCookie[] cs = request.getCookies();for (int i = 0; i < cs.length; i++) {System.out.println("key:"+cs[i].getName()+"value:"+cs[i].getValue());}}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

执行结果：

doFilter...

key:geek99value:www.geek99.com

其中：设置Cookies是通过response返回给客户端，获取Cookies是通过请求request获取。

给客户端设置Session和取出Session：

1》设置Session：

package com.demo;import java.io.IOException;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 javax.servlet.http.HttpSession;/** * Servlet implementation class SetSessionServlet */@WebServlet("/SetSessionServlet")public class SetSessionServlet extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public SetSessionServlet() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubHttpSession session = request.getSession();session.setAttribute("session_name", "session_value");}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

执行结果：

doFilter...

2》获取Session：

package com.demo;import java.io.IOException;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 javax.servlet.http.HttpSession;/** * Servlet implementation class GetSessionServlet */@WebServlet("/GetSessionServlet")public class GetSessionServlet extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public GetSessionServlet() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubHttpSession session = request.getSession();String value = (String) session.getAttribute("session_name");System.out.println("name:"+value);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

执行结果：

doFilter...

name:session_value

总结：区别于Cookies：Sesion的设置和获取都是通过request进行的。因此，Cookies是设置在浏览器本地的。不随着浏览器关闭而被清除。Session浏览器关闭则清除，同一浏览器不同页签可使用。

6、共享变量

变量的作用域
ServletContext整个应用程序
HttpSession当前浏览器
HttpServletRequest当前请求

1》设置共享变量和获取共享变量：

package com.shareVar;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;import javax.servlet.http.HttpSession;/** * Servlet implementation class SetShareVar * @WebServlet("/SetShareVar") */public class SetShareVar extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public SetShareVar() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stub//1.ServletContext//2.HttpSession//3.HttpServletRequestServletContext ctx = this.getServletContext();//1、ServletContext共享变量ctx.setAttribute("ctx_name", "ctx_value");HttpSession session = request.getSession();//2、HttpSession共享变量session.setAttribute("session_name", "session_value");request.setAttribute("request_name", "request_value");//3.上一行代码注释：HttpServletRequest携带信息到／getShareVar 的mapping指向的页面request.getRequestDispatcher("getShareVar").forward(request, response);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

getShareVar如下：

package com.shareVar;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;import javax.servlet.http.HttpSession;/** * Servlet implementation class getShareVar * @WebServlet("/getShareVar") */public class getShareVar extends HttpServlet {private static final long serialVersionUID = 1L;           /**     * @see HttpServlet#HttpServlet()     */    public getShareVar() {        super();        // TODO Auto-generated constructor stub    }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubServletContext ctx = this.getServletContext();String ctxValue = (String) ctx.getAttribute("ctx_name");HttpSession session = request.getSession();String sessionValue = (String)session.getAttribute("session_name");String requestValue = (String)request.getAttribute("request_name");System.out.println("ctxValue:"+ctxValue);System.out.println("sessionValue:"+sessionValue);System.out.println("requestValue:"+requestValue);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}}

web.xml配置略；

执行/setShare打印日志如下：

ctxValue:ctx_value
sessionValue:session_value
requestValue:request_value

打开新浏览器如下：其中ctx的值携带了，而session，request_value并没有。

ctxValue:ctx_value

sessionValue:null

requestValue:null

7、监听：

监听web工程状态的改变
1）整个工程状态的改变
servletContextListener ServletContextAttributeListener
2）session属性的变化
httpSessionListener httpSessionAttributeListener
3）request属性的变化
servletRequestListener servletRequestAttributeListener

1》对整个Servlet监听：前置性的工作可置于此处contextInitialized

implements ServletContextListener

写一个MyListener继承自ServletContextListener（还有其他的可以供继承），MyListener

package com.listener;import javax.servlet.ServletContext;import javax.servlet.ServletContextEvent;import javax.servlet.ServletContextListener;import javax.servlet.annotation.WebListener;/** * Application Lifecycle Listener implementation class MyListener * */public class MyListener implements ServletContextListener {    /**     * Default constructor.      */    public MyListener() {        // TODO Auto-generated constructor stub    }/**     * @see ServletContextListener#contextDestroyed(ServletContextEvent)     */    public void contextDestroyed(ServletContextEvent arg0)  {          // TODO Auto-generated method stub    }/**     * @see ServletContextListener#contextInitialized(ServletContextEvent)     */    public void contextInitialized(ServletContextEvent arg0)  {          System.out.println("contextInitialized...");         ServletContext sctx = arg0.getServletContext();         sctx.setAttribute("onLineCount", 0);    }}

在web.xml下配置：和上面的servlet平级处配置：

<listener>    <listener-class>com.listener.MyListener</listener-class>  </listener>

启动初始化服务的时候，会执行监听。打印：

contextInitialized...

attributeAdded...

2》对Session进行监听：

写一个类SessionListenerimplements HttpSessionListener ：

package com.listener;import javax.servlet.ServletContext;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener;public class SessionListener implements HttpSessionListener {@Overridepublic void sessionCreated(HttpSessionEvent arg0) {// TODO Auto-generated method stubServletContext ctx = arg0.getSession().getServletContext();Integer numSessions = (Integer) ctx.getAttribute("numSessions");if (numSessions == null) {numSessions = new Integer(1);} else {int count = numSessions.intValue();numSessions = new Integer(count + 1);}ctx.setAttribute("numSessions", numSessions);System.out.println("当前在线人数：" + numSessions);}@Overridepublic void sessionDestroyed(HttpSessionEvent arg0) {// TODO Auto-generated method stubServletContext ctx = arg0.getSession().getServletContext();Integer numSessions = (Integer) ctx.getAttribute("numSessions");if (numSessions == null) {numSessions = new Integer(0);} else {int count = numSessions.intValue();numSessions = new Integer(count - 1);}ctx.setAttribute("numSessions", numSessions);System.out.println("当前在线人数：" + numSessions);}}

在web.xml中添加：

<listener>  <listener-class>com.listener.SessionListener</listener-class>  </listener>

每打开一个浏览器，调用前面的/GetSessionServlet 会触发导致在线人数加1.

当前在线人数：0

doFilter...

当前在线人数：1

name:null

doFilter...

当前在线人数：2

3》对attribute的监听：

package com.listener;import javax.servlet.ServletContextAttributeEvent;import javax.servlet.ServletContextAttributeListener;/** * Application Lifecycle Listener implementation class MyServletContextAttributeListener * */public class MyServletContextAttributeListener implements ServletContextAttributeListener {    /**     * Default constructor.      */    public MyServletContextAttributeListener() {        // TODO Auto-generated constructor stub    }/**     * @see ServletContextAttributeListener#attributeAdded(ServletContextAttributeEvent)     */    public void attributeAdded(ServletContextAttributeEvent arg0)  {          // TODO Auto-generated method stub    System.out.println("attributeAdded...");    }/**     * @see ServletContextAttributeListener#attributeRemoved(ServletContextAttributeEvent)     */    public void attributeRemoved(ServletContextAttributeEvent arg0)  {          // TODO Auto-generated method stub    System.out.println("attributeRemoved...");    }/**     * @see ServletContextAttributeListener#attributeReplaced(ServletContextAttributeEvent)     */    public void attributeReplaced(ServletContextAttributeEvent arg0)  {          // TODO Auto-generated method stub    System.out.println("attributeReplaced...");    }}

添加web.xml配置略。

第一次执行/SetShareVar提示

attributeAdded...

第二次执行/SetShareVar提示

attributeReplaced...

销毁时执行

attributeRemoved...