一，Servlet访问URL映射配置
由于客户端是通过URL地址访问web服务器中的资源，所以Servlet程序若想被外界访问，必须把servlet程序映射到一个URL地址上，这个工作在web.xml文件中使用<servlet>元素和<servlet-mapping>元素完成。
Servlet：该元素用于注册Servlet，它包含有两个主要的子元素：<servlet-name>和<servlet-class>，分别用于设置Servlet的注册名称和Servlet的完整类名。
servlet-mapping：该元素用于映射一个已注册的Servlet的一个对外访问路径，它包含有两个子元素：<servlet-name>和<url-pattern>，分别用于指定Servlet的注册名称和Servlet的对外访问路径。
<servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello.html</url-pattern><!-- servlet访问路径 --></servlet-mapping>
同一个Servlet可以被映射到多个URL上，即多个<servlet-mapping>元素的<servlet-name>子元素的设置值可以是同一个Servlet的注册名
<servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello.html</url-pattern><!-- servlet访问路径 --></servlet-mapping><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello1.html</url-pattern><!-- servlet访问路径 --></servlet-mapping><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello2.html</url-pattern><!-- servlet访问路径 --></servlet-mapping>
二，ServletURL通配符：*
在Servlet映射到的URL中也可以使用*通配符，但是只能有两种固定的格式：
格式一：*.扩展名"
<servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>*.html</url-pattern><!-- servlet访问路径 --></servlet-mapping>
格式二：以正斜杠（/）开头并以"/*"结尾
<servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello/*</url-pattern><!-- servlet访问路径 --></servlet-mapping>
三，缺省Servlet
Servlet的映射路径仅仅为一个正斜杠（/），那么这个Servlet就成为当前Web应用程序的缺省Servlet。 凡是在web.xml文件中找不到匹配的<servlet-mapping>元素的URL，它们的访问请求都将交给缺省Servlet处理。
<servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/</url-pattern><!-- servlet访问路径 --></servlet-mapping>
四，ServletConfig
ServletConfig代表当前Servlet在web.xml中的配置信息。
1，Servlet初始化参数
在Servlet的配置文件中，可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。
例如：
<servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --><init-param><param-name>name</param-name><param-value>Zender</param-value></init-param><init-param><param-name>encode</param-name><param-value>UTF-8</param-value></init-param></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello.html</url-pattern><!-- servlet访问路径 --></servlet-mapping>
2，通过ServletConfig获取Servlet的初始化参数
当servlet配置了初始化参数后，web容器在创建servlet实例对象时，会自动将这些初始化参数封装到ServletConfig对象中，并在调用servlet的init方法时，将ServletConfig对象传递给servlet。进而，通过ServletConfig对象就可以得到当前servlet的初始化参数信息。
public class HelloServlet extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//获取ServletConfigServletConfig config = this.getServletConfig();//获取初始化参数String name = config.getInitParameter("name");String encode = config.getInitParameter("encode");//设置编码格式，否则中文会出现乱码resp.setCharacterEncoding("GBK");PrintWriter out = resp.getWriter();out.println("name:" + name);out.println("encode:" + encode);}}
五，ServletContext
WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。
ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext()方法获得ServletContext对象。
由于一个WEB应用中的所有Servlet共享同一个ServletContext对象，因此Servlet对象之间可以通过ServletContext对象来实现通讯。ServletContext对象通常也被称之为context域对象。
1，获取WEB应用的初始化参数
在web.xml文件中使用<context-param>标签配置WEB应用的初始化参数：
<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="/2001/XMLSchema-instance"xmlns="/xml/ns/javaee"xsi:schemaLocation="/xml/ns/javaee /xml/ns/javaee/web-app_3_0.xsd"id="WebApp_ID" version="3.0"><display-name>ServletDemo</display-name><welcome-file-list><welcome-file>index.jsp</welcome-file></welcome-file-list><!-- 配置WEB应用的初始化参数 --><context-param><param-name>name</param-name><param-value>Zender</param-value></context-param><context-param><param-name>encode</param-name><param-value>UTF-8</param-value></context-param><servlet><servlet-name>HelloServlet</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.HelloServlet</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>HelloServlet</servlet-name><url-pattern>/hello.html</url-pattern><!-- servlet访问路径 --></servlet-mapping></web-app>
获取Web应用的初始化参数：
public class HelloServlet extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//获取ServletContextServletContext context = this.getServletContext();//获取初始化参数String name = context.getInitParameter("name");String encode = context.getInitParameter("encode");//设置编码格式，否则中文会出现乱码resp.setCharacterEncoding("GBK");PrintWriter out = resp.getWriter();out.println("name:" + name);out.println("encode:" + encode);}}
2，多个Servlet通过ServletContext对象实现数据共享
例如 ：ServletTwo获得ServletOne所共享的数据
ServletOne：
public class ServletOne extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//共享的数据String name = "zender";//获取ServletContextServletContext context = this.getServletContext();//将name存储到ServletContext对象中context.setAttribute("name", name);}}
ServletTwo：
public class ServletTwo extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//获取ServletContextServletContext context = this.getServletContext();//获取初始化参数String name = (String) context.getAttribute("name");PrintWriter out = resp.getWriter();out.println("name:" + name);}}
Web.xml：
<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="/2001/XMLSchema-instance"xmlns="/xml/ns/javaee"xsi:schemaLocation="/xml/ns/javaee /xml/ns/javaee/web-app_3_0.xsd"id="WebApp_ID" version="3.0"><display-name>ServletDemo</display-name><welcome-file-list><welcome-file>index.jsp</welcome-file></welcome-file-list><servlet><servlet-name>ServletOne</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.ServletOne</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>ServletOne</servlet-name><url-pattern>/ServletOne.html</url-pattern><!-- servlet访问路径 --></servlet-mapping><servlet><servlet-name>ServletTwo</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.ServletTwo</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>ServletTwo</servlet-name><url-pattern>/ServletTwo.html</url-pattern><!-- servlet访问路径 --></servlet-mapping></web-app>
先访问：http://localhost:8081/ServletDemo/ServletOne.html
再访问：http://localhost:8081/ServletDemo/ServletTwo.html
3，用servletContext实现请求转发
context.getRequestDispatcher()方法实现请求转发
ServletOne：
public class ServletOne extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//共享的数据String name = "zender";//获取ServletContextServletContext context = this.getServletContext();//将name存储到ServletContext对象中context.setAttribute("name", name);context.getRequestDispatcher("/ServletTwo.html").forward(req, resp);}}
ServletTwo：
public class ServletTwo extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//获取ServletContextServletContext context = this.getServletContext();//获取初始化参数String name = (String) context.getAttribute("name");PrintWriter out = resp.getWriter();out.println("name:" + name);}}
Web.xml：
<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="/2001/XMLSchema-instance"xmlns="/xml/ns/javaee"xsi:schemaLocation="/xml/ns/javaee /xml/ns/javaee/web-app_3_0.xsd"id="WebApp_ID" version="3.0"><display-name>ServletDemo</display-name><welcome-file-list><welcome-file>index.jsp</welcome-file></welcome-file-list><servlet><servlet-name>ServletOne</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.ServletOne</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>ServletOne</servlet-name><url-pattern>/ServletOne.html</url-pattern><!-- servlet访问路径 --></servlet-mapping><servlet><servlet-name>ServletTwo</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.ServletTwo</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>ServletTwo</servlet-name><url-pattern>/ServletTwo.html</url-pattern><!-- servlet访问路径 --></servlet-mapping></web-app>
访问：http://localhost:8081/ServletDemo/ServletOne.html
访问的是ServletOne.html，浏览器显示的却是ServletTwo.html的内容，这就是使用ServletContext实现了请求转发。
4，利用ServletContext对象读取资源文件
获取上图的config.properties资源文件内容：
GetProperties.Java：
public class GetProperties extends HttpServlet{private static final long serialVersionUID = 3903946972744326948L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {this.doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//设置浏览器编码格式为UTF-8resp.setHeader("content-type","text/html;charset=UTF-8");resp.getWriter().println("读取src目录下的config.properties配置文件：");//读取src目录下的properties配置文件readProperties(resp,"/WEB-INF/classes/config.properties");resp.getWriter().println("<hr/>读取WebRoot目录下的config.properties配置文件：");//读取WebRoot目录下的properties配置文件readProperties(resp,"config.properties");}/*** * @方法名: readProperties* @描述: 获取properties配置文件内容* @param resp* @param fileUrl* @throws IOException * @创建人 zender*/protected void readProperties(HttpServletResponse resp, String fileUrl) throws IOException{//2种方式都能获取到config.properties//InputStream in = new FileInputStream(this.getServletContext().getRealPath(fileUrl));InputStream in = this.getServletContext().getResourceAsStream(fileUrl);Properties prop = new Properties();prop.load(in);String url = prop.getProperty("database.url");String username = prop.getProperty("database.username");String password = prop.getProperty("database.password");resp.getWriter().println(MessageFormat.format("url={0},username={1},password={2}", url,username, password));}}
Web.xml：
<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="/2001/XMLSchema-instance"xmlns="/xml/ns/javaee"xsi:schemaLocation="/xml/ns/javaee /xml/ns/javaee/web-app_3_0.xsd"id="WebApp_ID" version="3.0"><display-name>ServletDemo</display-name><welcome-file-list><welcome-file>index.jsp</welcome-file></welcome-file-list><servlet><servlet-name>GetProperties</servlet-name><!-- servlet名称 --><servlet-class>com.zender.servlet.GetProperties</servlet-class><!-- servlet关联的类 --></servlet><servlet-mapping><servlet-name>GetProperties</servlet-name><url-pattern>/GetProperties.html</url-pattern><!-- servlet访问路径 --></servlet-mapping></web-app>
访问路径：http://localhost:8081/ServletDemo/GetProperties.html
