2.深入学习Servlet的ServletContext对象

一.建立项目servlet01

在入门Servlet项目中建立一个子项目模块（此处不再赘述如何建立），补全maven项目中的java和resources文件夹，添加类HelloServlet.java，添加web.xml中的映射关系，添加tomcat （注意保持只有一个jar包方式把所有项目的jar都打进来了，删除其他项目的jar包，添加自己的jar包）,运行验证是否搭建正确！

二.ServletContext对象

在web容器启动时，它会为每一个web程序创建一个对应的ServletContext对象，用来代表当前的web的应用。

ServletContext对象的几个重点所用：

1. 共享数据：对于几个不同的Servlet之间可以相互传递数据

2. 获取初始化参数：在web.xml中配置的参数，可以被或得到

3. 请求转发：获得请求转发给别的Servlet

4. 读取资源文件：此处需要注意配置资源过滤防止读取不了资源文件

1.共享数据： 在servlet中保存的数据可以在另一个servlet中被访问

例子：

在HelloServlet中代码

 @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

 //        this.getInitParameter();  初始化参数
 //        this.getServletConfig();  Servlet配置
 //        this.getServletContext(); Servlet上下文

         ServletContext context = this.getServletContext();

         String username = "小王";
         context.setAttribute("username",username);  //将数据存入ServletContext中

     }

在GetServlet中的代码

 public class GetServlet extends HttpServlet {
     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

         ServletContext context = this.getServletContext();

         String username = (String) context.getAttribute("username");  //获取别的Servlet所保存的参数，实现数据共享

         resp.setContentType("text/html");
         resp.setCharacterEncoding("utf-8");
         resp.getWriter().println("名字"+ username);

     }

     @Override
     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         doGet(req, resp);
     }
 }

配置web.xml

 <!--注册Servlet-->
     <servlet>
         <servlet-name>getc</servlet-name>
         <servlet-class>ustc.wzh.servlet.GetServlet</servlet-class>
     </servlet>

     <!--Servlet的请求路径-->
     <servlet-mapping>
         <servlet-name>getc</servlet-name>
         <url-pattern>/getc</url-pattern>
     </servlet-mapping>

启动tomcat执行程序

先执行http://localhost:8080/servlet01/hello 此时已经存入数据了，在执行http://localhost:8080/servlet01/getc 显示数据

执行成功!

2.获取初始化参数配置

新建Java类为GetParam

 public class GetParam extends HttpServlet {
     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

         ServletContext context = this.getServletContext();

         String url = (String) context.getInitParameter("url");
         resp.getWriter().println(url);

     }

     @Override
     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         doGet(req, resp);
     }
 }

在web.xml中编写初始化配置项和映射路径

     <!--配置一些web应用初始化参数-->
     <context-param>
         <param-name>url</param-name>
         <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
     </context-param>

     <!--注册Servlet-->
     <servlet>
         <servlet-name>getParam</servlet-name>
         <servlet-class>ustc.wzh.servlet.GetParam</servlet-class>
     </servlet>

     <!--Servlet的请求路径-->
     <servlet-mapping>
         <servlet-name>getParam</servlet-name>
         <url-pattern>/getParam</url-pattern>
     </servlet-mapping>

重新部署tomcat,获取参数成功！

3.请求转发

新建Java类为GetRequestForward

 public class GetRequestForward extends HttpServlet {

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

         System.out.println("开始请求转发！");
         ServletContext context = this.getServletContext();

         //设置请求转发路径，再调用forward实现请求转发
         context.getRequestDispatcher("/getParam").forward(req,resp);

         System.out.println("转发成功！");
     }

     @Override
     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         doGet(req, resp);
     }
 }

在web.xml中编写映射路径

     <!--注册Servlet-->
     <servlet>
         <servlet-name>getReqFor</servlet-name>
         <servlet-class>ustc.wzh.servlet.GetRequestForward</servlet-class>
     </servlet>

     <!--Servlet的请求路径-->
     <servlet-mapping>
         <servlet-name>getReqFor</servlet-name>
         <url-pattern>/getReqFor</url-pattern>
     </servlet-mapping>

重新部署tomcat,转发成功！

4.读取资源文件

预处理：

由于我们在java目录下建立的properties和resources目录下建立的properties都被打报道同一路径下：classes,我们俗称这个路径为classpath;

1.建立properties文件:在resources目录下建立db.properties，在java目录下建立aa.properties

username=wzh password=123456

username=wzh02 password=123456890

2.修改当前项目下的pom.xml文件，添加配置

         <!--在build中配置resources,来防止我们资源导出失败的问题-->
         <resources>
             <resource>
                 <directory>src/main/resources</directory>
                 <includes>
                     <include>**/*.properties</include>
                     <include>**/*.xml</include>
                 </includes>
                 <filtering>true</filtering>
             </resource>
             <resource>
                 <directory>src/main/java</directory>
                 <includes>
                     <include>**/*.properties</include>
                     <include>**/*.xml</include>
                 </includes>
                 <filtering>true</filtering>
             </resource>
         </resources>

3.编写读取配置文件的类：GetText

 public class GetText extends HttpServlet {

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

         //读取resources目录下的db.properties内容
         InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");

         Properties properties = new Properties();
         properties.load(in);
         String username = properties.getProperty("username");
         String password = properties.getProperty("password");

         resp.getWriter().println(username + ":" + password);

         //读取java目录下的aa.properties内容
         InputStream in2 = this.getServletContext().getResourceAsStream("/WEB-INF/classes/ustc/wzh/servlet/aa.properties");

         Properties properties2 = new Properties();
         properties2.load(in2);
         String username2 = properties2.getProperty("username");
         String password2 = properties2.getProperty("password");

         resp.getWriter().println(username2 + ":" + password2);

     }

     @Override
     protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
         doGet(req, resp);
     }
 }

注意：两个文件名是Rebuild project之后产生的

修改web.xml中的映射关系

     <!--注册Servlet-->
     <servlet>
         <servlet-name>getText</servlet-name>
         <servlet-class>ustc.wzh.servlet.GetText</servlet-class>
     </servlet>

     <!--Servlet的请求路径-->
     <servlet-mapping>
         <servlet-name>getText</servlet-name>
         <url-pattern>/getText</url-pattern>
     </servlet-mapping>

重新部署tomcat,读取资源文件成功！