1.什么是ServletContext？

servletContext接口是Servlet中最大的一个接口，呈现了web应用的Servlet视图。ServletContext实例是通过 getServletContext()方法获得的，由于HttpServlet继承GenericServlet的关系，GenericServlet类和HttpServlet类同时具有该方法。

2.ServletContext的作用？

• ServletContext可以实现共享数据
• 可以再不同的请求中，拿到其他的参数

1.将数据放入ServletContext中,实现数据共享

//将数据放入ServletContext中
public class HelloServlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {//this.getInitParameter();   初始化参数//this.getServletConfig();   Servlet配置ServletContext servletContext = this.getServletContext();// 上下文servletContext.setAttribute("qxd","山有木兮木有枝");     //这样数据就放入了servletContext当中servletContext.setAttribute("qxd1","hello");            //我们就可以在另外一个servlet中取出来了System.out.println("Hello");String contextPath = req.getContextPath();//System.out.println(req.getServerPort());}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doGet(req, resp);}
}

2. 在另一个类中将ServletContext取出来

//将ServletContext取出来
public class Hello1Servlet extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {resp.setCharacterEncoding("UTF-8");req.setCharacterEncoding("UTF-8");resp.setContentType("text/html;charset=utf-8");ServletContext servletContext = this.getServletContext();String qxd = (String)servletContext.getAttribute("qxd");String qxd1 = (String) servletContext.getAttribute("qxd1");//将他最前端页面打印出来PrintWriter writer = resp.getWriter();writer.println(qxd);writer.println(qxd1);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doGet(req, resp);}
}

结果

拿到初始化参数和读取properties文件的数据

他们都是从ServletContext中读出来的

//如何读取初始化参数，和properties的数据
public class ServletContext1 extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();String url = servletContext.getInitParameter("url");resp.getWriter().println(url);InputStream is = servletContext.getResourceAsStream("/WEB-INF/classes/qxd.properties");  //将properties的数据写入is中Properties properties = new Properties();properties.load(is);    //将is读到的数据写入到properties类中String username = properties.getProperty("username");   //从properties的存入的数据中，读出数据String password = properties.getProperty("password");resp.getWriter().println(username);     //展示给前端resp.getWriter().println(password);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doGet(req, resp);}
}

properties文件的数据

username=qxd
password=123456# classpath:

结果:

servletContext实现请求转发

public class ServletDispatchar extends HttpServlet {@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {ServletContext servletContext = this.getServletContext();servletContext.getRequestDispatcher("/context").forward(req,resp); //转发的请求路径 转发到了另一个路径}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doGet(req, resp);}
}

web.xml文件

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"version="4.0"><!--配置一些web的初始化参数--><context-param><param-name>url</param-name><param-value>jdbc:mysql://localhost:3306/mybatis</param-value></context-param><servlet><servlet-name>hello</servlet-name><servlet-class>com.qin.HelloServlet</servlet-class></servlet><servlet-mapping><servlet-name>hello</servlet-name><url-pattern>/hello</url-pattern></servlet-mapping><servlet><servlet-name>hello1</servlet-name><servlet-class>com.qin.Hello1Servlet</servlet-class>
</servlet><servlet-mapping><servlet-name>hello1</servlet-name><url-pattern>/hello1</url-pattern></servlet-mapping><servlet><servlet-name>hello2</servlet-name><servlet-class>com.qin.ServletContext1</servlet-class></servlet><servlet-mapping><servlet-name>hello2</servlet-name><url-pattern>/context</url-pattern></servlet-mapping><servlet><servlet-name>hello3</servlet-name><servlet-class>com.qin.ServletDispatchar</servlet-class></servlet><servlet-mapping><servlet-name>hello3</servlet-name><url-pattern>/dis</url-pattern></servlet-mapping></web-app>

总结

ServletContext可以实现，

1. 数据共享 servletContext.getAttribute取出数据，servletContext.setAttribute存入数据
2. 请求转发 servletContext.getRequestDispatcher("/context").forward(req,resp);
3. 获取初始化参数 servletContext.getInitParameter
4. 获取properties文件中的数据
   servletContext.getResourceAsStream，获取资源文件的数据流，properties.load放入到properties中，
   properties.getPropertyj将数据取出来