ServletContext 学习

ServletContext

web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用；

1、共享数据

​ 在这个Servlet中保存了数据，就可以在另外一个servlet取到

首先将数据存入HelloServlet中，然后可以在另外一个Servlet类中取出

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    @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 name = "逍遥子";//数据
        context.setAttribute("name",name);//将一个数据保存在了 ServletContext 中
        System.out.println("数据已经存放！！！！");
    }

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

@WebServlet("/getc")
public class ReadServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //数据
        ServletContext context = this.getServletContext();
        String name = (String) context.getAttribute("name");
        //响应，设置编码
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("名字"+name);
    }

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

2、获取初始化参数

可以获得在xml中初始化保存的参数

<?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应用的初始化参数 -->
        <context-param>
            <param-name>url</param-name>
            <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
        </context-param>

</web-app>

@WebServlet("/gp")
public class ServletDeom03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //数据
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }

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

3、请求转发

转发并不会改变请求的路径，重定向才会改变请求的路径

转发是间接获取到资源，重定向是直接拿到资源

@WebServlet("/sd4")
public class Servletdemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //数据
        System.out.println("进入了demo04页面");
        ServletContext context = this.getServletContext();
        //RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//转发路径
        //requestDispatcher.forward(req,resp);//调用forward实现请求转发
        context.getRequestDispatcher("/gp").forward(req,resp);//请求转发
    }

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

4、读取资源文件

Properties

• 在java目录下新建properties
• 在resource目录下新建properties

发现：都被打包到了同一路径下：classes，我们俗称这个路径为classpath

注：在java目录下创建的properties文件，若想导出成功，还需配置xml文件

<!--  在build中配置resource，来防止我们资源导出失败的问题-->
  <build>
    <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>
  </build>

读文件思路：需要一个文件流；

@WebServlet("/sd5")
public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        /**
         * 1、导入properties流，根据项目的相对地址
         * 2、创建properties对象
         * 3、将刚刚的流加载到properties对象中
         * 4、成功引入properties文件
         */
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(is);
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");
        resp.getWriter().print("username = "+username);
        resp.getWriter().print("password = "+password);
    }

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