Java Web学习Day02 ServletContext对象

Java Web学习Day02 ServletContext对象

1.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，它代表了我当前的web应用

1、共享数据

• 我在这个Servlet保存的应用，可以在另一个servlet中拿到：

  //设置数据
  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 username = "蓝天";
          context.setAttribute("username",username);  //将一个数据保存在ServletContext中，名字为：username 值为username
          System.out.println("Hello！");
      }
  
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
  

  //拿取数据
  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");
          resp.setContentType("text/html");
          resp.setCharacterEncoding("utf-8");
          resp.getWriter().print("名字"+username);
      }
      @Override
      protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
          doGet(req, resp);
      }
  }
  

      <!--HelloServlet注册-->
      <servlet>
          <servlet-name>hello</servlet-name>
          <servlet-class>com.lantian.servlet.HelloServlet</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>hello</servlet-name>
          <url-pattern>/hello</url-pattern>
      </servlet-mapping>
  
      <!--GetServlet注册-->
      <servlet>
          <servlet-name>getc</servlet-name>
          <servlet-class>com.lantian.servlet.GetServlet</servlet-class>
      </servlet>
      <servlet-mapping>
          <servlet-name>getc</servlet-name>
          <url-pattern>/getc</url-pattern>
      </servlet-mapping>
  

  测试访问结果：直接访问getc会返回null 因为此时hello中setAttribute还会设置数据；访问hello后再此访问getc得到返回数据 名字蓝天

2、获取初始化参数

    <!--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>
    <!--ServletDemo03注册-->
    <servlet>
        <servlet-name>demo3</servlet-name>
        <servlet-class>com.lantian.servlet.ServletDemo03</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>demo3</servlet-name>
        <url-pattern>/demo3</url-pattern>
    </servlet-mapping>

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();  //创建ServletContext对象
       String url = context.getInitParameter("url");		//获取初始化参数
       resp.getWriter().print(url);
    }
}

3、请求转发

public class ServletDemo04 extends HelloServlet{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        System.out.println("进入方法Demo04");
/*      RequestDispatcher redis = context.getRequestDispatcher("/demo3");       //转发的请求路径
        redis.forward(req,resp);        //调用forward实现请求转发；
        */
        context.getRequestDispatcher("/demo3").forward(req,resp);

    }
    @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resources目录下新建properties

发现： 都被打包到同一路径下：classes， 称这个路径为 类路径

思路： 需要一个文件流

username=root
password=123456

public class ServletDemo05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        InputStream  is =this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(is);
        String user = prop.getProperty("username");
        String pwd = prop.getProperty("password");
        resp.getWriter().print("用户名: "+user+"\n"+"密码："+pwd);

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

访问测试