Servlet03_ServletContext

1.简介：

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。每个应用都会有一个ServletContext对象与之关联,当容器分布在多个虚拟机上时，web应用在所分布的每个虚拟机上都拥有一个ServletContext实例。缺省情况下，ServletContext不是分布式的，并且只存在于一个虚拟机上。

2.共享数据：

//存数据
public class ServletTest01 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("test01");

        //this.getInitParameter()   初始化参数
        //this.getServletConfig()   servlet配置
        //this.getServletContext()  servlet上下文
        ServletContext servletContext = this.getServletContext();

        String username="秦疆";
        servletContext.setAttribute("username",username);//将一个数据保存在了ServletContext中，键值对username:"秦疆"
    }
}

//取数据
public class ServletTest02 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String username = (String) servletContext.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);
    }
}

<!--web.xml中是配置我们web的核心应用-->
  <!--注册Servlet-->
  <servlet>
    <servlet-name>test01</servlet-name>
    <servlet-class>com.beyond.servlet.ServletTest01</servlet-class>
  </servlet>
  <!--一个Servlet对应一个mapping(映射)-->
  <servlet-mapping>
    <servlet-name>test01</servlet-name>
    <url-pattern>/test01</url-pattern>
  </servlet-mapping>

  <servlet>
    <servlet-name>test02</servlet-name>
    <servlet-class>com.beyond.servlet.ServletTest02</servlet-class>
  </servlet>
  <!--一个Servlet对应一个mapping(映射)-->
  <servlet-mapping>
    <servlet-name>test02</servlet-name>
    <url-pattern>/test02</url-pattern>
  </servlet-mapping>

3.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>

public class ServletTest03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();

        String url = servletContext.getInitParameter("url");
        resp.getWriter().print(url);
    }

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

4.请求转发

public class ServletTest04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();

//        RequestDispatcher requestDispatcher = servletContext.getRequestDispatcher("/test03");//转发的请求路径
//        requestDispatcher.forward(req,resp);//调用forward方法实现请求转发
        servletContext.getRequestDispatcher("/test03").forward(req,resp);

        System.out.println("进入了ServletTest04");
    }

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

5.读取资源文件

• classpath：
  • 在java目录下建立的资源文件和在resources目录下建立的资源文件，最终都会被打包到target/servlet-项目-SNAPSHOT/WEB-INF/classes目录下，这几个目录都被称为classpath。

public class PropertiesTest05 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/com/beyond/servlet/test05.properties");
        InputStream is = this.getServletContext().getResourceAsStream("/WEB-INF/classes/pt5.properties");

        Properties prop=new Properties();
        prop.load(is);
        String user=prop.getProperty("username");
        String pwd=prop.getProperty("password");

        resp.getWriter().print(user+":"+pwd);
    }

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