Context的用途

提示：这里所用的编译器为idea，其tomcat和maven都已配置完成。

文章目录

Context的用途
一、共享数据
- 代码示例：
二、获取初始化参数
- 代码示例
三、请求转发
- 代码示例
四、读取资源文件
- 代码示例
总结

提示：以下是本篇文章正文内容，下面案例可供参考

一、共享数据

示例：创建一个helloservlet.java通过调用context.setAttribute()方法，将数据保存到了ServletContext中，然后再创建一个GetContext.java从ServletContext中读取里面的存过内容。

代码示例：

public class helloservlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        String name="大大眼";
        context.setAttribute("name",name);//将数据保存到了ServletContext中，名字为：name，值为：name
        resp.getWriter().print("进行存入操作");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

在web.xml中配置servlet和mapping

<!--web.xml中是配置我们web的核心应用-->
  <!--注册Servlet-->
<servlet>
  <servlet-name>hello</servlet-name>
  <servlet-class>com.llp.servlet.helloservlet</servlet-class>
</servlet>
  <servlet-mapping>
    <servlet-name>hello</servlet-name>
    <url-pattern>/maintext</url-pattern>
  </servlet-mapping>

public class GetContext extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        String name=(String) context.getAttribute("name");
        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("姓名："+name);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

在web.xml中配置servlet和mapping

<servlet>
    <servlet-name>getContext</servlet-name>
    <servlet-class>com.llp.servlet.GetContext</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>getContext</servlet-name>
    <url-pattern>/get</url-pattern>
  </servlet-mapping>

在这里插入图片描述

二、获取初始化参数

代码示例

public class ContextDm1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        String url=context.getInitParameter("url");
        resp.getWriter().print(url);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

在web.xml中配置servlet和mapping（这里的mysql://localhost:3306/databaseName是一个随机设置的）

    <context-param>
        <param-name>url</param-name>
        <param-value>mysql://localhost:3306/databaseName</param-value>
    </context-param>
    <servlet>
       <servlet-name>ContextDm1</servlet-name>
       <servlet-class>com.llp.servlet.ContextDm1</servlet-class>
    </servlet>  
    <servlet-mapping>
       <servlet-name>ContextDm1</servlet-name>
       <url-pattern>/Dm1</url-pattern>
    </servlet-mapping>

三、请求转发

请求转发，是服务器的行为，请求由服务器转发给另外一个页面处理，如何转发，何时转发，转发几次，客户端是不知道的。
请求转发时，从发送第一次到最后一次请求的过程中，web容器创建一次request和response对象，新的页面继续处理同一个请求。也可以理解为服务器将request对象在页面之间传递。

代码示例

public class ContextDm2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context=this.getServletContext();
        context.getRequestDispatcher("/Dm1").forward(req,resp);//转发请求路径、调用forward请求转发
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

在web.xml中配置servlet和mapping

<servlet>
    <servlet-name>ContextDm2</servlet-name>
    <servlet-class>com.llp.servlet.ContextDm2</servlet-class>
</servlet>
  <servlet-mapping>
    <servlet-name>ContextDm2</servlet-name>
    <url-pattern>/Dm2</url-pattern>
  </servlet-mapping>

四、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：都被打包到了同一个路径下：classes，我们俗称这个路径为classpath
思路：需要一个文件流；

代码示例

public class ContextDm3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        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.setCharacterEncoding("utf-8");
        resp.setContentType("text/html");
        resp.getWriter().println("<h1>username : "+username+"</h1>");
        resp.getWriter().println("<h1>password : "+password+"</h2>");
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}

在web.xml中配置servlet和mapping

<servlet>
    <servlet-name>ContextDm3</servlet-name>
    <servlet-class>com.llp.servlet.ContextDm3</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>ContextDm3</servlet-name>
    <url-pattern>/Dm3</url-pattern>
  </servlet-mapping>