javaweb开发（四）：servlet应用上下文与jsp

• 4大作用域对象

PageContext: 当我们访问不同的servlet时，可以获取该对象，当访问其他的servlet，即切换不同的页面时，就会销毁该对象
ServletRequest：当我们发送不同的请求时，可以获取该对象，访问其他请求时，则该对象销毁
HttpSession: 当我们打开某个网站时，只要该会话没有过期，则可以随时从该对象中获取数据
ServletContext: 当tomcat服务器启动后该对象就一直存在，且所有的servlet都可以从该作用域中获取值

• 构建一个web项目，导入依赖
  
  
  

• 之后查看导入的jar包
  

• web.xml中添加如下

   <context-param>
        <param-name>url</param-name>
        <param-value>xdclass.net</param-value>
    </context-param>

    <context-param>
        <param-name>username</param-name>
        <param-value>jack</param-value>
    </context-param>

• 编写UserServlet: 先获取web.xml中数据，再向servletContext中添加数据

@WebServlet(name = "userServlet",urlPatterns = {"/user"})
public class UserServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter printWriter = resp.getWriter();
        printWriter.write("<div> doGet </div>");
        // 获取servletContext中的数据
        ServletContext servletContext = req.getServletContext();
        String url = servletContext.getInitParameter("url");
        System.out.println(url);
        // 向servletContext中添加数据
        servletContext.setAttribute("appInfo","xdclass.net info");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter printWriter = resp.getWriter();
        printWriter.write("<div> doPost </div>");
    }

}

• 编写TestServlet: 获取在UserServlet中向servletContext中添加的数据

@WebServlet(name = "testServlet",urlPatterns = {"/test"})
public class TestServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException, IOException {
        resp.setContentType("text/html;charset=utf-8");
        PrintWriter printWriter = resp.getWriter();
        printWriter.write("<div> TestServlet doGet </div>");
        // 获取servletContext中的数据
        ServletContext servletContext = req.getServletContext();
        String appInfo = (String)servletContext.getAttribute("appInfo");
        System.out.println("TestServlet appInfo="+appInfo);
    }

}

• 测试先访问testServlet: servletContext中没有数据

[2022-04-12 09:27:28,239] Artifact web02:war exploded: Artifact is deployed successfully
[2022-04-12 09:27:28,239] Artifact web02:war exploded: Deploy took 502 milliseconds
TestServlet appInfo=null

• 再访问userServlet: 获取到web.xml中的数据

12-Apr-2022 21:27:37.215 淇℃伅 [Catalina-utility-2] org.apache.catalina.startup.HostConfig.deployDirectory 鎶妛eb 搴旂敤绋嬪簭閮ㄧ讲鍒扮洰褰� [C:\Program Files\Apache Software Foundation\Tomcat 10.0\webapps\manager]
12-Apr-2022 21:27:37.271 淇℃伅 [Catalina-utility-2] org.apache.catalina.startup.HostConfig.deployDirectory Web搴旂敤绋嬪簭鐩綍[C:\Program Files\Apache Software Foundation\Tomcat 10.0\webapps\manager]鐨勯儴缃插凡鍦╗56]姣鍐呭畬鎴�
xdclass.net

• 再访问testServlet: 获取到在userServlet中向servletContext中添加的数据
  

12-Apr-2022 21:27:37.215 淇℃伅 [Catalina-utility-2] org.apache.catalina.startup.HostConfig.deployDirectory 鎶妛eb 搴旂敤绋嬪簭閮ㄧ讲鍒扮洰褰� [C:\Program Files\Apache Software Foundation\Tomcat 10.0\webapps\manager]
12-Apr-2022 21:27:37.271 淇℃伅 [Catalina-utility-2] org.apache.catalina.startup.HostConfig.deployDirectory Web搴旂敤绋嬪簭鐩綍[C:\Program Files\Apache Software Foundation\Tomcat 10.0\webapps\manager]鐨勯儴缃插凡鍦╗56]姣鍐呭畬鎴�
xdclass.net
TestServlet appInfo=xdclass.net info

• JSP本身就是⼀种Servlet, JSP在第⼀次被访问的时候会被编译为HttpJspPage类,是HttpServlet的⼀个⼦类