Javaweb | ServletContext对象

概述

• 全局对象，拥有作用域，对应Tomcat的Web应用
• Web服务器启动时，会为每一个Web应用程序创建一块共享的存储区域（ServletContext）
• ServletContext在Web服务器启动时创建，服务器关闭时销毁

获取ServletContext对象

• GenericServlet提供 getServletContext(); 方法 this.getServletContext();
• HttpServletRequest提供 getServletContext(); 方法
• HttpSession提供 getServletContext(); 方法

//获取ServletContext对象
ServletContext servletContext = this.getServletContext();

//通过request对象，获取ServletContext对象
ServletContext servletContext1 = request.getServletContext();

//通过request获取session对象，获取ServletContext对象
HttpSession session = request.getSession();
ServletContext servletContext2 = session.getServletContext();

ServletContext 作用

• 获取项目真实路径（获取当前项目在服务器发布的真实路径）

String realpath = servletContext.getRealPath("/")；

        //获取ServletContext对象
        ServletContext servletContext = this.getServletContext();
        //获取项目真实路径
        System.out.println(servletContext.getRealPath("/"));

		/*
		输出结果
		D:\Eclipse\apache-tomcat-8.5.82-windows-x64\apache-tomcat-8.5.82\webapps\Servlet_Projects_war\
		*/

• 获取项目上下文路径（获取当前项目上下文路径：应用程序名称）

System.out.println(servletContext.getContextPath());
System.out.println(request.getContextPath());

//获取ServletContext对象
ServletContext servletContext = this.getServletContext();
//获取项目上下文路径
System.out.println(servletContext.getContextPath());
System.out.println(request.getContextPath());
/*
输出结果：
/Servlet_Projects_war
/Servlet_Projects_war
*/

• 全局容器

  ServletContext拥有作用域，可以存储数据到全局容器中

  • 存储数据： servletContext.setAttribute("name","value");
  • 获取数据： servletContext.getAttribute("name");
  • 移除数据： servletContext.removeAttribute("name");

• ServletContextController类

@WebServlet(name = "ServletContextController", value = "/ctxController")
public class ServletContextController extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext servletContext = this.getServletContext();

        //获取项目真实路径
        System.out.println(servletContext.getRealPath("/"));

        //存储数据
        servletContext.setAttribute("context","info");
    }
}

• ShowContextController类

@WebServlet(name = "ShowContextController", value = "/ShowController")
public class ShowContextController extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext servletContext = this.getServletContext();
        
        //获取数据
        String context = (String) servletContext.getAttribute("context");
        System.out.println(context);
        
         /*
        输出结果：
        D:\Eclipse\apache-tomcat-8.5.82-windows-x64\apache-tomcat-8.5.82\webapps\Servlet_Projects_war\
        info
        */
    }
}

ServletContext 特点

• 唯一性：一个应用对应一个ServletContext
• 生命周期：只要容器不关闭或者应用不卸载，ServletContext就一直存在

ServletContext 应用场景

• CounterController类

@WebServlet(name = "CounterController", value = "/counterController")
public class CounterController extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext servletContext = request.getServletContext();

        //获取计数器
        Integer counter = (Integer) servletContext.getAttribute("counter");

        //计数器初始化
        if (counter == null) {
            counter = 1;
            //counter存入计数器
            servletContext.setAttribute("counter", counter);
        } else {
            counter++;
            servletContext.setAttribute("counter", counter);
        }
        
        System.out.println("counter:" + counter);

    }
}

• ShowCounterController类

@WebServlet(name = "ShowCounterController", value = "/ShowCounterController")
public class ShowCounterController extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取ServletContext对象
        ServletContext servletContext = request.getServletContext();

        //获取计数器
        Integer counter = (Integer) servletContext.getAttribute("counter");
        
        //计数器初始化
        if (counter == null) {
            counter = 1;
            //counter存入计数器
            servletContext.setAttribute("counter", counter);
        } else {
            counter++;
            servletContext.setAttribute("counter", counter);
        }

        System.out.println("show:" + counter);
    }
}

• 结果

counter:1
show:2
show:3
show:4
show:5
show:6
show:7

作用域总结

• HttpServletRequest：一次请求，请求响应之前有效
• HttpSession：一次会话开始，浏览器不关闭或不超时之前有效
• ServletContext：服务器启动开始，服务器停止之前有效