Web 应用基础 - ServletContext：获取与应用

Web 应用程序一旦部署到容器上之后，容器就要为Web应用实例化一个 ServletContext 对象， ServletContext 是一个全局的储存信息的空间，包含了整个Web应用程序的运行环境信息，服务器关闭时才销毁。

ServletContext：所有用户共用一个
Session: 一个用户一个
Request：一个请求一个

ServletContext 获取

Listener 中获取 ServletContext

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 public class StartupServletListener implements ServletContextListener {
        public void contextInitialized(ServletContextEvent servletContextEvent) {
            ServletContext servletContext = servletContextEvent.getServletContext();
        }
    }

Filter 中获取 ServletContext

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 public class SetCharacterEncodingFilter implements Filter {
        public void init(FilterConfig filterConfig) throws ServletException {
            ServletContext servletContext = filterConfig.getServletContext();
        }
    }

Servlet 中获取 ServletContext

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public class LoginServlet extends HttpServlet {
        public void doPost(HttpServletRequest request, HttpServletResponse response) throws
                IOException, ServletException {
            ServletContext servletContext = this.getServletContext();
        }
    }

HttpServletRequest 获取

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 ServletContext servletContext = request.getSession().getServletContext();

ServletContext 应用

读取 web.xml 参数

web.xml

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<context-param>
   <param-name>driverClass</param-name>
   <param-value>com.mysql.jdbc.Driver</param-value>
</context-param>

Listener启动时读取指定参数

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public class StartupServletListener implements ServletContextListener {

       public void contextInitialized(ServletContextEvent servletContextEvent) {
              System.out.println("初始化数据库数据参数");
              String driverClass = servletContextEvent.getServletContext().getInitParameter("driverClass");
               System.out.println(driverClass);
       }

       public void contextDestroyed(ServletContextEvent servletContextEvent) {
       }
}

Listener启动时读取所有参数

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public class StartupServletListener implements ServletContextListener {
       public void contextInitialized(ServletContextEvent servletContextEvent) {
             ServletContext servletContext = servletContextEvent.getServletContext();
             Enumeration<String> enumer = servletContext.getInitParameterNames();
             while (enumer.hasMoreElements()) {
                       String name = enumer.nextElement();
                       String value = servletContext.getInitParameter(name);
                       System.out.println(name + " = " + value);
            }
      }
     public void contextDestroyed(ServletContextEvent servletContextEvent) {
     }
}

读取资源文件

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