小谈-—ServletConfig对象和servletContext对象

一、servletContext概述

　　servletContext对象是Servlet三大域对象之一，每个Web应用程序都拥有一个ServletContext对象，该对象是Web应用程序的全局对象或者上下文。Tomcat服务器在启动时，会自动创建一个ServletContext对象，在关闭时，会自动销毁这个ServletContext对象。每个Web应用程序只拥有一个ServletContext对象，ServletContext对象可以在整个Web应用中共享数据资源。

生命周期很长
每个web应用都有一个唯一的servletContext对象.
在每个应用加载的时候，服务器就会创建servletContext对象。
ServletContext对象是一个域对象(领域)
获得servletContext的方式

在自定义Servlet中有以下几种方式获取到ServletContext对象：

通过ServletConfig对象的this.getServletContext()方法获取。
采用request.getSession().getServletContext()方法获取。
通过继承GenericServlet类或HttpServlet类，调用GenericServlet类或HttpServlet类的getServletContext()方法获取。

 import java.io.IOException;

 import javax.servlet.ServletConfig;

 import javax.servlet.ServletContext;

 import javax.servlet.ServletException;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 //演示获取servletContext对象

 /**

  * 有三种方式获取servletContext对象

  *             1. 采用servletConfig对象获取

  *             2. 采用servlet实例对象获取

  *             3. 采用request对象获取

  * @author Administrator

  *

  */

 public class ServletContext1 extends HttpServlet {

     ServletContext sc ; 

     @Override

     public void init(ServletConfig config) throws ServletException {

         super.init(config) ;

         sc = config.getServletContext() ;//1.

     }

     public void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         //第二种方式

         ServletContext sc1 = this.getServletContext() ;

         System.out.println(sc);

         System.out.println(sc1 == sc);

         //第三种方式

         ServletContext sc2 = request.getSession().getServletContext() ;

         System.out.println(sc2 == sc);

     }

     public void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doGet(request, response);

     }

 }

应用

1) 实现数据共享

2) 获取全局配置参数

3) 请求转发

a) 拿取请求转发器，然后转发

4) 获取资源文件

5) 用来获得文件的MIME的类型.

案例

下面我将通过例子说明：登录成功后,5秒后跳转到某个页面,在页面中显示您是第x次登录成功

创建一个VisitServlet用来获取访问次数，并存储在ServletContext对象中。

 public class VisitServlet extends HttpServlet {

     @Override

     public void init() throws ServletException {

         ServletContext context = getServletContext();

         context.setAttribute("times", 0);

     }

     public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

         ServletContext context = getServletContext();

         int times = (Integer)context.getAttribute("times");

         times ++;

         context.setAttribute("times", times);

response.setHeader("Refresh", "5;url=/test/showTimeServlet");

     }

     public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

         doGet(request, response);

     }

 }

创建一个ShowTimeServlet用来显示访问次数。

 public class ShowTimeServlet extends HttpServlet {

     public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

         ServletContext context = getServletContext();

        //获取访问的次数

         int times = (Integer)context.getAttribute("times");

         response.setContentType("text/html;charset=utf-8");

         PrintWriter out = response.getWriter();

         out.println("<h1>VisitServlet共被访问了"+times+"次</h1>");

     }

     public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

         doGet(request, response);

     }

 }


获得Servlet的web.xml配置信息

 <context-param>

         <param-name>username</param-name>

         <param-value>苍老师</param-value>

 </context-param>

 <context-param>

       <param-name>password</param-name>

       <param-value>123</param-value>

 </context-param>

获取web.xml配置信息中

import java.io.IOException;

import java.util.Enumeration;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

/**

 * 获取web应用的初始化信息

 */

public class ServletContextDemo extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

            //获得ServletContext域对象

        ServletContext context = this.getServletContext();

            //获得单个数据（username）

              String name = context.getInitParameter("username") ;

//        System.out.println(name);

             //获得所有数据

        Enumeration enumeration = context.getInitParameterNames();

        while(enumeration.hasMoreElements()){

            String name = (String)                     enumeration.nextElement();

            String value = context.getInitParameter(name);

            System.out.println(name+":"+value);

        }

    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        doGet(request, response);

    }

}

打开浏览器，在地址栏输入http://localhost:8080/test/visit，访问VisitServlet。
再新打开浏览器，在地址栏输入http://localhost:8080/test/show，显示访问次数。

二、servletConfig概述

　　在javax.servlet包中，定义了ServletConfig接口。Servlet容器使用ServletConfig对象在Servlet初始化时向其传递配置信息。

所谓的Serlvet配置信息，就是在Web应用程序中web.xml文件中配置有关Servlet的内容。

<servlet>

    <description>This is the description of my J2EE component</description>

    <display-name>This is the display name of my J2EE component</display-name>

    <servlet-name>ServletConfigTest</servlet-name>

    <servlet-class>app.java.servlet.FirstServlet</servlet-class>

  </servlet>

  <servlet-mapping>

    <servlet-name>FirstServlet</servlet-name>

    <url-pattern>/servlet/ServletConfigTest</url-pattern>

  </servlet-mapping>

    <init-param>

    <param-name>name</param-name>

    <param-value>yl</param-value>

    </init-param>

    <init-param>

    <param-name>blog</param-name>

    <param-value>http://www.longestory.com</param-value>

    </init-param>

getServletName()方法：获取web.xml文件中配置的Servlet名称。

public void service(ServletRequest req, ServletResponse res)throws ServletException, IOException {

    ServletConfig servletConfig = this.getServletConfig();

    String servletName = servletConfig.getServletName();

    System.out.println(servletName);

}

运行Web应用程序，在控制台中打印“ConfigServletTest”。

getInitParameter(String name)方法：返回指定名称的初始化参数的值，如果参数不存在则返回null值。

public void service(ServletRequest req, ServletResponse res)

            throws ServletException, IOException {

    ServletConfig servletConfig = getServletConfig();

    String name = servletConfig.getInitParameter("name");

    String blog = servletConfig.getInitParameter("blog");

    System.out.println(name + "'s blog is " + blog);

}

运行Web应用程序，在控制台中打印“yl's blog is http://www.longestory.com”。

getInitParameterNames()方法：返回Servlet配置的所有初始化参数名称的枚举集合. Enumeration是Iterator的前身，用法与Iterator一致。

public void service(ServletRequest req, ServletResponse res)

            throws ServletException, IOException {

    ServletConfig servletConfig = getServletConfig();

    Enumeration enumeration = servletConfig.getInitParameterNames();

    while (enumeration.hasMoreElements()) {

        String element = (String) enumeration.nextElement();

        String value = servletConfig.getInitParameter(element);

        System.out.println(element + ": " + value);

    }

}

ServletConfig有哪些实际作用呢？在struts 1 框架中就运行了ServletConfig内容。

<!-- Standard Action Servlet Configuration (with debugging) -->

<servlet>

    <servlet-name>action</servlet-name>

    <servlet-class>org.apache.struts.action.ActionServlet</servlet-class>

    <init-param>

      <param-name>config</param-name>

      <param-value>

          /WEB-INF/struts-config.xml,

          /WEB-INF/struts-config-Wildcard.xml

       </param-value>

    </init-param>

    <load-on-startup>2</load-on-startup>

</servlet>

<!-- Standard Action Servlet Mapping -->

<servlet-mapping>

    <servlet-name>action</servlet-name>

    <url-pattern>*.do</url-pattern>

</servlet-mapping>

2017-05-12