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。

1. 生命周期很长
2. 每个web应用都有一个唯一的servletContext对象.
3. 在每个应用加载的时候，服务器就会创建servletContext对象。
4. ServletContext对象是一个域对象(领域)
5. 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