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。
1 import java.io.IOException;
2
3 import javax.servlet.ServletConfig;
4 import javax.servlet.ServletContext;
5 import javax.servlet.ServletException;
6 import javax.servlet.http.HttpServlet;
7 import javax.servlet.http.HttpServletRequest;
8 import javax.servlet.http.HttpServletResponse;
9 //演示获取servletContext对象
10 /**
11 * 有三种方式获取servletContext对象
12 * 1. 采用servletConfig对象获取
13 * 2. 采用servlet实例对象获取
14 * 3. 采用request对象获取
15 * @author Administrator
16 *
17 */
18 public class ServletContext1 extends HttpServlet {
19
20 ServletContext sc ;
21
22 @Override
23 public void init(ServletConfig config) throws ServletException {
24 super.init(config) ;
25 sc = config.getServletContext() ;//1.
26 }
27
28 public void doGet(HttpServletRequest request, HttpServletResponse response)
29 throws ServletException, IOException {
30 //第二种方式
31 ServletContext sc1 = this.getServletContext() ;
32 System.out.println(sc);
33 System.out.println(sc1 == sc);
34 //第三种方式
35 ServletContext sc2 = request.getSession().getServletContext() ;
36 System.out.println(sc2 == sc);
37 }
38
39 public void doPost(HttpServletRequest request, HttpServletResponse response)
40 throws ServletException, IOException {
41 doGet(request, response);
42 }
43 }
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。
1 public class VisitServlet extends HttpServlet {
2 @Override
3 public void init() throws ServletException {
4 ServletContext context = getServletContext();
5 context.setAttribute("times", 0);
6 }
7 public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
8 ServletContext context = getServletContext();
9
10 int times = (Integer)context.getAttribute("times");
11 times ++;
12 context.setAttribute("times", times);
response.setHeader("Refresh", "5;url=/test/showTimeServlet");
13 }
14 public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
15 doGet(request, response);
16 }
17 }
- 创建一个ShowTimeServlet用来显示访问次数。
1 public class ShowTimeServlet extends HttpServlet {
2 public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
3 ServletContext context = getServletContext();
4 //获取访问的次数
5 int times = (Integer)context.getAttribute("times");
6
7 response.setContentType("text/html;charset=utf-8");
8 PrintWriter out = response.getWriter();
9 out.println("<h1>VisitServlet共被访问了"+times+"次</h1>");
10 }
11 public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
12 doGet(request, response);
13 }
14 }
获得Servlet的web.xml配置信息
1 <context-param> 2 <param-name>username</param-name> 3 <param-value>苍老师</param-value> 4 </context-param> 5 <context-param> 6 <param-name>password</param-name> 7 <param-value>123</param-value> 8 </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
时间： 2024-10-09 23:16:16