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使用XML文件來實現(xiàn)對Servlet的配置
我們在Web應(yīng)用中可以使用xml來配置servlet,給其提供初始化參數(shù),如下例:
我們創(chuàng)建的Servlet為:ServletDemo.java,代碼如下:
/*
* Created on 2005-8-29
*
* TODO To change the template for this generated file go to
* Window - Preferences - Java - Code Style - Code Templates
*/
package zy.pro.wd.servlet;
import java.io.IOException;
import java.io.PrintWriter;
import javax.sql.DataSource;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
* @author zhangyi
*
* TODO To change the template for this generated type comment go to
* Window - Preferences - Java - Code Style - Code Templates
*/
public class ServletDemo extends HttpServlet {
String message;
DataSource ds;
/**
* Constructor of the object.
*/
public ServletDemo() {
super();
}
/**
* Destruction of the servlet. <br>
*/
public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}
/**
* The doGet method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to get.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out
.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");
out.println("<HTML>");
out.println(" <HEAD><TITLE>A Servlet</TITLE></HEAD>");
out.println(" <BODY>");
out.print(" This is ");
out.print(this.getClass());
out.println(", using the GET method<br>");
out.println(this.getServletConfig().getInitParameter("message"));
out.println(" </BODY>");
out.println("</HTML>");
out.flush();
out.close();
}
/**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void init() throws ServletException {
// Put your code here
}
}
在此Servlet中我們定義了兩個屬性message和ds。我們現(xiàn)在在web.xml中作如下配置：
<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>ServletDemo</servlet-name>
<servlet-class>zy.pro.wd.servlet.ServletDemo</servlet-class>
<init-param>
<description>initialize the field of message</description>
<param-name>message</param-name>
<param-value>
welcome here ,thank you for visiting !!!
</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>ServletDemo</servlet-name>
<url-pattern>/servlet/ServletDemo</url-pattern>
</servlet-mapping>
加粗的部分是我們要作的配置。在其中我們給message屬性設(shè)置了初始值：
welcome here ,thank you for visiting !!!
注意：此處我們不能同時給ds設(shè)置初始值，因為web.xml的DTD中約定了只能定義一個屬性也就是在配置文件中只允許聲明一個參數(shù)值對。這樣，在我們的servlet中就可以這樣來訪問此屬性：
this.getServletConfig().getInitParameter("message")。
但是，有時候我們需要同時對多個屬性用XML來初始化，那么我們就需要自己來寫XML文件，同時自己來解析了。
使用XML來配置Servlet的好處：
如果不在XML中對Servlet配置，那么我們修改Servlet的屬性的話就要重新啟動服務(wù)器，而如果使用XML來配置的話就不需要重新啟動服務(wù)器而可以自動生效。服務(wù)器可以自動監(jiān)視其改變而重新裝入文檔。對企業(yè)來說，系統(tǒng)的連續(xù)運營是很重要的。
XML來配置Servlet主要用在初始化參數(shù)在運行過程中需要改變的情況下。
（如果可以實現(xiàn)配置多屬性的話，那么不是有點象Spring中的動態(tài)注入？？？）
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