在Servlet的配置文件中,可以使用一个或多个<init-param>标签为servlet配置一些初始化参数。当servlet配置了初始化参数后,web容器在创建servlet实例对象时,会自动将这些初始化参数封装到ServletConfig对象中,并在调用servlet的init方法时,ServletConfig对象传递给servlet。进而,程序员通过ServletConfig对象就可以得到当前servlet的初始化参数信息。
示例代码如下:
package com.yyz.servletconfig; import java.io.IOException; import java.util.Enumeration; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletConfigDemo1 extends HttpServlet { ServletConfig config; public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //获取指定的初始化参数 String value = config.getInitParameter("xxx"); response.getOutputStream().write(value.getBytes()); //获取所有的初始化参数 Enumeration e = cofig.getInitParameterNames(); while(e.hasMoreElements()){ String name = (String) e.nextElement(); value = config.getInitParameter(name); response.getOutputStream().write((name+"="+value+"<br/>").getBytes()); } } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request,response); } @Override public void init(ServletConfig config) throws ServletException { this.config = config; } }
相应的web.xml如下:
<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5"> <servlet> <servlet-name>ServletConfigDemo1</servlet-name> <servlet-class>com.yyz.servletconfig.ServletConfigDemo1</servlet-class> <init-param> <param-name>xxx</param-name> <param-value>yyy</param-value> </init-param> <init-param> <param-name>name</param-name> <param-value>yyz</param-value> </init-param> <init-param> <param-name>password</param-name> <param-value>yyy</param-value> </init-param> </servlet> <servlet-mapping> <servlet-name>ServletConfigDemo1</servlet-name> <url-pattern>/servlet/ServletConfigDemo1</url-pattern> </servlet-mapping> </web-app>
测试结果如下:
在上面的代码中,ServletConfigDemo1对象中有一个ServletConfig对象,其实这是不必要的。因为ServletConfigDemo1继承了HttpServlet,HttpServlet又继承了GenericServlet 。GenericServlet 已经在内部维护了一个ServletConfig对象。相关实现如下:
public abstract class GenericServlet implements Servlet, ServletConfig, java.io.Serializable { … … private transient ServletConfig config; public ServletConfig getServletConfig() { return config; } }
因而我们可以通过我们写的Servlet对象的getServletConfig()方法直接拿到ServletConfig对象,示例代码如下:
package com.yyz.servletconfig; import java.io.IOException; import java.io.PrintWriter; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ServletConfigDemo2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String value = this.getServletConfig().getInitParameter("name"); System.out.println(value); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { doGet(request, response); } }
web.xml文件:
<?xml version="1.0" encoding="UTF-8"?><web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5"> <servlet> <servlet-name>ServletConfigDemo2</servlet-name> <servlet-class>com.yyz.servletconfig.ServletConfigDemo2</servlet-class> <init-param> <param-name>name</param-name> <param-value>yyz</param-value> </init-param> </servlet> <servlet-mapping> <servlet-name>ServletConfigDemo2</servlet-name> <url-pattern>/servlet/ServletConfigDemo2</url-pattern> </servlet-mapping> </web-app>