<context-param>标签，

比如

<context-param>
        <param-name>breed</param-name>
        <param-value>test</param-value>

</context-param>

应用在容器中启动之后，整个应用任何一个servlet都可以通过getContextConfig获取breed的值。

但是如果我们需要在项目启动的时候获取一个对象呢，而不仅仅是一个字符串呢？

我们可以通过写一个类实现ServletContextListener接口实现这一点。

package javax.servlet;import java.util.EventListener;public abstract interface ServletContextListener extends EventListener
{public abstract void contextInitialized(ServletContextEvent paramServletContextEvent);public abstract void contextDestroyed(ServletContextEvent paramServletContextEvent);
}

在web.xml中配置

<listener>
        <listener-class>
            servlet.ServletContextListenerTest
        </listener-class>
  </listener>

ServletContextListenerTest实现一个ServletContextListener的接口

package servlet;import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;public class ServletContextListenerTest implements ServletContextListener {@Overridepublic void contextDestroyed(ServletContextEvent event) {}@Overridepublic void contextInitialized(ServletContextEvent event) {System.out.println("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa");ServletContext sc = event.getServletContext();String dogBreed  = sc.getInitParameter("breed");System.out.println("aaaaaaaaaaaaaa"+dogBreed);Dog dog = new Dog(dogBreed);event.getServletContext().setAttribute("dog", dog);}}

上面的代码主要是实现了在应用启动的时候，将一个dog对象塞进ServletContext的attribute属性中，

这样应用的所有servlet就可以通过getAttribute共享这个dog对象了。

Dog类

package servlet;public class Dog {public String breed;public String getBreed() {return breed;}public void setBreed(String breed) {this.breed = breed;}public Dog(String breed) {this.breed=breed;}public void getDogBreed() {System.out.println("the breed of the dog is"+breed);}
}

写一个情景类

package servlet;import java.io.IOException;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(urlPatterns={"/servletContextListenerTestClient"})
public class ServletContextListenerTestClient extends HttpServlet{private static final long serialVersionUID = -5169172101637034615L;@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {doPost(req, resp);}@Overrideprotected void doPost(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {Dog dog = (Dog)this.getServletContext().getAttribute("dog");dog.getDogBreed();}}