用listener,filter,servlet在web应用启动时进行初始化

当web应用启动时,有三种方式能够进行初始化工做。

初始化顺序是 <context-param> - <listener> - <filter> - <servlet>.

 

1. listener

    (1) 实现javax.servlet.ServletContextListener接口.       

         

@Override 
public void contextInitialized(ServletContextEvent event) { 
}

    (2) web.xml  

        

<listener>
   <listener-class>org.blueleek.Listener</listener-class>
</listener>

        

    (3) depends on the order of your <listener> elements in the web.xml

 

2. filter

   (1)实现javax.servlet.Filter接口.  

    

@Override 
public void init(FilterConfig config) throws ServletException { 
}

   (2)web.xml   

<filter>
      <filter-name>ofilter</filter-name>
      <filter-class>order.OFilter</filter-class>
</filter>
<filter-mapping>
      <filter-name>ofilter</filter-name>
      <url-pattern>/*</url-pattern>
</filter-mapping>

   (3) The container uses the <filter-mapping> declarations to decide which filters to apply to a request,and in what order.              

3. servlet

    (1)继承javax.servlet.http.HttpServlet类.  

   

@Override 
public void init() throws ServletException { 
 super.init(); 
}

   (2)web.xml         

<servlet>
      <servlet-name>oservlet</servlet-name>
      <servlet-class>org.blueleek..OServlet</servlet-class>
      <load-on-startup></load-on-startup>
</servlet>

    (3) The load-on-startup element indicates that this servlet should be loaded (instantiated and have its init() called) on the startup of the web application. The optional contents of these element must be an integer indicating the order in which the servlet should be loaded. If the value is a negative integer, or the element is not present, the container is free to load the servlet whenever it chooses. If the value is a positive integer or 0, the container must load and initialize the servlet as the application is deployed. The container must guarantee that servlets marked with lower integers are loaded before servlets marked with higher integers. The container may choose the order of loading of servlets with the same load-on-start-up value.

 

结论来自: http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd 以及本身动手实验.