学步园

      filter的生命周期和servlet相似：init()->doFilter()->destroy()，主要用于对用户请求进行预处理，不能直接向用户生成响应。

一、下面看一个小例子:

      1. xml的配置，主要配置filter的classpath和过滤的路径：

<filter>  
     <filter-name>UserFilter</filter-name>  
     <filter-class>JspServletJavaBean.UserFilter</filter-class>  
</filter>  
<filter-mapping>  
     <filter-name>UserFilter</filter-name>  
     <url-pattern>/jspservletjavabean/*</url-pattern>  
</filter-mapping>  

      2. UserFilter代码：

import java.io.IOException;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class UserFilter implements Filter {

    @Override
    public void destroy() {
        // TODO Auto-generated method stub
        
    }

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse arg1,
            FilterChain arg2) throws IOException, ServletException {
        
        HttpServletRequest req=(HttpServletRequest)arg0;
        HttpServletResponse res=(HttpServletResponse)arg1;
        HttpSession session=req.getSession();
        if(session.getAttribute("username")==null){
            System.out.println("null~~");
            res.sendRedirect("../temp/login.jsp");
        }else{
            arg2.doFilter(arg0,arg1);
        }
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub
        
    }

}

这个filter的作用就是对/jspservletjavabean/*下的请求进行过滤，根据session判断用户是否登陆。

 

二、多个filter和servlet之间的执行顺序

      过滤链的好处是，执行过程中任何时候都可以打断，只要不执行chain.doFilter()就不会再执行后面的过滤器和请求的内容。而在实际使用时，就要特别注意过滤链的执行顺序问题。多个filter就组成了一个filter chain，filter过滤连的示意图如下:

 

关于访问的顺序：

filter1.java

public void doFilter(ServletRequest arg0, ServletResponse arg1,
FilterChain arg2) throws IOException, ServletException {
    // TODO Auto-generated method stub
    System.out.println("in filter 1.");
    arg2.doFilter(arg0, arg1);
    System.out.println("Getting out of filter 1");
}

filter2.java

public void doFilter(ServletRequest arg0, ServletResponse arg1,
FilterChain arg2) throws IOException, ServletException {
    // TODO Auto-generated method stub
    System.out.println("in filter 2.");
    arg2.doFilter(arg0, arg1);
    System.out.println("Getting out of filter 2");
}

servlet.java

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    System.out.println("hello the world....");
    response.setStatus(HttpServletResponse.SC_MOVED_PERMANENTLY);
    response.setHeader("Location", "../show.jsp");
    System.out.println("hello world is coming!!!!!!!!!");
}

xml的配置

<filter>
    <filter-name>Filter1</filter-name>
    <filter-class>Filter1</filter-class>
</filter>
<filter>
    <filter-name>Filter2</filter-name>
    <filter-class>Filter2</filter-class>
</filter>

<filter-mapping>
    <filter-name>Filter1</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>
<filter-mapping>
    <filter-name>Filter2</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

访问这个Servlet，会看到控制台输出如下信息：
in simple filter 1.
in simple filter 2.
hello the world....
hello world is coming!!!!!!!!!
Getting out of simple filter 2
Getting out of simple filter 1                            ---------@
in simple filter 1.
in simple filter 2.
Getting out of simple filter 2
Getting out of simple filter 1

分析：在@以上出现得说明了filter chain访问了Servlet，最后是Servlet输出内容，然后再回到filter chain的。

而@后面的就是Servlet转向的show.jsp页面。

总结：在doFilter()后的总在最后访问完才运行。而且对于Filter来说，顺序就像栈那样，FILO。