filter过滤器

[快乐风男�]

public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
chain.doFilter(req, resp);
System.out.println("f2");
chain.doFilter(req,resp);
}

<filter>
<filter-name>Filter2</filter-name>
<filter-class>com.test.Filter2</filter-class>
</filter>
<filter-mapping>
<filter-name>Filter2</filter-name>
<url-pattern>/s1</url-pattern>
</filter-mapping>


@WebServlet(name = "Servlet1", urlPatterns = "/s1")
public class Servlet1 extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.getWriter().println("suc");
}

<form action="s1" method="post">
<div>
<input type="submit" value="确认">
</div>
请教下各位大佬，控制台输出4个f2，页面打印两个suc，是因为啥？按照我的理解，点击按钮，跳转/s1，对/s1资源请求过滤，输出一次f2，一次suc，不知道哪里出了问题？

[书断华]

过滤器，他过滤来也过滤回，一进一出就 两次就4个了