判断session是否存在的过滤器

在web.xml中有如下配置：

<filter>
		<filter-name>myFilter</filter-name>
		<filter-class>com.test.MyFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>myFilter</filter-name>
		<url-pattern>*.jsp</url-pattern>
	</filter-mapping>

凡是访问以jsp为后缀的页面都会经过过滤器com.test.MyFilter，代码如下：

public class MyFilterimplements Filter {
	
	protected FilterConfig filterConfig;

	public void doFilter(ServletRequest req, ServletResponse res,
			FilterChain chain) throws IOException, ServletException {
		HttpServletRequest hreq = (HttpServletRequest) req;
		HttpServletResponse hres = (HttpServletResponse) res;
		String requestUrl = hreq.getRequestURI();
		if (requestUrl.indexOf("index.jsp") > -1) {
			chain.doFilter(req, res);
		} else {
			if (null == hreq.getSession().getAttribute("session_user"))
				hres.sendRedirect(hreq.getContextPath()+ "/"+"index.jsp");
			else
				chain.doFilter(req, res);
		}
	}

	public void init(FilterConfig filterConfig) throws ServletException {
		this.filterConfig = filterConfig;
	}
	
	public void destroy() {
		this.filterConfig = null;
	}
	
	public void setFilterConfig(FilterConfig filterConfig) {
		this.filterConfig = filterConfig;
	}
}