过滤器判断session过期跳转问题

判断session过期就跳转,但是执行了跳转那一步,但是不跳,直接空指针异常...public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
HttpServletRequest req = (HttpServletRequest) request;
HttpServletResponse res = (HttpServletResponse) response;

String str = req.getServletPath();
int i = str.lastIndexOf(".");
if(i!=-1){
String str3 = str.substring(i+1);
if("jsp".equals(str3)){
if(str.length()>7){
String str2 = str.substring(1, 7);
if(str.length()>12){
String str1 = str.substring(8,12);
if("CRM_os".equals(str2)&&!"news".equals(str1)){
//req.getSession().invalidate();
Object obj = req.getSession(true).getAttribute("users");
if(null==obj){
System.out.println("aaa");
res.sendRedirect("/login/kf_login.jsp");
}
}
}
}
}
}
chain.doFilter(request, response); }11:04:31,265 INFO  [STDOUT] doThreadWorkWed Nov 03 11:04:31 CST 2010
11:11:18,250 INFO  [STDOUT] aaa
11:11:18,265 ERROR [[jsp]] Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at org.apache.jsp.CRM_005fos.question.OnlineQuestions_jsp._jspService(OnlineQuestions_jsp.java:102)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:373)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:336)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at csp.util.LoginFilter.doFilter(LoginFilter.java:49)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at emsp.encode.filter.EncodeFilter.doFilter(EncodeFilter.java:48)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:230)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:182)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:84)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.jboss.web.tomcat.service.jca.CachedConnectionValve.invoke(CachedConnectionValve.java:157)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:262)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:446)
at java.lang.Thread.run(Thread.java:619)