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用到SPRING SECURITY3项目部署到tomcat下运行，登陆没有出问题，到第二天第一次登陆，出现空指针异常，返回后重新登陆，没问题；第三天第一次登陆问题又出现；发现只有没天第一次登陆会出现问题；错误如下
type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
java.lang.NullPointerException
org.pmp.dao.admin.BaseDAO.getInstance(BaseDAO.java:215)
org.pmp.dao.impl.admin.UserDAO.getUserByUsername(UserDAO.java:83)
org.pmp.security.MyUserDetailsService.loadUserByUsername(MyUserDetailsService.java:35)
org.springframework.security.authentication.dao.DaoAuthenticationProvider.retrieveUser
(DaoAuthenticationProvider.java:83)
org.springframework.security.authentication.dao.AbstractUserDetailsAuthenticationProvider.authenticate
(AbstractUserDetailsAuthenticationProvider.java:125)
org.springframework.security.authentication.ProviderManager.doAuthentication(ProviderManager.java:121)
org.springframework.security.authentication.AbstractAuthenticationManager.authenticate
(AbstractAuthenticationManager.java:49)
org.springframework.security.authentication.ProviderManager.doAuthentication(ProviderManager.java:139)
org.springframework.security.authentication.AbstractAuthenticationManager.authenticate
(AbstractAuthenticationManager.java:49)
org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter.attemptAuthentication
(UsernamePasswordAuthenticationFilter.java:98)
org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter
(AbstractAuthenticationProcessingFilter.java:200)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:356)
org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:106)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:356)
org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter
(SecurityContextPersistenceFilter.java:80)
org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:356)
org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:150)
org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:237)
org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:167)
org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
note The full stack trace of the root cause is available in the Apache Tomcat/7.0.27 logs.
