SpringSecurity 本质是一个过滤器链：

从启动是可以获取到过滤器链：

org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter
org.springframework.security.web.context.SecurityContextPersistenceFilter
org.springframework.security.web.header.HeaderWriterFilter
org.springframework.security.web.csrf.CsrfFilter
org.springframework.security.web.authentication.logout.LogoutFilter
org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter
org.springframework.security.web.authentication.ui.DefaultLoginPageGeneratingFilter
org.springframework.security.web.authentication.ui.DefaultLogoutPageGeneratingFilter
org.springframework.security.web.savedrequest.RequestCacheAwareFilter
org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter
org.springframework.security.web.authentication.AnonymousAuthenticationFilter
org.springframework.security.web.session.SessionManagementFilter
org.springframework.security.web.access.ExceptionTranslationFilter
org.springframework.security.web.access.intercept.FilterSecurityInterceptor

代码底层流程：重点看三个过滤器：
FilterSecurityInterceptor：是一个方法级的权限过滤器, 基本位于过滤链的最底部。

ExceptionTranslationFilter：是个异常过滤器，用来处理在认证过程中抛出的异常

UsernamePasswordAuthenticationFilter：对/login的POST请求做拦截，校验表单中用户名，密码。