RememberMeAuthenticationFilter过滤器对应的类路径为
org.springframework.security.web.authentication.rememberme.RememberMeAuthenticationFilter
看主要的doFilter方法

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        //判断SecurityContext实例中是否存在Authentication认证信息
         //没有认证的情况，才会做autoLogin的操作
        if (SecurityContextHolder.getContext().getAuthentication() == null) {
            //具体autoLogin过程后面分析，主要返回一个认证实体Authentication
            Authentication rememberMeAuth = rememberMeServices.autoLogin(request, response);

            if (rememberMeAuth != null) {
                // Attempt authenticaton via AuthenticationManager
                try {
                    //通过authenticationManager对该认证实体验证
                       //之前登录的Filter已经说明了，authenticationManager将
                       //将具体的认证工作交给provider完成
                       //而provider只负责自己关心的认证实体
                       //所以返回的认证实体RememberMeAuthenticationToken由
                       //RememberMeAuthenticationProvider完成认证的工作
                       //而这个provider也仅仅判断key是否相同，然后直接返回Authentication
                    rememberMeAuth = authenticationManager.authenticate(rememberMeAuth);

                    //设置认证信息到SecurityContext中
                    SecurityContextHolder.getContext().setAuthentication(rememberMeAuth);
                    //登录成功后的处理
                    onSuccessfulAuthentication(request, response, rememberMeAuth);
                    // Fire event
                    if (this.eventPublisher != null) {
                        eventPublisher.publishEvent(new InteractiveAuthenticationSuccessEvent(
                                SecurityContextHolder.getContext().getAuthentication(), this.getClass()));
                    }
                } catch (AuthenticationException authenticationException) {
                ……
                }
            }

            chain.doFilter(request, response);
        } else {
            ……
            chain.doFilter(request, response);
        }
    }

下面看具体的autoLogin执行过程。
autoLogin是由RememberMeServices接口定义的方法，那么在RememberMeAuthenticationFilter类中的RememberMeServices实例也是通过解析http标签时注入到Filter中的。具体解析过程代码就不分析了。
如果remember-me标签有data-source-ref或token-repository-ref属性，RememberMeServices接口实现类为
PersistentTokenBasedRememberMeServices
其他情况下，没有设置services-ref属性，RememberMeServices接口实现类为
TokenBasedRememberMeServices
这两个实现类都继承了AbstractRememberMeServices抽象类

而autoLogin是AbstractRememberMeServices类完成的，看源码

public final Authentication autoLogin(HttpServletRequest request, HttpServletResponse response) {
        //从cookie中获取remember me的相关信息
         //cookie名称是SPRING_SECURITY_REMEMBER_ME_COOKIE
        //cookie的value实际上是经过Base64做了两次编码算法处理的结果
         //对应的是数据库中persistent_logins表中的series、token的数据
         //数据库中保存的值是经过一次Base64编码的处理
        String rememberMeCookie = extractRememberMeCookie(request);

        if (rememberMeCookie == null) {
            return null;
        }

        logger.debug("Remember-me cookie detected");

        UserDetails user = null;

        try {
            //所以这里只要解码一次就可以了
              //这个数组中包含两个字符串，分别对应persistent_logins
            //表中的series、token的数据
            String[] cookieTokens = decodeCookie(rememberMeCookie);
            //执行autoLogin的处理
            user = processAutoLoginCookie(cookieTokens, request, response);
            //账号状态检查，主要检查是否被锁，是否可用等
            userDetailsChecker.check(user);

            logger.debug("Remember-me cookie accepted");
            //返回UserDetails的实现类RememberMeAuthenticationToken
            //注意这里构造的不是之前登录的那个UsernamePasswordAuthenticationToken了
            return createSuccessfulAuthentication(request, user);
        } catch (CookieTheftException cte) {
            cancelCookie(request, response);
            throw cte;
        } catch (UsernameNotFoundException noUser) {
            logger.debug("Remember-me login was valid but corresponding user not found.", noUser);
        } catch (InvalidCookieException invalidCookie) {
            logger.debug("Invalid remember-me cookie: " + invalidCookie.getMessage());
        } catch (AccountStatusException statusInvalid) {
            logger.debug("Invalid UserDetails: " + statusInvalid.getMessage());
        } catch (RememberMeAuthenticationException e) {
            logger.debug(e.getMessage());
        }

        cancelCookie(request, response);
        return null;
    }

继续跟踪processAutoLoginCookie方法，该方法由具体子类实现，下面的代码从
PersistentTokenBasedRememberMeServices中截取

protected UserDetails processAutoLoginCookie(String[] cookieTokens, HttpServletRequest request, HttpServletResponse response) {

        if (cookieTokens.length != 2) {
            throw new InvalidCookieException("Cookie token did not contain " + 2 +
                    " tokens, but contained '" + Arrays.asList(cookieTokens) + "'");
        }
        final String presentedSeries = cookieTokens[0];
        final String presentedToken = cookieTokens[1];
        //根据cookie中获取的Series数据从持久化载体（一般为数据库）中
         //获取该Series对应的登录信息，构造PersistentRememberMeToken实例
        PersistentRememberMeToken token = tokenRepository.getTokenForSeries(presentedSeries);
        //此处省略一部分验证……
         //重新产生一个token并由当前时间构造新的PersistentRememberMeToken实例
        PersistentRememberMeToken newToken = new PersistentRememberMeToken(token.getUsername(),
                token.getSeries(), generateTokenData(), new Date());

        try {
            //根据Series更新persistent_logins
            tokenRepository.updateToken(newToken.getSeries(), newToken.getTokenValue(), newToken.getDate());
            //重新设置cookie
            addCookie(newToken, request, response);
        } catch (DataAccessException e) {
            logger.error("Failed to update token: ", e);
            throw new RememberMeAuthenticationException("Autologin failed due to data access problem");
        }
        //直接根据username通过UserDetailsService实例的loadUserByUsername方法
         //获取UserDetails对象并返回
        UserDetails user = getUserDetailsService().loadUserByUsername(token.getUsername());

        return user;
    }

这里需要注意的是getUserDetailsService方法返回的userDetailsService变量从哪里注入进来的呢？通过解析remember标签构造bean时注入的么？不是的，进一步观察remember标签解析代码

public BeanDefinition parse(Element element, ParserContext pc) {
        //此处省略了获取标签属性、判断等代码
        boolean isPersistent = dataSourceSet | tokenRepoSet;
        //如果配置了data-source-ref或token-repository-ref属性
        if (isPersistent) {
            Object tokenRepo;
            services = new RootBeanDefinition(PersistentTokenBasedRememberMeServices.class);

            if (tokenRepoSet) {
                tokenRepo = new RuntimeBeanReference(tokenRepository);
            } else {
                //设置data-source-ref属性的情况
                tokenRepo = new RootBeanDefinition(JdbcTokenRepositoryImpl.class);
                ((BeanDefinition)tokenRepo).getPropertyValues().addPropertyValue("dataSource",
                        new RuntimeBeanReference(dataSource));
            }
            //tokenRepository主要处理存储的功能
            services.getPropertyValues().addPropertyValue("tokenRepository", tokenRepo);
        } else if (!servicesRefSet) {
            services = new RootBeanDefinition(TokenBasedRememberMeServices.class);
        }

        if (services != null) {
            //注意：这里仅仅判断是否配置了user-service-ref属性
              //如果配置了该属性，就会注入userDetailsService
            //实际上很少配置该标签
            if (userServiceSet) {
                services.getPropertyValues().addPropertyValue("userDetailsService", new RuntimeBeanReference(userServiceRef));
            }

            if ("true".equals(element.getAttribute(ATT_SECURE_COOKIE))) {
                services.getPropertyValues().addPropertyValue("useSecureCookie", true);
            }

            if (tokenValiditySet) {
                Integer tokenValidity = new Integer(tokenValiditySeconds);
                if (tokenValidity.intValue() < 0 && isPersistent) {
                    pc.getReaderContext().error(ATT_TOKEN_VALIDITY + " cannot be negative if using" +
                            " a persistent remember-me token repository", source);
                }
                services.getPropertyValues().addPropertyValue("tokenValiditySeconds", tokenValidity);
            }
            services.setSource(source);
            services.getPropertyValues().addPropertyValue("key", key);
            servicesName = pc.getReaderContext().generateBeanName(services);
            pc.registerBeanComponent(new BeanComponentDefinition(services, servicesName));
        } else {
            servicesName = rememberMeServicesRef;
        }

        if (StringUtils.hasText(element.getAttribute(ATT_SERVICES_ALIAS))) {
            pc.getRegistry().registerAlias(servicesName, element.getAttribute(ATT_SERVICES_ALIAS));
        }

        BeanDefinition filter = createFilter(pc, source);
        pc.popAndRegisterContainingComponent();

        return filter;
    }

上面的解析过程已经很明确了，确实没有在这里注入userDetailsService。如果细心的话，会发现在解析http标签时，有这么一段代码

authBldr.createUserServiceInjector();

没错，就是这个方法完成动态注入的了

void createUserServiceInjector() {
        BeanDefinitionBuilder userServiceInjector = BeanDefinitionBuilder.rootBeanDefinition(UserDetailsServiceInjectionBeanPostProcessor.class);
        userServiceInjector.addConstructorArgValue(x509ProviderId);
        userServiceInjector.addConstructorArgValue(rememberMeServicesId);
        userServiceInjector.addConstructorArgValue(openIDProviderId);
        userServiceInjector.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        pc.getReaderContext().registerWithGeneratedName(userServiceInjector.getBeanDefinition());
    }

UserDetailsServiceInjectionBeanPostProcessor实现了两个接口BeanPostProcessor, BeanFactoryAware，既可以动态改变bean，又能直接获取到BeanFactory对象再从ioc容器中获取具体的bean。
主要看postProcessBeforeInitialization方法

public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        if(beanName == null) {
            return bean;
        }
        if (beanName.equals(x509ProviderId)) {
            injectUserDetailsServiceIntoX509Provider((PreAuthenticatedAuthenticationProvider) bean);
        } else if (beanName.equals(rememberMeServicesId)) {
            injectUserDetailsServiceIntoRememberMeServices(bean);
        } else if (beanName.equals(openIDProviderId)) {
            injectUserDetailsServiceIntoOpenIDProvider(bean);
        }

        return bean;
    }

只对x509ProviderId、rememberMeServicesId、openIDProviderId的bean动态注入userDetailsService实例。