源码剖析 Spring Security 的实现原理

Spring Security 是一个轻量级的安全框架，可以和 Spring 项目很好地集成，提供了丰富的身份认证和授权相关的功能，而且还能防止一些常见的网络攻击。我在工作中有很多项目都使用了 Spring Security 框架，但基本上都是浅尝辄止，按照说明文档配置好就完事了，一直没有时间深入地研究过。最近在 Reflectoring 上看到了一篇文章 Getting started with Spring Security and Spring Boot，写得非常全面仔细，感觉是一篇不错的 Spring Security 入门文章，于是花了一点时间拜读了一番，结合着 官方文档 和 源码 系统地学习一下 Spring Security 的实现原理。

入门示例

我们先从一个简单的例子开始，这里我直接使用了 使用 Spring 项目脚手架 中的 Hello World 示例。为了让这个示例程序开启 Spring Security 功能，我们在 pom.xml 文件中引入 spring-boot-starter-security 依赖即可：

<dependency>``  ``<groupId>org.springframework.boot</groupId>``  ``<artifactId>spring-boot-starter-security</artifactId>``</dependency>

启动程序，会在控制台日志中看到类似下面这样的信息：

2023-05-15 06:52:52.418 INFO 8596 --- [      main] .s.s.UserDetailsServiceAutoConfiguration : ` `Using generated security password: eeb386a9-e16a-4b9b-bbc6-c054c8d263b0

这个是由 Spring Security 随机生成的密码。访问 /hello 页面，可以看到出现了一个登录页面：

输入用户名（默认为 user）和密码（控制台日志）登录成功后我们才能正常访问页面。默认的用户名和密码可以使用下面的配置进行修改：

spring.security.user.name=admin``spring.security.user.password=123456

为了后续更好地对 Spring Security 进行配置，理解 Spring Security 的实现原理，我们需要进一步学习 Spring Security 的三大核心组件：

• 过滤器（Servlet Filters）
• 认证（Authentication）
• 授权（Authorization）

Servlet Filters：Spring Security 的基础

我们知道，在 Spring MVC 框架中，DispatcherServlet 负责对用户的 Web 请求进行分发和处理，在请求到达 DispatcherServlet 之前，会经过一系列的 Servlet Filters，这被称之为过滤器，主要作用是拦截请求并对请求做一些前置或后置处理。这些过滤器串在一起，形成一个过滤器链（FilterChain）：

我们可以在配置文件中加上下面的日志配置：

logging.level.org.springframework.boot.web.servlet.ServletContextInitializerBeans=TRACE

然后重新启动服务，会在控制台输出类似下面这样的日志（为了方便查看，我做了一点格式化）：

2023-05-18 07:08:14.805 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Added existing Filter initializer bean 'webMvcMetricsFilter'; order=-2147483647, ``  ``resource=class path resource [org/springframework/boot/actuate/autoconfigure/metrics/web/servlet/WebMvcMetricsAutoConfiguration.class]``2023-05-18 07:08:14.806 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Added existing Filter initializer bean 'securityFilterChainRegistration'; order=-100, ``  ``resource=class path resource [org/springframework/boot/autoconfigure/security/servlet/SecurityFilterAutoConfiguration.class]``2023-05-18 07:08:14.808 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Added existing Servlet initializer bean 'dispatcherServletRegistration'; order=2147483647, ``  ``resource=class path resource [org/springframework/boot/autoconfigure/web/servlet/DispatcherServletAutoConfiguration$DispatcherServletRegistrationConfiguration.class]``2023-05-18 07:08:14.810 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Added existing Filter initializer bean 'errorPageSecurityFilter'; order=2147483647, ``  ``resource=class path resource [org/springframework/boot/autoconfigure/security/servlet/SpringBootWebSecurityConfiguration$ErrorPageSecurityFilterConfiguration.class]``2023-05-18 07:08:14.813 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Added existing ServletContextInitializer initializer bean 'servletEndpointRegistrar'; order=2147483647, ``  ``resource=class path resource [org/springframework/boot/actuate/autoconfigure/endpoint/web/ServletEndpointManagementContextConfiguration$WebMvcServletEndpointManagementContextConfiguration.class]``2023-05-18 07:08:14.828 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Created Filter initializer for bean 'characterEncodingFilter'; order=-2147483648, ``  ``resource=class path resource [org/springframework/boot/autoconfigure/web/servlet/HttpEncodingAutoConfiguration.class]  ``2023-05-18 07:08:14.831 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Created Filter initializer for bean 'formContentFilter'; order=-9900, ``  ``resource=class path resource [org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.class]``2023-05-18 07:08:14.834 TRACE 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Created Filter initializer for bean 'requestContextFilter'; order=-105, ``  ``resource=class path resource [org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter.class]``2023-05-18 07:08:14.842 DEBUG 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Mapping filters: ``    ``filterRegistrationBean urls=[/*] order=-2147483647, ``    ``springSecurityFilterChain urls=[/*] order=-100, ``    ``filterRegistrationBean urls=[/*] order=2147483647, ``    ``characterEncodingFilter urls=[/*] order=-2147483648, ``    ``formContentFilter urls=[/*] order=-9900, ``    ``requestContextFilter urls=[/*] order=-105``2023-05-18 07:08:14.844 DEBUG 10020 --- [      main] o.s.b.w.s.ServletContextInitializerBeans : ``  ``Mapping servlets: dispatcherServlet urls=[/]

这里显示了应用开启的所有 Filter 以及对应的自动配置类，可以看到 Spring Security 自动注入了两个 FilterRegistrationBean：

• 来自配置类 SecurityFilterAutoConfiguration 的 securityFilterChainRegistration
• 来自配置类 ErrorPageSecurityFilterConfiguration 的 errorPageSecurityFilter

DelegatingFilterProxy：Servlet Filter 与 Spring Bean 的桥梁

注意上面显示的并非 Filter 的名字，而是 FilterRegistrationBean 的名字，这是一种 RegistrationBean，它实现了 ServletContextInitializer 接口，用于在程序启动时，将 Filter 或 Servlet 注入到 ServletContext 中：

public abstract class RegistrationBean implements ServletContextInitializer, Ordered {` `  ``@Override``  ``public final void onStartup(ServletContext servletContext) throws ServletException {``    ``...``    ``register(description, servletContext);``  ``}` `}

其中 securityFilterChainRegistration 的定义如下：

@Bean``@ConditionalOnBean(name = DEFAULT_FILTER_NAME)``public DelegatingFilterProxyRegistrationBean securityFilterChainRegistration(``    ``SecurityProperties securityProperties) {``  ``DelegatingFilterProxyRegistrationBean registration = new DelegatingFilterProxyRegistrationBean(``      ``DEFAULT_FILTER_NAME);``  ``registration.setOrder(securityProperties.getFilter().getOrder());``  ``registration.setDispatcherTypes(getDispatcherTypes(securityProperties));``  ``return registration;``}

这个 RegistrationBean 的类型为 DelegatingFilterProxyRegistrationBean，由它注入的 Filter 叫 DelegatingFilterProxy：

public class DelegatingFilterProxyRegistrationBean extends AbstractFilterRegistrationBean<DelegatingFilterProxy> {``  ``...``}

这是一个非常重要的 Servlet Filter，它充当着 Servlet 容器和 Spring 上下文之间的桥梁，由于 Servlet 容器有着它自己的标准，在注入 Filter 时并不知道 Spring Bean 的存在，所以我们可以通过 DelegatingFilterProxy 来实现 Bean Filter 的延迟加载：

看一下 DelegatingFilterProxy 的实现：

public class DelegatingFilterProxy extends GenericFilterBean {` `  ``@Override``  ``public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)``      ``throws ServletException, IOException {` `    ``// Lazily initialize the delegate if necessary.``    ``Filter delegateToUse = this.delegate;``    ``if (delegateToUse == null) {``      ``synchronized (this.delegateMonitor) {``        ``delegateToUse = this.delegate;``        ``if (delegateToUse == null) {``          ``WebApplicationContext wac = findWebApplicationContext();``          ``if (wac == null) {``            ``throw new IllegalStateException("No WebApplicationContext found: " +``                ``"no ContextLoaderListener or DispatcherServlet registered?");``          ``}``          ``delegateToUse = initDelegate(wac);``        ``}``        ``this.delegate = delegateToUse;``      ``}``    ``}` `    ``// Let the delegate perform the actual doFilter operation.``    ``invokeDelegate(delegateToUse, request, response, filterChain);``  ``}``}

这段代码很容易理解，首先判断代理的 Bean Filter 是否存在，如果不存在则根据 findWebApplicationContext() 找到 Web 应用上下文，然后从上下文中获取 Bean Filter 并初始化，最后再调用该 Bean Filter。

FilterChainProxy：Spring Security 的统一入口

那么接下来的问题是，这个 DelegatingFilterProxy 代理的 Bean Filter 是什么呢？我们从上面定义 DelegatingFilterProxyRegistrationBean 的地方可以看出，代理的 Bean Filter 叫做 DEFAULT_FILTER_NAME，查看它的定义就知道，实际上就是 springSecurityFilterChain：

public static final String DEFAULT_FILTER_NAME = "springSecurityFilterChain";

那么这个 springSecurityFilterChain 是在哪定义的呢？我们可以在 WebSecurityConfiguration 配置类中找到答案：

public class WebSecurityConfiguration {` `  ``@Bean(name = AbstractSecurityWebApplicationInitializer.DEFAULT_FILTER_NAME)``  ``public Filter springSecurityFilterChain() throws Exception {``    ``boolean hasConfigurers = this.webSecurityConfigurers != null && !this.webSecurityConfigurers.isEmpty();``    ``boolean hasFilterChain = !this.securityFilterChains.isEmpty();``    ``if (!hasConfigurers && !hasFilterChain) {``      ``WebSecurityConfigurerAdapter adapter = this.objectObjectPostProcessor``          ``.postProcess(new WebSecurityConfigurerAdapter() {``          ``});``      ``this.webSecurity.apply(adapter);``    ``}``    ``for (SecurityFilterChain securityFilterChain : this.securityFilterChains) {``      ``this.webSecurity.addSecurityFilterChainBuilder(() -> securityFilterChain);``      ``for (Filter filter : securityFilterChain.getFilters()) {``        ``if (filter instanceof FilterSecurityInterceptor) {``          ``this.webSecurity.securityInterceptor((FilterSecurityInterceptor) filter);``          ``break;``        ``}``      ``}``    ``}``    ``for (WebSecurityCustomizer customizer : this.webSecurityCustomizers) {``      ``customizer.customize(this.webSecurity);``    ``}``    ``return this.webSecurity.build();``  ``}``}

很显然，springSecurityFilterChain 经过一系列的安全配置，最后通过 this.webSecurity.build() 构建出来的，进一步深入到 webSecurity 的源码我们就可以发现它的类型是 FilterChainProxy：

public final class WebSecurity extends AbstractConfiguredSecurityBuilder<Filter, WebSecurity>``    ``implements SecurityBuilder<Filter>, ApplicationContextAware, ServletContextAware {` `  ``@Override``  ``protected Filter performBuild() throws Exception {` `    ``int chainSize = this.ignoredRequests.size() + this.securityFilterChainBuilders.size();``    ``List<SecurityFilterChain> securityFilterChains = new ArrayList<>(chainSize);``    ``List<RequestMatcherEntry<List<WebInvocationPrivilegeEvaluator>>> requestMatcherPrivilegeEvaluatorsEntries = new ArrayList<>();``    ``for (RequestMatcher ignoredRequest : this.ignoredRequests) {``      ``SecurityFilterChain securityFilterChain = new DefaultSecurityFilterChain(ignoredRequest);``      ``securityFilterChains.add(securityFilterChain);``      ``requestMatcherPrivilegeEvaluatorsEntries``          ``.add(getRequestMatcherPrivilegeEvaluatorsEntry(securityFilterChain));``    ``}``    ``for (SecurityBuilder<? extends SecurityFilterChain> securityFilterChainBuilder : this.securityFilterChainBuilders) {``      ``SecurityFilterChain securityFilterChain = securityFilterChainBuilder.build();``      ``securityFilterChains.add(securityFilterChain);``      ``requestMatcherPrivilegeEvaluatorsEntries``          ``.add(getRequestMatcherPrivilegeEvaluatorsEntry(securityFilterChain));``    ``}` `    ``FilterChainProxy filterChainProxy = new FilterChainProxy(securityFilterChains);``    ``if (this.httpFirewall != null) {``      ``filterChainProxy.setFirewall(this.httpFirewall);``    ``}``    ``if (this.requestRejectedHandler != null) {``      ``filterChainProxy.setRequestRejectedHandler(this.requestRejectedHandler);``    ``}``    ``filterChainProxy.afterPropertiesSet();` `    ``Filter result = filterChainProxy;` `    ``this.postBuildAction.run();``    ``return result;``  ``}``}

从 FilterChainProxy 的名字可以看出来，它也是一个代理类，它代理的类叫做 SecurityFilterChain，它包含了多个 Security Filters 形成一个过滤器链，这和 Servlet Filters 有点类似，只不过这些 Security Filters 都是普通的 Spring Bean：

使用 FilterChainProxy 来代理 Security Filters 相比于直接使用 Servlet Filters 或使用 DelegatingFilterProxy 来代理有几个明显的好处：

1. FilterChainProxy 作为 Spring Security 对 Servlet 的支持入口，方便理解和调试；
2. FilterChainProxy 可以对 Spring Security 做一些集中处理，比如统一清除 SecurityContext 防止内存泄漏，以及统一使用 HttpFirewall 对应用进行保护等；
3. 支持多个 SecurityFilterChain，传统的 Servlet Filters 只能通过 URL 来匹配，使用 FilterChainProxy 可以配合 RequestMatcher 更灵活地控制调用哪个 SecurityFilterChain；

构建 SecurityFilterChain

上面讲到，FilterChainProxy 是通过 webSecurity 构建的，一个 FilterChainProxy 里包含一个或多个 SecurityFilterChain，那么 SecurityFilterChain 是由谁构建的呢？答案是 httpSecurity。我们可以在 SecurityFilterChainConfiguration 配置类中看到 SecurityFilterChain 的构建过程：

@Configuration(proxyBeanMethods = false)``@ConditionalOnDefaultWebSecurity``static class SecurityFilterChainConfiguration {` `  ``@Bean``  ``@Order(SecurityProperties.BASIC_AUTH_ORDER)``  ``SecurityFilterChain defaultSecurityFilterChain(HttpSecurity http) throws Exception {``    ``http.authorizeRequests().anyRequest().authenticated();``    ``http.formLogin();``    ``http.httpBasic();``    ``return http.build();``  ``}``}

深入到 http.build() 的源码，可以看到过滤器链的默认实现为 DefaultSecurityFilterChain：

public final class HttpSecurity extends AbstractConfiguredSecurityBuilder<DefaultSecurityFilterChain, HttpSecurity>``    ``implements SecurityBuilder<DefaultSecurityFilterChain>, HttpSecurityBuilder<HttpSecurity> {` `  ``@SuppressWarnings("unchecked")``  ``@Override``  ``protected DefaultSecurityFilterChain performBuild() {` `    ``this.filters.sort(OrderComparator.INSTANCE);``    ``List<Filter> sortedFilters = new ArrayList<>(this.filters.size());``    ``for (Filter filter : this.filters) {``      ``sortedFilters.add(((OrderedFilter) filter).filter);``    ``}``    ``return new DefaultSecurityFilterChain(this.requestMatcher, sortedFilters);``  ``}``}

构建 Security Filters

通过上面的梳理，我们大概清楚了 SecurityFilterChain 的构建过程，接下来，我们继续看 Security Filters 的构建过程。我们知道，一个SecurityFilterChain 中包含了多个 Security Filters，那么这些 Security Filters 是从哪里来的呢？

在 HttpSecurity 的代码里可以找到这么几个方法：

• public HttpSecurity addFilter(Filter filter)
• public HttpSecurity addFilterBefore(Filter filter, Class<? extends Filter> beforeFilter)
• public HttpSecurity addFilterAfter(Filter filter, Class<? extends Filter> afterFilter)
• public HttpSecurity addFilterAt(Filter filter, Class<? extends Filter> atFilter)

我们不妨在 addFilter 方法内下个断点，然后以调试模式启动程序，每次触发断点时，我们将对应的 Filter 记录下来，并通过堆栈找到该 Filter 是从何处添加的：

序号	Filter	来源
1	WebAsyncManagerIntegrationFilter	HttpSecurityConfiguration.httpSecurity()
2	CsrfFilter	CsrfConfigurer.configure()
3	ExceptionTranslationFilter	ExceptionHandlingConfigurer.configure()
4	HeaderWriterFilter	HeadersConfigurer.configure()
5	SessionManagementFilter	SessionManagementConfigurer.configure()
6	DisableEncodeUrlFilter	SessionManagementConfigurer.configure()
7	SecurityContextPersistenceFilter	SecurityContextConfigurer.configure()
8	RequestCacheAwareFilter	RequestCacheConfigurer.configure()
9	AnonymousAuthenticationFilter	AnonymousConfigurer.configure()
10	SecurityContextHolderAwareRequestFilter	ServletApiConfigurer.configure()
11	DefaultLoginPageGeneratingFilter	DefaultLoginPageConfigurer.configure()
12	DefaultLogoutPageGeneratingFilter	DefaultLoginPageConfigurer.configure()
13	LogoutFilter	LogoutConfigurer.configure()
14	FilterSecurityInterceptor	AbstractInterceptUrlConfigurer.configure()
15	UsernamePasswordAuthenticationFilter	AbstractAuthenticationFilterConfigurer.configure()
16	BasicAuthenticationFilter	HttpBasicConfigurer.configure()

除了第一个 WebAsyncManagerIntegrationFilter 是在创建 HttpSecurity 的时候直接添加的，其他的 Filter 都是通过 XXXConfigurer 这样的配置器添加的。我们继续深挖下去可以发现，生成这些配置器的地方有两个，第一个地方是在 HttpSecurityConfiguration 配置类中创建 HttpSecurity 时，如下所示：

class HttpSecurityConfiguration {` `  ``@Bean(HTTPSECURITY_BEAN_NAME)``  ``@Scope("prototype")``  ``HttpSecurity httpSecurity() throws Exception {``    ``WebSecurityConfigurerAdapter.LazyPasswordEncoder passwordEncoder = new WebSecurityConfigurerAdapter.LazyPasswordEncoder(``        ``this.context);``    ``AuthenticationManagerBuilder authenticationBuilder = new WebSecurityConfigurerAdapter.DefaultPasswordEncoderAuthenticationManagerBuilder(``        ``this.objectPostProcessor, passwordEncoder);``    ``authenticationBuilder.parentAuthenticationManager(authenticationManager());``    ``authenticationBuilder.authenticationEventPublisher(getAuthenticationEventPublisher());``    ``HttpSecurity http = new HttpSecurity(this.objectPostProcessor, authenticationBuilder, createSharedObjects());``    ``// @formatter:off``    ``http``      ``.csrf(withDefaults())``      ``.addFilter(new WebAsyncManagerIntegrationFilter())``      ``.exceptionHandling(withDefaults())``      ``.headers(withDefaults())``      ``.sessionManagement(withDefaults())``      ``.securityContext(withDefaults())``      ``.requestCache(withDefaults())``      ``.anonymous(withDefaults())``      ``.servletApi(withDefaults())``      ``.apply(new DefaultLoginPageConfigurer<>());``    ``http.logout(withDefaults());``    ``// @formatter:on``    ``applyDefaultConfigurers(http);``    ``return http;``  ``}``}

另外一个地方则是在上面的 SecurityFilterChainConfiguration 配置类中使用 http.build() 构建 SecurityFilterChain 之前（参见上面 defaultSecurityFilterChain 的代码），至此，我们大概理清了所有的 Security Filters 是如何创建的，下面再以表格的形式重新整理下：

序号	Filter	httpSecurity 配置
1	WebAsyncManagerIntegrationFilter	http.addFilter(new WebAsyncManagerIntegrationFilter())
2	CsrfFilter	http.csrf(withDefaults())
3	ExceptionTranslationFilter	http.exceptionHandling(withDefaults())
4	HeaderWriterFilter	http.headers(withDefaults())
5	SessionManagementFilter	http.sessionManagement(withDefaults())
6	DisableEncodeUrlFilter	http.sessionManagement(withDefaults())
7	SecurityContextPersistenceFilter	http.securityContext(withDefaults())
8	RequestCacheAwareFilter	http.requestCache(withDefaults())
9	AnonymousAuthenticationFilter	http.anonymous(withDefaults())
10	SecurityContextHolderAwareRequestFilter	http.servletApi(withDefaults())
11	DefaultLoginPageGeneratingFilter	http.apply(new DefaultLoginPageConfigurer<>())
12	DefaultLogoutPageGeneratingFilter	http.apply(new DefaultLoginPageConfigurer<>())
13	LogoutFilter	http.logout(withDefaults())
14	FilterSecurityInterceptor	http.authorizeRequests().anyRequest().authenticated()
15	UsernamePasswordAuthenticationFilter	http.formLogin()
16	BasicAuthenticationFilter	http.httpBasic()

其实，如果仔细观察我们的程序输出的日志，也可以看到 Spring Security 默认的过滤器链为 DefaultSecurityFilterChain，以及它注入的所有 Security Filters：

2023-05-17 08:16:18.173 INFO 3936 --- [      main] o.s.s.web.DefaultSecurityFilterChain   : Will secure any request with [``    ``org.springframework.security.web.session.DisableEncodeUrlFilter@1d6751e3, ``    ``org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter@2d258eff, ``    ``org.springframework.security.web.context.SecurityContextPersistenceFilter@202898d7, ``    ``org.springframework.security.web.header.HeaderWriterFilter@2c26ba07, ``    ``org.springframework.security.web.csrf.CsrfFilter@52d3fafd, ``    ``org.springframework.security.web.authentication.logout.LogoutFilter@235c997d, ``    ``org.springframework.security.web.authentication.UsernamePasswordAuthenticationFilter@5d5c41e5, ``    ``org.springframework.security.web.authentication.ui.DefaultLoginPageGeneratingFilter@50b93353, ``    ``org.springframework.security.web.authentication.ui.DefaultLogoutPageGeneratingFilter@6dca31eb, ``    ``org.springframework.security.web.authentication.www.BasicAuthenticationFilter@22825e1e, ``    ``org.springframework.security.web.savedrequest.RequestCacheAwareFilter@2c719bd4, ``    ``org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter@53aa38be, ``    ``org.springframework.security.web.authentication.AnonymousAuthenticationFilter@4a058df8, ``    ``org.springframework.security.web.session.SessionManagementFilter@42ea7565, ``    ``org.springframework.security.web.access.ExceptionTranslationFilter@77cb452c, ``    ``org.springframework.security.web.access.intercept.FilterSecurityInterceptor@8054fe2]

在某些低版本中，可能会显示 DefaultSecurityFilterChain: Will not secure any request 这样的日志，这可能是 Spring Security 的 BUG，升级到最新版本即可。

其中有几个 Security Filters 比较重要，是实现认证和授权的基础：

• CsrfFilter：默认开启对所有接口的 CSRF 防护，关于 CSRF 的详细信息，可以参考 Configuring CSRF/XSRF with Spring Security；
• DefaultLoginPageGeneratingFilter：用于生成 /login 登录页面；
• DefaultLogoutPageGeneratingFilter：用于生成 /login?logout 登出页面；
• LogoutFilter：当用户退出应用时被调用，它通过注册的 LogoutHandler 删除会话并清理 SecurityContext，然后通过 LogoutSuccessHandler 将页面重定向到 /login?logout；
• UsernamePasswordAuthenticationFilter：实现基于用户名和密码的安全认证，当认证失败，抛出 AuthenticationException 异常；
• BasicAuthenticationFilter：实现 Basic 安全认证，当认证失败，抛出 AuthenticationException 异常；
• AnonymousAuthenticationFilter：如果 SecurityContext 中没有 Authentication 对象时，它自动创建一个匿名用户 anonymousUser，角色为 ROLE_ANONYMOUS；
• FilterSecurityInterceptor：这是 Spring Security 的最后一个 Security Filters，它从 SecurityContext 中获取 Authentication 对象，然后对请求的资源做权限判断，当授权失败，抛出 AccessDeniedException 异常；
• ExceptionTranslationFilter：用于处理过滤器链中抛出的 AuthenticationException 和 AccessDeniedException 异常，AuthenticationException 异常由 AuthenticationEntryPoint 来处理，AccessDeniedException 异常由 AccessDeniedHandler 来处理；

认证和授权

有了 Security Filters，我们就可以实现各种 Spring Security 的相关功能了。应用程序的安全性归根结底包括了两个主要问题：认证（Authentication） 和 授权（Authorization） 。认证解决的是 你是谁？ 的问题，而授权负责解决 你被允许做什么？ ，授权也被称为 访问控制（Access Control）。这一节将深入学习 Spring Security 是如何实现认证和授权的。

跳转到 /login 页面

让我们回到第一节的例子，当访问 /hello 时，可以看到浏览器自动跳转到了 /login 登录页面，那么 Spring Security 是如何实现的呢？为了一探究竟，我们可以将 Spring Security 的日志级别调到 TRACE：

logging.level.org.springframework.security=TRACE

这样我们就能完整地看到这个请求经过 Security Filters 的处理过程：

2023-05-20 09:37:38.558 DEBUG 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Securing GET /hello``2023-05-20 09:37:38.559 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking DisableEncodeUrlFilter (1/17)``2023-05-20 09:37:38.559 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking WebAsyncManagerIntegrationFilter (2/17)``2023-05-20 09:37:38.560 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking SecurityContextPersistenceFilter (3/17)``2023-05-20 09:37:38.561 TRACE 6632 --- [nio-8080-exec-9] w.c.HttpSessionSecurityContextRepository : No HttpSession currently exists``2023-05-20 09:37:38.561 TRACE 6632 --- [nio-8080-exec-9] w.c.HttpSessionSecurityContextRepository : Created SecurityContextImpl [Null authentication]``2023-05-20 09:37:38.562 DEBUG 6632 --- [nio-8080-exec-9] s.s.w.c.SecurityContextPersistenceFilter : Set SecurityContextHolder to empty SecurityContext``2023-05-20 09:37:38.562 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking HeaderWriterFilter (4/17)``2023-05-20 09:37:38.562 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking CorsFilter (5/17)``2023-05-20 09:37:38.566 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking CsrfFilter (6/17)``2023-05-20 09:37:38.567 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.csrf.CsrfFilter     : Did not protect against CSRF since request did not match CsrfNotRequired [TRACE, HEAD, GET, OPTIONS]``2023-05-20 09:37:38.568 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking LogoutFilter (7/17)``2023-05-20 09:37:38.571 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.logout.LogoutFilter      : Did not match request to Ant [pattern='/logout', POST]    ``2023-05-20 09:37:38.573 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking UsernamePasswordAuthenticationFilter (8/17)``2023-05-20 09:37:38.574 TRACE 6632 --- [nio-8080-exec-9] w.a.UsernamePasswordAuthenticationFilter : Did not match request to Ant [pattern='/login', POST]``2023-05-20 09:37:38.576 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking DefaultLoginPageGeneratingFilter (9/17)``2023-05-20 09:37:38.578 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking DefaultLogoutPageGeneratingFilter (10/17)``2023-05-20 09:37:38.582 TRACE 6632 --- [nio-8080-exec-9] .w.a.u.DefaultLogoutPageGeneratingFilter : Did not render default logout page since request did not match [Ant [pattern='/logout', GET]]``2023-05-20 09:37:38.583 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking BasicAuthenticationFilter (11/17)``2023-05-20 09:37:38.584 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.www.BasicAuthenticationFilter : Did not process authentication request since failed to find username and password in Basic Authorization header``2023-05-20 09:37:38.587 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking RequestCacheAwareFilter (12/17)``2023-05-20 09:37:38.588 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.s.HttpSessionRequestCache    : No saved request``2023-05-20 09:37:38.590 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking SecurityContextHolderAwareRequestFilter (13/17)   ``2023-05-20 09:37:38.591 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking AnonymousAuthenticationFilter (14/17)``2023-05-20 09:37:38.592 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.AnonymousAuthenticationFilter : Set SecurityContextHolder to AnonymousAuthenticationToken [Principal=anonymousUser, Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=null], Granted Authorities=[ROLE_ANONYMOUS]]``2023-05-20 09:37:38.593 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking SessionManagementFilter (15/17)``2023-05-20 09:37:38.593 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking ExceptionTranslationFilter (16/17)``2023-05-20 09:37:38.594 TRACE 6632 --- [nio-8080-exec-9] o.s.security.web.FilterChainProxy    : Invoking FilterSecurityInterceptor (17/17)``2023-05-20 09:37:38.596 TRACE 6632 --- [nio-8080-exec-9] edFilterInvocationSecurityMetadataSource : Did not match request to EndpointRequestMatcher includes=[health], excludes=[], includeLinks=false - [permitAll] (1/2)``2023-05-20 09:37:38.610 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.i.FilterSecurityInterceptor  : Did not re-authenticate AnonymousAuthenticationToken [Principal=anonymousUser, Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=null], Granted Authorities=[ROLE_ANONYMOUS]] before authorizing``2023-05-20 09:37:38.619 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.i.FilterSecurityInterceptor  : Authorizing filter invocation [GET /hello] with attributes [authenticated]``2023-05-20 09:37:38.626 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.expression.WebExpressionVoter : Voted to deny authorization``2023-05-20 09:37:38.632 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.i.FilterSecurityInterceptor  : Failed to authorize filter invocation [GET /hello] with attributes [authenticated] using AffirmativeBased [DecisionVoters=[org.springframework.security.web.access.expression.WebExpressionVoter@f613067], AllowIfAllAbstainDecisions=false]``2023-05-20 09:37:38.640 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.a.ExceptionTranslationFilter   : Sending AnonymousAuthenticationToken [Principal=anonymousUser, Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=null], Granted Authorities=[ROLE_ANONYMOUS]] ``to authentication entry point since access is denied` `org.springframework.security.access.AccessDeniedException: Access is denied``    ``at org.springframework.security.access.vote.AffirmativeBased.decide(AffirmativeBased.java:73) ~[spring-security-core-5.7.8.jar:5.7.8]` `2023-05-20 09:37:38.691 DEBUG 6632 --- [nio-8080-exec-9] o.s.s.w.s.HttpSessionRequestCache    : Saved request http://localhost:8080/hello to session``2023-05-20 09:37:38.693 DEBUG 6632 --- [nio-8080-exec-9] s.w.a.DelegatingAuthenticationEntryPoint : Trying to match using And [Not [RequestHeaderRequestMatcher [expectedHeaderName=X-Requested-With, expectedHeaderValue=XMLHttpRequest]], MediaTypeRequestMatcher [contentNegotiationStrategy=org.springframework.web.accept.HeaderContentNegotiationStrategy@4b95451, matchingMediaTypes=[application/xhtml+xml, image/*, text/html, text/plain], useEquals=false, ignoredMediaTypes=[*/*]]]    ``2023-05-20 09:37:38.701 DEBUG 6632 --- [nio-8080-exec-9] s.w.a.DelegatingAuthenticationEntryPoint : Match found! Executing org.springframework.security.web.authentication.LoginUrlAuthenticationEntryPoint@168ad26f``2023-05-20 09:37:38.709 DEBUG 6632 --- [nio-8080-exec-9] o.s.s.web.DefaultRedirectStrategy    : Redirecting to http://localhost:8080/login``2023-05-20 09:37:38.712 TRACE 6632 --- [nio-8080-exec-9] o.s.s.w.header.writers.HstsHeaderWriter : Not injecting HSTS header since it did not match request to [Is Secure]``2023-05-20 09:37:38.720 DEBUG 6632 --- [nio-8080-exec-9] w.c.HttpSessionSecurityContextRepository : Did not store empty SecurityContext``2023-05-20 09:37:38.730 DEBUG 6632 --- [nio-8080-exec-9] w.c.HttpSessionSecurityContextRepository : Did not store empty SecurityContext``2023-05-20 09:37:38.731 DEBUG 6632 --- [nio-8080-exec-9] s.s.w.c.SecurityContextPersistenceFilter : Cleared SecurityContextHolder to complete request

这个过程中有两点比较重要：第一点是经过 AnonymousAuthenticationFilter 时，将当前用户设置为 anonymousUser，角色为 ROLE_ANONYMOUS；第二点是经过 FilterSecurityInterceptor 时，校验当前用户是否有访问 /hello 页面的权限，在上面的 defaultSecurityFilterChain 中，可以看到 http.authorizeRequests().anyRequest().authenticated() 这样的代码，这说明 Spring Security 默认对所有的页面都开启了鉴权，所以会抛出 AccessDeniedException 异常，而这个异常被 ExceptionTranslationFilter 拦截，并将这个异常交给 LoginUrlAuthenticationEntryPoint 处理，从而重定向到 /login 页面，整个过程的示意图如下：

接下来，浏览器开始访问重定向后的 /login 页面，这时请求又会再一次经历一系列的 Security Filters，和上面的 /hello 请求不一样的是，/login 请求经过 DefaultLoginPageGeneratingFilter 时，会生成上面我们看到的登录页面并结束整个调用链：

public class DefaultLoginPageGeneratingFilter extends GenericFilterBean {` `  ``private void doFilter(HttpServletRequest request, HttpServletResponse response, FilterChain chain)``      ``throws IOException, ServletException {``    ``boolean loginError = isErrorPage(request);``    ``boolean logoutSuccess = isLogoutSuccess(request);``    ``if (isLoginUrlRequest(request) || loginError || logoutSuccess) {``      ``String loginPageHtml = generateLoginPageHtml(request, loginError, logoutSuccess);``      ``response.setContentType("text/html;charset=UTF-8");``      ``response.setContentLength(loginPageHtml.getBytes(StandardCharsets.UTF_8).length);``      ``response.getWriter().write(loginPageHtml);``      ``return;``    ``}``    ``chain.doFilter(request, response);``  ``}``}

AuthenticationManager：剖析认证流程

接下来，输入用户名和密码并提交，请求会再一次经历 Security Filters，这一次，请求在 UsernamePasswordAuthenticationFilter 这里被拦截下来，并开始了用户名和密码的认证过程：

public class UsernamePasswordAuthenticationFilter extends AbstractAuthenticationProcessingFilter {` `  ``@Override``  ``public Authentication attemptAuthentication(HttpServletRequest request, HttpServletResponse response)``      ``throws AuthenticationException {``    ``if (this.postOnly && !request.getMethod().equals("POST")) {``      ``throw new AuthenticationServiceException("Authentication method not supported: " + request.getMethod());``    ``}``    ``String username = obtainUsername(request);``    ``username = (username != null) ? username.trim() : "";``    ``String password = obtainPassword(request);``    ``password = (password != null) ? password : "";``    ``UsernamePasswordAuthenticationToken authRequest = UsernamePasswordAuthenticationToken.unauthenticated(username,``        ``password);``    ``// Allow subclasses to set the "details" property``    ``setDetails(request, authRequest);``    ``return this.getAuthenticationManager().authenticate(authRequest);``  ``}``}

这里将遇到 Spring Security 中处理认证的核心接口：AuthenticationManager：

public interface AuthenticationManager {` `  ``Authentication authenticate(Authentication authentication) throws AuthenticationException;``}

这个接口只有一个 authenticate() 方法，它的入参是一个未认证的 Authentication，从 UsernamePasswordAuthenticationFilter 的代码中可以看到使用了 UsernamePasswordAuthenticationToken，它的返回有三种情况：

• 如果认证成功，则返回认证成功后的 Authentication（通常带有 authenticated=true）；
• 如果认证失败，则抛出 AuthenticationException 异常；
• 如果无法判断，则返回 null；

AuthenticationManager 接口最常用的一个实现是 ProviderManager 类，它包含了一个或多个 AuthenticationProvider 实例：

public class ProviderManager implements AuthenticationManager {` `  ``private List<AuthenticationProvider> providers;``}

AuthenticationProvider 有点像 AuthenticationManager，但它有一个额外的方法 boolean supports(Class<?> authentication)：

public interface AuthenticationProvider {` `  ``Authentication authenticate(Authentication authentication) throws AuthenticationException;` `  ``boolean supports(Class<?> authentication);``}

Spring Security 会遍历列表中所有的 AuthenticationProvider，并通过 supports() 方法来选取合适的 AuthenticationProvider 实例来实现认证，从上文中我们知道，UsernamePasswordAuthenticationFilter 在认证时使用的 Authentication 类型为 UsernamePasswordAuthenticationToken，对于这个 Authentication，默认使用的 AuthenticationProvider 是 DaoAuthenticationProvider，它继承自抽象类 AbstractUserDetailsAuthenticationProvider：

public abstract class AbstractUserDetailsAuthenticationProvider``    ``implements AuthenticationProvider, InitializingBean, MessageSourceAware {` `  ``@Override``  ``public Authentication authenticate(Authentication authentication) throws AuthenticationException {` `    ``String username = determineUsername(authentication);``    ``UserDetails user = retrieveUser(username, (UsernamePasswordAuthenticationToken) authentication);``    ` `    ``this.preAuthenticationChecks.check(user);``    ``additionalAuthenticationChecks(user, (UsernamePasswordAuthenticationToken) authentication);``    ``this.postAuthenticationChecks.check(user);``    ` `    ``Object principalToReturn = user;``    ``if (this.forcePrincipalAsString) {``      ``principalToReturn = user.getUsername();``    ``}``    ``return createSuccessAuthentication(principalToReturn, authentication, user);``  ``}``}

其中，最关键的代码有两行，第一行是通过 retrieveUser() 方法获取 UserDetails：

public class DaoAuthenticationProvider extends AbstractUserDetailsAuthenticationProvider {` `  ``@Override``  ``protected final UserDetails retrieveUser(String username, UsernamePasswordAuthenticationToken authentication)``      ``throws AuthenticationException {``    ` `    ``UserDetails loadedUser = this.getUserDetailsService().loadUserByUsername(username);``    ``return loadedUser;``  ``}``}

进入 retrieveUser() 方法内部，可以看到它是通过 UserDetailsService 的 loadUserByUsername() 方法来获取 UserDetails 的，而这个 UserDetailsService 默认实现是 InMemoryUserDetailsManager：

public class InMemoryUserDetailsManager implements UserDetailsManager, UserDetailsPasswordService {` `  ``@Override``  ``public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {``    ``UserDetails user = this.users.get(username.toLowerCase());``    ``if (user == null) {``      ``throw new UsernameNotFoundException(username);``    ``}``    ``return new User(user.getUsername(), user.getPassword(), user.isEnabled(), user.isAccountNonExpired(),``        ``user.isCredentialsNonExpired(), user.isAccountNonLocked(), user.getAuthorities());``  ``}``}

它的实现非常简单，就是从 users 这个 Map 中直接获取 UserDetails，那么 users 这个 Map 又是从哪来的呢？ 答案就是我们在配置文件中配置的 spring.security.user，我们可以从自动配置类 UserDetailsServiceAutoConfiguration 中找到 InMemoryUserDetailsManager 的初始化代码：

public class UserDetailsServiceAutoConfiguration {` `  ``@Bean``  ``@Lazy``  ``public InMemoryUserDetailsManager inMemoryUserDetailsManager(SecurityProperties properties,``      ``ObjectProvider<PasswordEncoder> passwordEncoder) {``    ``SecurityProperties.User user = properties.getUser();``    ``List<String> roles = user.getRoles();``    ``return new InMemoryUserDetailsManager(User.withUsername(user.getName())``      ``.password(getOrDeducePassword(user, passwordEncoder.getIfAvailable()))``      ``.roles(StringUtils.toStringArray(roles))``      ``.build());``  ``}``}

另一行关键代码是通过 additionalAuthenticationChecks() 方法对 UserDetails 和 UsernamePasswordAuthenticationToken 进行校验，一般来说，就是验证密码是否正确：

public class DaoAuthenticationProvider extends AbstractUserDetailsAuthenticationProvider {` `  ``@Override``  ``protected void additionalAuthenticationChecks(UserDetails userDetails,``      ``UsernamePasswordAuthenticationToken authentication) throws AuthenticationException {``    ` `    ``String presentedPassword = authentication.getCredentials().toString();``    ``if (!this.passwordEncoder.matches(presentedPassword, userDetails.getPassword())) {``      ``this.logger.debug("Failed to authenticate since password does not match stored value");``      ``throw new BadCredentialsException(this.messages``          ``.getMessage("AbstractUserDetailsAuthenticationProvider.badCredentials", "Bad credentials"));``    ``}``  ``}``}

一旦用户名和密码都验证通过，就调用 createSuccessAuthentication() 方法创建并返回一个认证成功后的 Authentication，然后经过一系列的后处理，整个认证的流程如下所示：

其中，SecurityContextHolder 将认证成功后的 Authentication 保存到安全上下文中供后续 Filter 使用；AuthenticationSuccessHandler 用于定义一些认证成功后的自定义逻辑，默认实现为 SimpleUrlAuthenticationSuccessHandler，它返回一个重定向，将浏览器转到登录之前用户访问的页面。

在我的测试中，SimpleUrlAuthenticationSuccessHandler 貌似并没有触发，新版本的逻辑有变动？

AccessDecisionManager：剖析授权流程

其实，在上面分析重定向 /login 页面的流程时已经大致了解了实现授权的逻辑，请求经过 FilterSecurityInterceptor 时，校验当前用户是否有访问页面的权限，如果没有，则会抛出 AccessDeniedException 异常。FilterSecurityInterceptor 的核心代码如下：

public class FilterSecurityInterceptor extends AbstractSecurityInterceptor implements Filter {``  ` `  ``public void invoke(FilterInvocation filterInvocation) throws IOException, ServletException {` `    ``InterceptorStatusToken token = super.beforeInvocation(filterInvocation);``    ``try {``      ``filterInvocation.getChain().doFilter(filterInvocation.getRequest(), filterInvocation.getResponse());``    ``}``    ``finally {``      ``super.finallyInvocation(token);``    ``}``    ``super.afterInvocation(token, null);``  ``}``}

可以看到，主要逻辑就包含在 beforeInvocation()、finallyInvocation() 和 afterInvocation() 这三个方法中，而对授权相关的部分则位于 beforeInvocation() 方法中：

public abstract class AbstractSecurityInterceptor``    ``implements InitializingBean, ApplicationEventPublisherAware, MessageSourceAware {` `  ``protected InterceptorStatusToken beforeInvocation(Object object) {``    ` `    ``Collection<ConfigAttribute> attributes = this.obtainSecurityMetadataSource().getAttributes(object);``    ` `    ``Authentication authenticated = authenticateIfRequired();``    ` `    ``// Attempt authorization``    ``attemptAuthorization(object, attributes, authenticated);``    ` `    ``if (this.publishAuthorizationSuccess) {``      ``publishEvent(new AuthorizedEvent(object, attributes, authenticated));``    ``}` `    ``return new InterceptorStatusToken(SecurityContextHolder.getContext(), false, attributes, object);``  ``}` `  ``private void attemptAuthorization(Object object, Collection<ConfigAttribute> attributes,``      ``Authentication authenticated) {``    ``try {``      ``this.accessDecisionManager.decide(authenticated, object, attributes);``    ``}``    ``catch (AccessDeniedException ex) {``      ``publishEvent(new AuthorizationFailureEvent(object, attributes, authenticated, ex));``      ``throw ex;``    ``}``  ``}``}

在这里，我们遇到了 Spring Security 实现授权的核心接口：AccessDecisionManager，Spring Security 就是通过该接口的 decide() 方法来决定用户是否有访问某个资源的权限。AccessDecisionManager 接口的默认实现为 AffirmativeBased，可以从 AbstractInterceptUrlConfigurer 中找到它的踪影：

public abstract class AbstractInterceptUrlConfigurer<C extends AbstractInterceptUrlConfigurer<C, H>, H extends HttpSecurityBuilder<H>>``    ``extends AbstractHttpConfigurer<C, H> {``  ` `  ``private AccessDecisionManager createDefaultAccessDecisionManager(H http) {``    ``AffirmativeBased result = new AffirmativeBased(getDecisionVoters(http));``    ``return postProcess(result);``  ``}``}

AffirmativeBased 实例中包含一个或多个 AccessDecisionVoter，它通过遍历所有的 AccessDecisionVoter 依次投票决定授权是否允许，只要有一个 AccessDecisionVoter 拒绝，则抛出 AccessDeniedException 异常：

public class AffirmativeBased extends AbstractAccessDecisionManager {` `  ``@Override``  ``@SuppressWarnings({ "rawtypes", "unchecked" })``  ``public void decide(Authentication authentication, Object object, Collection<ConfigAttribute> configAttributes)``      ``throws AccessDeniedException {``    ``int deny = 0;``    ``for (AccessDecisionVoter voter : getDecisionVoters()) {``      ``int result = voter.vote(authentication, object, configAttributes);``      ``switch (result) {``      ``case AccessDecisionVoter.ACCESS_GRANTED:``        ``return;``      ``case AccessDecisionVoter.ACCESS_DENIED:``        ``deny++;``        ``break;``      ``default:``        ``break;``      ``}``    ``}``    ``if (deny > 0) {``      ``throw new AccessDeniedException(``          ``this.messages.getMessage("AbstractAccessDecisionManager.accessDenied", "Access is denied"));``    ``}``    ``// To get this far, every AccessDecisionVoter abstained``    ``checkAllowIfAllAbstainDecisions();``  ``}``}

默认情况下，AffirmativeBased 实例中只有一个 AccessDecisionVoter，那就是 WebExpressionVoter：

public class WebExpressionVoter implements AccessDecisionVoter<FilterInvocation> {` `  ``@Override``  ``public int vote(Authentication authentication, FilterInvocation filterInvocation,``      ``Collection<ConfigAttribute> attributes) {``    ` `    ``WebExpressionConfigAttribute webExpressionConfigAttribute = findConfigAttribute(attributes);``    ` `    ``EvaluationContext ctx = webExpressionConfigAttribute.postProcess(``        ``this.expressionHandler.createEvaluationContext(authentication, filterInvocation), filterInvocation);` `    ``boolean granted = ExpressionUtils.evaluateAsBoolean(webExpressionConfigAttribute.getAuthorizeExpression(), ctx);``    ``if (granted) {``      ``return ACCESS_GRANTED;``    ``}``    ``return ACCESS_DENIED;``  ``}``}

WebExpressionVoter 将授权转换为 SpEL 表达式，检查授权是否通过，就是看执行 SpEL 表达式的结果是否为 true，这里的细节还有很多，详细内容还是参考 官方文档 吧。