<filter>
    <filter-name>springSecurityFilterChain</filter-name>
    <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
</filter>

<filter-mapping>
    <filter-name>springSecurityFilterChain</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

@EnableWebSecurity
public class SecurityConfiguration extends WebSecurityConfigurerAdapter {
    ...
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import({ 
    WebSecurityConfiguration.class,
    SpringWebMvcImportSelector.class,
    OAuth2ImportSelector.class,
    HttpSecurityConfiguration.class
})
@EnableGlobalAuthentication
@Configuration
public @interface EnableWebSecurity {

   /**
    * Controls debugging support for Spring Security. Default is false.
    * @return if true, enables debug support with Spring Security
    */
   boolean debug() default false;

}

private WebSecurity webSecurity;

// 注入 bean
@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);
   }

   // 重點(diǎn)關(guān)注
   return this.webSecurity.build();
}
@Autowired(required = false)
public void setFilterChainProxySecurityConfigurer(ObjectPostProcessor<Object> objectPostProcessor,
      @Value("#{@autowiredWebSecurityConfigurersIgnoreParents.getWebSecurityConfigurers()}") List<SecurityConfigurer<Filter, WebSecurity>> webSecurityConfigurers)
      throws Exception {
   this.webSecurity = objectPostProcessor.postProcess(new WebSecurity(objectPostProcessor));
   if (this.debugEnabled != null) {
      this.webSecurity.debug(this.debugEnabled);
   }
   webSecurityConfigurers.sort(AnnotationAwareOrderComparator.INSTANCE);
   Integer previousOrder = null;
   Object previousConfig = null;
   for (SecurityConfigurer<Filter, WebSecurity> config : webSecurityConfigurers) {
      Integer order = AnnotationAwareOrderComparator.lookupOrder(config);
      if (previousOrder != null && previousOrder.equals(order)) {
         throw new IllegalStateException("@Order on WebSecurityConfigurers must be unique. Order of " + order
               + " was already used on " + previousConfig + ", so it cannot be used on " + config + " too.");
      }
      previousOrder = order;
      previousConfig = config;
   }
   for (SecurityConfigurer<Filter, WebSecurity> webSecurityConfigurer : webSecurityConfigurers) {
      this.webSecurity.apply(webSecurityConfigurer);
   }
   this.webSecurityConfigurers = webSecurityConfigurers;
}

   // 重點(diǎn)關(guān)注
   return this.webSecurity.build();

@Override
protected Filter performBuild() throws Exception {
   ...
   int chainSize = this.ignoredRequests.size() + this.securityFilterChainBuilders.size();
   List<SecurityFilterChain> securityFilterChains = new ArrayList<>(chainSize);
   for (RequestMatcher ignoredRequest : this.ignoredRequests) {
      securityFilterChains.add(new DefaultSecurityFilterChain(ignoredRequest));
   }

   // ① 調(diào)用 securityFilterChainBuilder 的 build() 方法
   for (SecurityBuilder<? extends SecurityFilterChain> securityFilterChainBuilder : this.securityFilterChainBuilders) {
      securityFilterChains.add(securityFilterChainBuilder.build());
   }
   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;
}

public class FilterChainProxy extends GenericFilterBean {

   // 維護(hù)的 spring security 過濾器鏈列表
   private List<SecurityFilterChain> filterChains;
   public FilterChainProxy(SecurityFilterChain chain) {
      this(Arrays.asList(chain));
   }
   public FilterChainProxy(List<SecurityFilterChain> filterChains) {
      this.filterChains = filterChains;
   }
    @Override
    public void doFilter(
        ServletRequest request,
        ServletResponse response,
        FilterChain chain) throws IOException, ServletException {
            boolean clearContext = request.getAttribute(FILTER_APPLIED) == null;
            if (!clearContext) {
                    doFilterInternal(request, response, chain);
                    return;
            }
            try {
                    request.setAttribute(FILTER_APPLIED, Boolean.TRUE);

                    // 真正起作用的函數(shù)
                    doFilterInternal(request, response, chain);
            }
            catch (RequestRejectedException ex) {
                    this.requestRejectedHandler.handle((HttpServletRequest) request, (HttpServletResponse) response, ex);
            }
    ...
    }
    private void doFilterInternal(
        ServletRequest request,
        ServletResponse response,
        FilterChain chain) throws IOException, ServletException {
            FirewalledRequest firewallRequest = this.firewall.getFirewalledRequest(
                (HttpServletRequest) request);
            HttpServletResponse firewallResponse = this.firewall.getFirewalledResponse(
                (HttpServletResponse) response);
            List<Filter> filters = getFilters(firewallRequest);
            if (filters == null || filters.size() == 0) {
                    ...
                    chain.doFilter(firewallRequest, firewallResponse);
                    return;
            }
            VirtualFilterChain virtualFilterChain = new VirtualFilterChain(
                firewallRequest, chain, filters);
            virtualFilterChain.doFilter(firewallRequest, firewallResponse);
    }
}

   for (RequestMatcher ignoredRequest : this.ignoredRequests) {
      securityFilterChains.add(new DefaultSecurityFilterChain(ignoredRequest));
   }
   // ① 調(diào)用 securityFilterChainBuilder 的 build() 方法
   for (SecurityBuilder<? extends SecurityFilterChain> securityFilterChainBuilder : this.securityFilterChainBuilders) {
      securityFilterChains.add(securityFilterChainBuilder.build());
   }