最近在整合微服务OAuth 2认证过程中，它是基于Spring Security之上，而本人对Spring Security架构原理并不太熟悉，导致很多配置搞不太清楚，遂咬牙啃完了Spring Security核心源码，花了差不多一星期，总体上来说，其代码确实比较晦涩，之前在学习Apache Shiro框架之前也曾经在相关论坛里了解过，相比Spring Security，Apache Shiro真的是相当轻量，代码研读起来容易很多，而Spring Security类继承结构复杂，大量使用了其所谓Builder和Configuer模式，其代码跟踪过程很痛苦，遂记录下，分享给有需要的人，由于本人能力有限，在文章中有不对之处，还请各位执教，在此谢谢各位了。

本人研读的Spring Security版本为：5.1.4.RELEASE

Spring Security在3.2版本之后支持Java Configuration，即：通过Java编码形式配置Spring Security，可不再依赖XML文件配置，本文采用Java Configuration方式。

在Spring Security官方文档中有一个最简配置例子：

import org.springframework.beans.factory.annotation.Autowired;

import org.springframework.context.annotation.*;
import org.springframework.security.config.annotation.authentication.builders.*;
import org.springframework.security.config.annotation.web.configuration.*;

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {

    @Autowired
    public void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
        auth
            .inMemoryAuthentication()
                .withUser("user").password("password").roles("USER");
    }
}

我们先不要看其它内容，先关注注解@EnableWebSecurity，它是初始化Spring Security的入口，打开其源码如下：

@Retention(value = java.lang.annotation.RetentionPolicy.RUNTIME)
@Target(value = { java.lang.annotation.ElementType.TYPE })
@Documented
@Import({ WebSecurityConfiguration.class,
        SpringWebMvcImportSelector.class,
        OAuth2ImportSelector.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;
}

该注解类通过@Configuration和@Import配合使用引入了一个配置类（WebSecurityConfiguration）和两个ImportSelector（SpringWebMvcImportSelector，OAuth2ImportSelector），我们重点关注下WebSecurityConfiguration，它是Spring Security的核心，正是它构建初始化了所有的Bean实例和相关配置，下面我们详细分析下。

打开WebSecurityConfiguration源码，发现它被@Configuration标记，说明它是配置类，

@Configuration
public class WebSecurityConfiguration implements ImportAware, BeanClassLoaderAware

该类中最重要的工作就是实例并注册FilterChainProxy，也就是我们在以前XML文件中配置的过滤器：

<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>

该过滤器负责拦截请求，并把请求通过一定的匹配规则（通过RequestMatcher匹配实现）路由（或者Delegate）到具体的SecurityFilterChain，源码如下：

/**
     * Creates the Spring Security Filter Chain
     * @return the {@link Filter} that represents the security filter chain
     * @throws Exception
     */
    @Bean(name = AbstractSecurityWebApplicationInitializer.DEFAULT_FILTER_NAME)
    public Filter springSecurityFilterChain() throws Exception {
        boolean hasConfigurers = webSecurityConfigurers != null
                && !webSecurityConfigurers.isEmpty();
        if (!hasConfigurers) {
            WebSecurityConfigurerAdapter adapter = objectObjectPostProcessor
                    .postProcess(new WebSecurityConfigurerAdapter() {
                    });
            webSecurity.apply(adapter);
        }
        return webSecurity.build();
    }

@Bean注解name属性值AbstractSecurityWebApplicationInitializer.DEFAULT_FILTER_NAME就是XML中定义的springSecurityFilterChain。

从源码中知道过滤器通过最后的webSecurity.build()创建，webSecurity的类型为：WebSecurity，它在setFilterChainProxySecurityConfigurer方法中优先被创建了：

    /**
     * Sets the {@code <SecurityConfigurer<FilterChainProxy, WebSecurityBuilder>}
     * instances used to create the web configuration.
     *
     * @param objectPostProcessor the {@link ObjectPostProcessor} used to create a
     * {@link WebSecurity} instance
     * @param webSecurityConfigurers the
     * {@code <SecurityConfigurer<FilterChainProxy, WebSecurityBuilder>} instances used to
     * create the web configuration
     * @throws Exception
     */
    @Autowired(required = false)
    public void setFilterChainProxySecurityConfigurer(
            ObjectPostProcessor<Object> objectPostProcessor,
            @Value("#{@autowiredWebSecurityConfigurersIgnoreParents.getWebSecurityConfigurers()}") List<SecurityConfigurer<Filter, WebSecurity>> webSecurityConfigurers)
            throws Exception {
        webSecurity = objectPostProcessor
                .postProcess(new WebSecurity(objectPostProcessor));
        if (debugEnabled != null) {
            webSecurity.debug(debugEnabled);
        }

        Collections.sort(webSecurityConfigurers, 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) {
            webSecurity.apply(webSecurityConfigurer);
        }
        this.webSecurityConfigurers = webSecurityConfigurers;
    }

从代码中可以看到，它是直接被new出来的：

webSecurity = objectPostProcessor
                .postProcess(new WebSecurity(objectPostProcessor));

setFilterChainProxySecurityConfigurer方法参数中需要被注入两个对象：objectPostProcessor和webSecurityConfigurers，objectPostProcessor是在ObjectPostProcessorConfiguration配置类中注册的，而webSecurityConfigurers则是使用了@Value注解方式，注解内容为：#{@autowiredWebSecurityConfigurersIgnoreParents.getWebSecurityConfigurers()}，通过源码了解，autowiredWebSecurityConfigurersIgnoreParents是在本类中被注册：

@Bean
    public static AutowiredWebSecurityConfigurersIgnoreParents autowiredWebSecurityConfigurersIgnoreParents(
            ConfigurableListableBeanFactory beanFactory) {
        return new AutowiredWebSecurityConfigurersIgnoreParents(beanFactory);
    }

在AutowiredWebSecurityConfigurersIgnoreParents中定义了方法：getWebSecurityConfigurers：

@SuppressWarnings({ "rawtypes", "unchecked" })
    public List<SecurityConfigurer<Filter, WebSecurity>> getWebSecurityConfigurers() {
        List<SecurityConfigurer<Filter, WebSecurity>> webSecurityConfigurers = new ArrayList<SecurityConfigurer<Filter, WebSecurity>>();
        Map<String, WebSecurityConfigurer> beansOfType = beanFactory
                .getBeansOfType(WebSecurityConfigurer.class);
        for (Entry<String, WebSecurityConfigurer> entry : beansOfType.entrySet()) {
            webSecurityConfigurers.add(entry.getValue());
        }
        return webSecurityConfigurers;
    }

它通过BeanFactory获取了类型为WebSecurityConfigurer的Bean实例列表。回到WebSecurityConfiguration类中的setFilterChainProxySecurityConfigurer方法，它把WebSecurityConfigurer列表设置到了WebSecurity中，源码如下：

for (SecurityConfigurer<Filter, WebSecurity> webSecurityConfigurer : webSecurityConfigurers) {
            webSecurity.apply(webSecurityConfigurer);
        }

通过apply方法，apply方法其实就是webSecurityConfigurer放入webSecurity维护的configurers属性中，configurers是个LinkedHashMap，源码如下：

    /**
     * Applies a {@link SecurityConfigurer} to this {@link SecurityBuilder} overriding any
     * {@link SecurityConfigurer} of the exact same class. Note that object hierarchies
     * are not considered.
     *
     * @param configurer
     * @return the {@link SecurityConfigurerAdapter} for further customizations
     * @throws Exception
     */
    public <C extends SecurityConfigurer<O, B>> C apply(C configurer) throws Exception {
        add(configurer);
        return configurer;
    }

其中代码add(configurer)就是将这些webSecurityConfigurer添加到webSecurity的configurers属性中。

现在webSecurity的初始化工作已经完成，现在回到springSecurityFilterChain方法中，它首先检查当前是否配置了webSecurityConfigurer，如果没有的会默认设置一个，并且调用上面提到的apply方法，源码如下：

        boolean hasConfigurers = webSecurityConfigurers != null
                && !webSecurityConfigurers.isEmpty();
        if (!hasConfigurers) {
            WebSecurityConfigurerAdapter adapter = objectObjectPostProcessor
                    .postProcess(new WebSecurityConfigurerAdapter() {
                    });
            webSecurity.apply(adapter);
        }

如果已经存在配置了webSecurityConfigurer，则调用webSecurity.build()进行构建。

在进入build方法之前，首先简单介绍下WebSecurity的继承结构，

它实现了SecurityBuilder接口，继承自AbstractConfiguredSecurityBuilder，AbstractConfiguredSecurityBuilder继承自AbstractSecurityBuilder，AbstractSecurityBuilder实现了SecurityBuilder，其中AbstractConfiguredSecurityBuilder实现了通过自定义SecurityConfigurer类来配置SecurityBuilder，上面提到的apply(SecurityConfigurer configurer)就是在该类中实现的，它把configurer保存在它维护的LinkedHashMap<Class<? extends SecurityConfigurer<O, B>>, List<SecurityConfigurer<O, B>>> configurers = new LinkedHashMap<Class<? extends SecurityConfigurer<O, B>>, List<SecurityConfigurer<O, B>>>()中。

调用webSecurity.build()后，首先调用的父类AbstractSecurityBuilder中的build方法：

public final O build() throws Exception {
        if (this.building.compareAndSet(false, true)) {
            this.object = doBuild();
            return this.object;
        }
        throw new AlreadyBuiltException("This object has already been built");
    }

然后调用doBuild()，doBuild()在子类中实现，AbstractConfiguredSecurityBuilder实现了该方法：

    @Override
    protected final O doBuild() throws Exception {
        synchronized (configurers) {
            buildState = BuildState.INITIALIZING;

            beforeInit();
            init();

            buildState = BuildState.CONFIGURING;

            beforeConfigure();
            configure();

            buildState = BuildState.BUILDING;

            O result = performBuild();

            buildState = BuildState.BUILT;

            return result;
        }
    }

在这里重点关注init()、configure()和performBuild()，下面逐个分析它们的作用。

init()方法在AbstractConfiguredSecurityBuilder实现：

private void init() throws Exception {
        Collection<SecurityConfigurer<O, B>> configurers = getConfigurers();

        for (SecurityConfigurer<O, B> configurer : configurers) {
            configurer.init((B) this);
        }

        for (SecurityConfigurer<O, B> configurer : configurersAddedInInitializing) {
            configurer.init((B) this);
        }
    }

它的工作是迭代调用所有配置的SecurityConfigrer的init方法，在这里其实是它的子类WebSecurityConfigurer，因为之前获取时指定的类型就是WebSecurityConfigurer，在上文中提到AutowiredWebSecurityConfigurersIgnoreParents.getWebSecurityConfigurers()中：

Map<String, WebSecurityConfigurer> beansOfType = beanFactory.getBeansOfType(WebSecurityConfigurer.class);

而实现了WebSecurityConfigurer接口的就是WebSecurityConfigurerAdapter，WebSecurityConfigurerAdapter.init()源码如下：

public void init(final WebSecurity web) throws Exception {
        final HttpSecurity http = getHttp();
        web.addSecurityFilterChainBuilder(http).postBuildAction(new Runnable() {
            public void run() {
                FilterSecurityInterceptor securityInterceptor = http
                        .getSharedObject(FilterSecurityInterceptor.class);
                web.securityInterceptor(securityInterceptor);
            }
        });
    }

它只要完成两件重要的事情：

初始化HttpSecurity对象；
设置HttpSecurity对象添加至WebSecurity的securityFilterChainBuilders列表中；

初始化HttpSecurity对象在getHttp()方法中实现：

protected final HttpSecurity getHttp() throws Exception {
        if (http != null) {
            return http;
        }

        DefaultAuthenticationEventPublisher eventPublisher = objectPostProcessor
                .postProcess(new DefaultAuthenticationEventPublisher());
        localConfigureAuthenticationBldr.authenticationEventPublisher(eventPublisher);

        AuthenticationManager authenticationManager = authenticationManager();
        authenticationBuilder.parentAuthenticationManager(authenticationManager);
        authenticationBuilder.authenticationEventPublisher(eventPublisher);
        Map<Class<? extends Object>, Object> sharedObjects = createSharedObjects();

        http = new HttpSecurity(objectPostProcessor, authenticationBuilder,
                sharedObjects);
        if (!disableDefaults) {
            // @formatter:off
            http
                .csrf().and()
                .addFilter(new WebAsyncManagerIntegrationFilter())
                .exceptionHandling().and()
                .headers().and()
                .sessionManagement().and()
                .securityContext().and()
                .requestCache().and()
                .anonymous().and()
                .servletApi().and()
                .apply(new DefaultLoginPageConfigurer<>()).and()
                .logout();
            // @formatter:on
            ClassLoader classLoader = this.context.getClassLoader();
            List<AbstractHttpConfigurer> defaultHttpConfigurers =
                    SpringFactoriesLoader.loadFactories(AbstractHttpConfigurer.class, classLoader);

            for (AbstractHttpConfigurer configurer : defaultHttpConfigurers) {
                http.apply(configurer);
            }
        }
        configure(http);
        return http;
    }

从代码中可以了解，HttpSecurity是直接被new出来的，在创建HttpSecurity之前，首先初始化了AuthenticationManagerBuilder对象，这里有段代码很重要就是： AuthenticationManager authenticationManager = authenticationManager();，它创建AuthenticationManager实例，打开authenticationManager()方法：

protected AuthenticationManager authenticationManager() throws Exception {
        if (!authenticationManagerInitialized) {
            configure(localConfigureAuthenticationBldr);
            if (disableLocalConfigureAuthenticationBldr) {
                authenticationManager = authenticationConfiguration
                        .getAuthenticationManager();
            }
            else {
                authenticationManager = localConfigureAuthenticationBldr.build();
            }
            authenticationManagerInitialized = true;
        }
        return authenticationManager;
    }

在初始化时，它会调用configure(localConfigureAuthenticationBldr);，默认的实现是：

protected void configure(AuthenticationManagerBuilder auth) throws Exception {
        this.disableLocalConfigureAuthenticationBldr = true;
    }

【1、个性化配置入口之configure(AuthenticationManagerBuilder auth)】
我们可以通过继承WebSecurityConfigurerAdapter并重写该方法来个性化配置AuthenticationManager。

构建完authenticationManager 实例后，将它设置为authenticationBuilder的父认证管理器：

authenticationBuilder.parentAuthenticationManager(authenticationManager);

并将该authenticationBuilder传入HttpSecurity构造器构建HttpSecurity实例。

构建完HttpSecurity实例后，默认情况下会添加默认的拦截其配置：

            http
                .csrf().and()
                .addFilter(new WebAsyncManagerIntegrationFilter())
                .exceptionHandling().and()
                .headers().and()
                .sessionManagement().and()
                .securityContext().and()
                .requestCache().and()
                .anonymous().and()
                .servletApi().and()
                .apply(new DefaultLoginPageConfigurer<>()).and()
                .logout();

最后调用configure(http);，这又是一个可个性化的配置入口，它的默认实现是：

protected void configure(HttpSecurity http) throws Exception {
        logger.debug("Using default configure(HttpSecurity). If subclassed this will potentially override subclass configure(HttpSecurity).");

        http
            .authorizeRequests()
                .anyRequest().authenticated()
                .and()
            .formLogin().and()
            .httpBasic();
    }

默认的配置是拦截所有的请求需要认证之后才能访问，如果没有认证，会自动生成一个认证表单要求输入用户名和密码。
【2、个性化配置入口之configure(HttpSecurity http)】
我们可以通过继承WebSecurityConfigurerAdapter并重写该方法来个性化配置HttpSecurity。

OK，目前为止HttpSecurity已经被初始化，接下去需要设置HttpSecurity对象添加至WebSecurity的securityFilterChainBuilders列表中：

web.addSecurityFilterChainBuilder(http).postBuildAction(new Runnable() {
            public void run() {
                FilterSecurityInterceptor securityInterceptor = http
                        .getSharedObject(FilterSecurityInterceptor.class);
                web.securityInterceptor(securityInterceptor);
            }
        });

打开HttpSecurity类结构，和WebSecurity一样，它也实现了SecurityBuilder接口，同样继承自AbstractConfiguredSecurityBuilder。

当所有的WebSecurityConfigurer的init方法被调用之后，webSecurity.init()工作就结束了。
接下去调用了webSecurity.configure()，该方法同样是在AbstractConfiguredSecurityBuilder中实现的：

private void configure() throws Exception {
        Collection<SecurityConfigurer<O, B>> configurers = getConfigurers();

        for (SecurityConfigurer<O, B> configurer : configurers) {
            configurer.configure((B) this);
        }
    }

它的主要工作是迭代调用所有WebSecurityConfigurer的configurer方法，参数是WebSeucrity本身，这又是另外一个重要的个性化入口：
【3、个性化配置入口之configure(WebSecurity web)】
我们可以通过继承WebSecurityConfigurerAdapter并重写该方法来个性化配置WebSecurity。

自此，三个重要的个性化入口都已经被调用，即在实现WebSecurityConfigurerAdapter经常需要重写的：

1、configure(AuthenticationManagerBuilder auth);

2、configure(WebSecurity web);

3、configure(HttpSecurity http);

回到webSecurity构建过程，接下去重要的的调用：

O result = performBuild();

该方法在WebSecurityConfigurerAdapter中实现，返回的就是过滤器FilterChainProxy，源码如下：

@Override
    protected Filter performBuild() throws Exception {
        Assert.state(
                !securityFilterChainBuilders.isEmpty(),
                () -> "At least one SecurityBuilder<? extends SecurityFilterChain> needs to be specified. "
                        + "Typically this done by adding a @Configuration that extends WebSecurityConfigurerAdapter. "
                        + "More advanced users can invoke "
                        + WebSecurity.class.getSimpleName()
                        + ".addSecurityFilterChainBuilder directly");
        int chainSize = ignoredRequests.size() + securityFilterChainBuilders.size();
        List<SecurityFilterChain> securityFilterChains = new ArrayList<>(
                chainSize);
        for (RequestMatcher ignoredRequest : ignoredRequests) {
            securityFilterChains.add(new DefaultSecurityFilterChain(ignoredRequest));
        }
        for (SecurityBuilder<? extends SecurityFilterChain> securityFilterChainBuilder : securityFilterChainBuilders) {
            securityFilterChains.add(securityFilterChainBuilder.build());
        }
        FilterChainProxy filterChainProxy = new FilterChainProxy(securityFilterChains);
        if (httpFirewall != null) {
            filterChainProxy.setFirewall(httpFirewall);
        }
        filterChainProxy.afterPropertiesSet();

        Filter result = filterChainProxy;
        if (debugEnabled) {
            logger.warn("\n\n"
                    + "********************************************************************\n"
                    + "**********        Security debugging is enabled.       *************\n"
                    + "**********    This may include sensitive information.  *************\n"
                    + "**********      Do not use in a production system!     *************\n"
                    + "********************************************************************\n\n");
            result = new DebugFilter(filterChainProxy);
        }
        postBuildAction.run();
        return result;
    }

首先计算出chainSize，也就是ignoredRequests.size() + securityFilterChainBuilders.size();，如果你不配置ignoredRequests，那就是securityFilterChainBuilders.size()，也就是HttpSecurity的个数，其本质上就是你一共配置几个WebSecurityConfigurerAdapter，因为每个WebSecurityConfigurerAdapter对应一个HttpSecurity，而所谓的ignoredRequests就是FilterChainProxy的请求，默认是没有的，如果你需要条跳过某些请求不需要认证或授权，可以如下配置：

@Override
    public void configure(WebSecurity web) throws Exception {
        web.ignoring().antMatchers("/statics/**");
    }

在上面配置中，所有以/statics开头请求都将被FilterChainProxy忽略。

计算完chainSize后，就会创建List<SecurityFilterChain> securityFilterChains = new ArrayList<>(chainSize);，遍历所有的HttpSecurity，调用HtppSecurity的build()构建其对应的过滤器链SecurityFilterChain实例，并将SecurityFilterChain添加到securityFilterChains 列表中：

for (SecurityBuilder<? extends SecurityFilterChain> securityFilterChainBuilder : securityFilterChainBuilders) {
            securityFilterChains.add(securityFilterChainBuilder.build());
        }

调用HtppSecurity的build()构建其实和调用WebSecurity的build()构建类类似，父类中方法一次被执行，最后执行本身的performBuild()方法，其源码如下：

@Override
    protected DefaultSecurityFilterChain performBuild() throws Exception {
        Collections.sort(filters, comparator);
        return new DefaultSecurityFilterChain(requestMatcher, filters);
    }

构建SecurityFilterChain主要是完成RequestMatcher和对应的过滤器列表，我们都知道在Spring Security中，过滤器执行按顺序顺序的，这个排序就是在performBuild()中完成的，也就是：

Collections.sort(filters, comparator);

它通过一个比较器实现了过滤器的排序，这个比较器就是FilterComparator，有兴趣的朋友可以自己去了解详情。
最后返回的是SecurityFilterChain的默认实现DefaultSecurityFilterChain。

构建完所有SecurityFilterChain后，创建最为重要的FilterChainProxy实例，

FilterChainProxy filterChainProxy = new FilterChainProxy(securityFilterChains);

构造器中传入SecurityFilterChain列表，如果开启了Debug模式，还会被包装成DebugFilter类型，共开发调试使用，默认是关闭的，可以通过过下面方式开启Debug模式：

@Override
    public void configure(WebSecurity web) throws Exception {
        web.debug(true);
    }

至此Spring Security 初始化完成，我们通过继承WebSecurityConfigurerAdapter来代达到个性化配置目的，文中提到了三个重要的个性化入口，并且WebSecurityConfigurerAdapter是可以配置多个的，其对应的接口就是会存在多个SecurityFilterChain实例，但是它们人仍然在同一个FilterChainProxy中，通过RequestMatcher来匹配并传入到对应的SecurityFilterChain中执行请求。