Spring Security 启动过程分析 从@EnableWebSecurity开始说起

准备环境

本文以oauth2+github登录为例，首先创建一个Spring Boot工程，版本为2.1.0.RELEASE，工程目录结构如下图：

pom.xml如下：

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-security</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.security</groupId>
    <artifactId>spring-security-oauth2-client</artifactId>
    <version>5.1.2.RELEASE</version>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
</dependency>

启动类SecurityApplication.java：

/**
 * @author iHelin
 */
@RestController
@SpringBootApplication
public class SecurityApplication {

    public static void main(String[] args) {
        SpringApplication.run(SecurityApplication.class, args);
    }

    @GetMapping({"/", "/user"})
    public Object get() {
        OAuth2AuthenticationToken authentication = (OAuth2AuthenticationToken) SecurityContextHolder.getContext().getAuthentication();
        OAuth2User principal = authentication.getPrincipal();
        return principal.getAttributes();
    }

}

SercurityConfig.java：

/**
 * @author iHelin
 * @date 2018-11-30 15:47
 */
@EnableWebSecurity(debug = true)
public class SercurityConfig extends WebSecurityConfigurerAdapter {

    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http.csrf().disable()
                .authorizeRequests().anyRequest().authenticated().and()
                .oauth2Login();
    }

}

配置文件application.properties

server.port=8080
logging.level.org.springframework.security=debug
logging.level.org.springframework.boot.autoconfigure.security=debug
spring.security.oauth2.client.registration.github.client-id=xxxxxx
spring.security.oauth2.client.registration.github.client-secret=xxxxxx

从@EnableWebSecurity开始说起

SercurityConfig是一个配置类，它继承了WebSecurityConfigurerAdapter，并标明了@EnableWebSecurity(debug = true)注解，查看这个注解发现，里面又导入(import)了WebSecurityConfiguration.class这个配置类，如下图：

WebSecurityConfiguration是一个自动配置类，它的主要作用创建过滤器链(securityFilterChains)并完成安全配置工作，而这一系列过程主要是通过webSecurity完成的。
系统启动时Spring上下文会首先调用它setFilterChainProxySecurityConfigurer方法进行webSecurity的初始化，这一步通过反射完成（当然，这不是我们的重点）。然后再调用springSecurityFilterChain进行webSecurity的配置，具体步骤如下：
首先进入springSecurityFilterChain方法

接着调用org.springframework.security.config.annotation.AbstractSecurityBuilder#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");
}

cas操作进入if语句，进入关键的org.springframework.security.config.annotation.AbstractConfiguredSecurityBuilder#doBuild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;
    }
}

里面是一个同步的代码块，不过这也不是重点，核心在init和performBuild方法，注意，到现在我们的主语还是webSecurity。首先来看看init方法：

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);
    }
}

这里主要看第一个for循环，里面会进行一些配置的初始化，其中会有一个我们继承的WebSecurityConfigurerAdapter的代理，其实也就是我们自己定义的安全配置类SercurityConfig，调用其init方法：

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);
        }
    });
}

看下getHttp：

protected final HttpSecurity getHttp() throws Exception {
    ...
    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
        ...
    }
    configure(http);
    return http;
}

里面首先进行默认的配置，这里添加了一个Filter：WebAsyncManagerIntegrationFilter，继续向下执行，会执行configure方法，它是一个模板方法，也就是这里会执行我们配置类里面覆盖的configure方法，这里就完成了httpSecurity的初始化。
以上步骤都只是webSecurity的init操作，也就是创建了许多的配置器，接下来进入webSecurity的performBuild方法使配置生效，具体过程是调用httpSecurity的config方法，里面会调用上面创建的众多配置器的configure方法，其目的是向过滤器链添加各种Filter，最后还会调用performBuild方法对过滤器进行排序，创建DefaultSecurityFilterChain过滤器链，这里以ExceptionHandlingConfigurer为例。

public void configure(H http) throws Exception {
    AuthenticationEntryPoint entryPoint = getAuthenticationEntryPoint(http);
    ExceptionTranslationFilter exceptionTranslationFilter = new ExceptionTranslationFilter(entryPoint, getRequestCache(http));
    AccessDeniedHandler deniedHandler = getAccessDeniedHandler(http);
    exceptionTranslationFilter.setAccessDeniedHandler(deniedHandler);
    exceptionTranslationFilter = postProcess(exceptionTranslationFilter);
    http.addFilter(exceptionTranslationFilter);
}

关键看最后的addFilter方法

public HttpSecurity addFilter(Filter filter) {
    Class <? extends Filter > filterClass = filter.getClass();
    if (!comparator.isRegistered(filterClass)) {
        throw new IllegalArgumentException("The Filter class " + filterClass.getName() + " does not have a registered order and cannot be added without a specified order. Consider using addFilterBefore or addFilterAfter instead.");
    }
    this.filters.add(filter);
    return this;
}

最终向httpSecurity对象的filters中添加filter。
然后再调用httpSecurity的performBuild方法对filters进行排序：

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

最后返回了一个DefaultSecurityFilterChain对象，至此http的配置宣告完成。再回到webSecurity的performBuild方法，它根据httpSecurity返回的securityFilterChain创建了一个securityFilterChains。

protected Filter performBuild() throws Exception {
    ...
    for (RequestMatcher ignoredRequest: ignoredRequests) {
        securityFilterChains.add(new DefaultSecurityFilterChain(ignoredRequest));
    }
    for (SecurityBuilder <? extends SecurityFilterChain > securityFilterChainBuilder: securityFilterChainBuilders) {
        securityFilterChains.add(securityFilterChainBuilder.build());
    }
    FilterChainProxy filterChainProxy = new FilterChainProxy(securityFilterChains);
    ...
    Filter result = filterChainProxy;
    ...
    return result;
}

可以看到filterChainProxy本质上也是一个filter，它最终返回给容器。这样webSecurity的配置也基本完成。

以上。