SpringCloud集成了很多第三方框架，把它的全部源码拿出来解析几本书都讲不完，也不太现实，本文带领读者分析其中一小部分源码（其余源码读者有兴趣可以继续跟进），包括eureka-server、config、zuul的starter部分，分析其启动原理。 
如果我们开发出一套框架，要和SpringBoot集成，就需要放到它的starter里。因此我们分析启动原理，直接从每个框架的starter开始分析即可。

Eureka-Server源码解析

我们知道，要实现注册与发现，需要在启动类加上@EnableEurekaServer注解，我们进入其源码：

@EnableDiscoveryClient//表示eurekaserver也是一个客户端服务
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(EurekaServerMarkerConfiguration.class)
public @interface EnableEurekaServer {

}

注意看@Import注解，这个注解导入了EurekaServerMarkerConfiguration类，继续跟进这个类：

/**
 * Responsible for adding in a marker bean to activate
 * {@link EurekaServerAutoConfiguration}
 *
 * @author Biju Kunjummen
 */
@Configuration
public class EurekaServerMarkerConfiguration {

    @Bean
    public Marker eurekaServerMarkerBean() {
        return new Marker();
    }

    class Marker {
    }
}

通过上面的注释，我们继续查看EurekaServerAutoConfiguration类的源码：

@Configuration
@Import(EurekaServerInitializerConfiguration.class)
@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)
@EnableConfigurationProperties({ EurekaDashboardProperties.class,
        InstanceRegistryProperties.class })
@PropertySource("classpath:/eureka/server.properties")
public class EurekaServerAutoConfiguration extends WebMvcConfigurerAdapter {
    /**
     * List of packages containing Jersey resources required by the Eureka server
     */
    private static String[] EUREKA_PACKAGES = new String[] { "com.netflix.discovery",
            "com.netflix.eureka" };

    @Autowired
    private ApplicationInfoManager applicationInfoManager;

    @Autowired
    private EurekaServerConfig eurekaServerConfig;

    @Autowired
    private EurekaClientConfig eurekaClientConfig;

    @Autowired
    private EurekaClient eurekaClient;

    @Autowired
    private InstanceRegistryProperties instanceRegistryProperties;

    public static final CloudJacksonJson JACKSON_JSON = new CloudJacksonJson();

    @Bean
    public HasFeatures eurekaServerFeature() {
        return HasFeatures.namedFeature("Eureka Server",
                EurekaServerAutoConfiguration.class);
    }

    //如果eureka.client.registerWithEureka=true，则把自己注册进去
    @Configuration
    protected static class EurekaServerConfigBeanConfiguration {
        @Bean
        @ConditionalOnMissingBean
        public EurekaServerConfig eurekaServerConfig(EurekaClientConfig clientConfig) {
            EurekaServerConfigBean server = new EurekaServerConfigBean();
            if (clientConfig.shouldRegisterWithEureka()) {
                // Set a sensible default if we are supposed to replicate
                server.setRegistrySyncRetries(5);
            }
            return server;
        }
    }

    //实例化eureka-server的界面
    @Bean
    @ConditionalOnProperty(prefix = "eureka.dashboard", name = "enabled", matchIfMissing = true)
    public EurekaController eurekaController() {
        return new EurekaController(this.applicationInfoManager);
    }

    static {
        CodecWrappers.registerWrapper(JACKSON_JSON);
        EurekaJacksonCodec.setInstance(JACKSON_JSON.getCodec());
    }

    @Bean
    public ServerCodecs serverCodecs() {
        return new CloudServerCodecs(this.eurekaServerConfig);
    }

    private static CodecWrapper getFullJson(EurekaServerConfig serverConfig) {
        CodecWrapper codec = CodecWrappers.getCodec(serverConfig.getJsonCodecName());
        return codec == null ? CodecWrappers.getCodec(JACKSON_JSON.codecName()) : codec;
    }

    private static CodecWrapper getFullXml(EurekaServerConfig serverConfig) {
        CodecWrapper codec = CodecWrappers.getCodec(serverConfig.getXmlCodecName());
        return codec == null ? CodecWrappers.getCodec(CodecWrappers.XStreamXml.class)
                : codec;
    }

    class CloudServerCodecs extends DefaultServerCodecs {

        public CloudServerCodecs(EurekaServerConfig serverConfig) {
            super(getFullJson(serverConfig),
                    CodecWrappers.getCodec(CodecWrappers.JacksonJsonMini.class),
                    getFullXml(serverConfig),
                    CodecWrappers.getCodec(CodecWrappers.JacksonXmlMini.class));
        }
    }

    @Bean
    public PeerAwareInstanceRegistry peerAwareInstanceRegistry(
            ServerCodecs serverCodecs) {
        this.eurekaClient.getApplications(); // force initialization
        return new InstanceRegistry(this.eurekaServerConfig, this.eurekaClientConfig,
                serverCodecs, this.eurekaClient,
                this.instanceRegistryProperties.getExpectedNumberOfRenewsPerMin(),
                this.instanceRegistryProperties.getDefaultOpenForTrafficCount());
    }

    @Bean
    @ConditionalOnMissingBean
    public PeerEurekaNodes peerEurekaNodes(PeerAwareInstanceRegistry registry,
            ServerCodecs serverCodecs) {
        return new PeerEurekaNodes(registry, this.eurekaServerConfig,
                this.eurekaClientConfig, serverCodecs, this.applicationInfoManager);
    }

    @Bean
    public EurekaServerContext eurekaServerContext(ServerCodecs serverCodecs,
            PeerAwareInstanceRegistry registry, PeerEurekaNodes peerEurekaNodes) {
        return new DefaultEurekaServerContext(this.eurekaServerConfig, serverCodecs,
                registry, peerEurekaNodes, this.applicationInfoManager);
    }

    @Bean
    public EurekaServerBootstrap eurekaServerBootstrap(PeerAwareInstanceRegistry registry,
            EurekaServerContext serverContext) {
        return new EurekaServerBootstrap(this.applicationInfoManager,
                this.eurekaClientConfig, this.eurekaServerConfig, registry,
                serverContext);
    }

    /**
     * Register the Jersey filter
     */
    @Bean
    public FilterRegistrationBean jerseyFilterRegistration(
            javax.ws.rs.core.Application eurekaJerseyApp) {
        FilterRegistrationBean bean = new FilterRegistrationBean();
        bean.setFilter(new ServletContainer(eurekaJerseyApp));
        bean.setOrder(Ordered.LOWEST_PRECEDENCE);
        bean.setUrlPatterns(
                Collections.singletonList(EurekaConstants.DEFAULT_PREFIX + "/*"));

        return bean;
    }

    /**
     * Construct a Jersey {@link javax.ws.rs.core.Application} with all the resources
     * required by the Eureka server.
     */
    @Bean
    public javax.ws.rs.core.Application jerseyApplication(Environment environment,
            ResourceLoader resourceLoader) {

        ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(
                false, environment);

        // Filter to include only classes that have a particular annotation.
        //
        provider.addIncludeFilter(new AnnotationTypeFilter(Path.class));
        provider.addIncludeFilter(new AnnotationTypeFilter(Provider.class));

        // Find classes in Eureka packages (or subpackages)
        //
        Set<Class<?>> classes = new HashSet<Class<?>>();
        for (String basePackage : EUREKA_PACKAGES) {
            Set<BeanDefinition> beans = provider.findCandidateComponents(basePackage);
            for (BeanDefinition bd : beans) {
                Class<?> cls = ClassUtils.resolveClassName(bd.getBeanClassName(),
                        resourceLoader.getClassLoader());
                classes.add(cls);
            }
        }

        // Construct the Jersey ResourceConfig
        //
        Map<String, Object> propsAndFeatures = new HashMap<String, Object>();
        propsAndFeatures.put(
                // Skip static content used by the webapp
                ServletContainer.PROPERTY_WEB_PAGE_CONTENT_REGEX,
                EurekaConstants.DEFAULT_PREFIX + "/(fonts|images|css|js)/.*");

        DefaultResourceConfig rc = new DefaultResourceConfig(classes);
        rc.setPropertiesAndFeatures(propsAndFeatures);

        return rc;
    }

    @Bean
    public FilterRegistrationBean traceFilterRegistration(
            @Qualifier("webRequestLoggingFilter") Filter filter) {
        FilterRegistrationBean bean = new FilterRegistrationBean();
        bean.setFilter(filter);
        bean.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
        return bean;
    }
}

这个类上有一个注解：@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)，这后面指定的类就是刚才那个类，而@ConditionalOnBean这个注解的作用是：仅仅在当前上下文中存在某个对象时，才会实例化一个Bean。 
因此，启动时就会实例化EurekaServerAutoConfiguration这个类。 
@EnableConfigurationProperties({ EurekaDashboardProperties.class, 
InstanceRegistryProperties.class }) 
这个注解就是定义了一些eureka的配置项。

Config源码解析

通过上面的方法，我们找到了ConfigServerAutoConfiguration类：

@Configuration
@ConditionalOnBean(ConfigServerConfiguration.Marker.class)
@EnableConfigurationProperties(ConfigServerProperties.class)
@Import({ EnvironmentRepositoryConfiguration.class, CompositeConfiguration.class, ResourceRepositoryConfiguration.class,
        ConfigServerEncryptionConfiguration.class, ConfigServerMvcConfiguration.class, TransportConfiguration.class })
public class ConfigServerAutoConfiguration {

}

可以发现这个类是空的，只是多了几个注解， 
@EnableConfigurationProperties(ConfigServerProperties.class)表示开启config配置属性。 
最核心的注解是：@Import，他将其他一些配置类导入进这个类，其中： 
EnvironmentRepositoryConfiguration：环境配置类，内置了以下几种环境配置： 
1.Native

@Configuration
    @Profile("native")
    protected static class NativeRepositoryConfiguration {

        @Autowired
        private ConfigurableEnvironment environment;

        @Bean
        public NativeEnvironmentRepository nativeEnvironmentRepository() {
            return new NativeEnvironmentRepository(this.environment);
        }
    }

2.git

@Configuration
    @Profile("git")
    protected static class GitRepositoryConfiguration extends DefaultRepositoryConfiguration {}

3.subversion

@Configuration
    @Profile("subversion")
    protected static class SvnRepositoryConfiguration {
        @Autowired
        private ConfigurableEnvironment environment;

        @Autowired
        private ConfigServerProperties server;

        @Bean
        public SvnKitEnvironmentRepository svnKitEnvironmentRepository() {
            SvnKitEnvironmentRepository repository = new SvnKitEnvironmentRepository(this.environment);
            if (this.server.getDefaultLabel()!=null) {
                repository.setDefaultLabel(this.server.getDefaultLabel());
            }
            return repository;
        }
    }

4.vault

@Configuration
    @Profile("subversion")
    protected static class SvnRepositoryConfiguration {
        @Autowired
        private ConfigurableEnvironment environment;

        @Autowired
        private ConfigServerProperties server;

        @Bean
        public SvnKitEnvironmentRepository svnKitEnvironmentRepository() {
            SvnKitEnvironmentRepository repository = new SvnKitEnvironmentRepository(this.environment);
            if (this.server.getDefaultLabel()!=null) {
                repository.setDefaultLabel(this.server.getDefaultLabel());
            }
            return repository;
    }
    }

//从代码可以看到Git是配置中心默认环境

@Bean
        public MultipleJGitEnvironmentRepository defaultEnvironmentRepository() {
            MultipleJGitEnvironmentRepository repository = new MultipleJGitEnvironmentRepository(this.environment);
            repository.setTransportConfigCallback(this.transportConfigCallback);
            if (this.server.getDefaultLabel()!=null) {
                repository.setDefaultLabel(this.server.getDefaultLabel());
            }
            return repository;
        }

我们进入MultipleJGitEnvironmentRepository类：

@ConfigurationProperties("spring.cloud.config.server.git")
public class MultipleJGitEnvironmentRepository extends JGitEnvironmentRepository {
}

这个类表示可以支持配置多个Git仓库，它继承自JGitEnvironmentRepository类：

public class JGitEnvironmentRepository extends AbstractScmEnvironmentRepository
        implements EnvironmentRepository, SearchPathLocator, InitializingBean {
/**
     * Get the working directory ready.
     */
    public String refresh(String label) {
        Git git = null;
        try {
            git = createGitClient();
            if (shouldPull(git)) {
                fetch(git, label);
                // checkout after fetch so we can get any new branches, tags,
                // ect.
                checkout(git, label);
                if (isBranch(git, label)) {
                    // merge results from fetch
                    merge(git, label);
                    if (!isClean(git)) {
                        logger.warn("The local repository is dirty. Resetting it to origin/" + label + ".");
                        resetHard(git, label, "refs/remotes/origin/" + label);
                    }
                }
            } else {
                // nothing to update so just checkout
                checkout(git, label);
            }
            // always return what is currently HEAD as the version
            return git.getRepository().getRef("HEAD").getObjectId().getName();
        } catch (RefNotFoundException e) {
            throw new NoSuchLabelException("No such label: " + label, e);
        } catch (NoRemoteRepositoryException e) {
            throw new NoSuchRepositoryException("No such repository: " + getUri(), e);
        } catch (GitAPIException e) {
            throw new NoSuchRepositoryException("Cannot clone or checkout repository: " + getUri(), e);
        } catch (Exception e) {
            throw new IllegalStateException("Cannot load environment", e);
        } finally {
            try {
                if (git != null) {
                    git.close();
                }
            } catch (Exception e) {
                this.logger.warn("Could not close git repository", e);
            }
        }
    }
}

refresh方法的作用就是configserver会从我们配置的git仓库拉取配置下来。

Zuul源码解析

同理，我们找到Zuul的配置类ZuulProxyAutoConfiguration：

@Configuration
@Import({ RibbonCommandFactoryConfiguration.RestClientRibbonConfiguration.class,
        RibbonCommandFactoryConfiguration.OkHttpRibbonConfiguration.class,
        RibbonCommandFactoryConfiguration.HttpClientRibbonConfiguration.class })
@ConditionalOnBean(ZuulProxyMarkerConfiguration.Marker.class)
public class ZuulProxyAutoConfiguration extends ZuulServerAutoConfiguration {

    @SuppressWarnings("rawtypes")
    @Autowired(required = false)
    private List<RibbonRequestCustomizer> requestCustomizers = Collections.emptyList();

    @Autowired
    private DiscoveryClient discovery;

    @Autowired
    private ServiceRouteMapper serviceRouteMapper;

    @Override
    public HasFeatures zuulFeature() {
        return HasFeatures.namedFeature("Zuul (Discovery)", ZuulProxyAutoConfiguration.class);
    }

    @Bean
    @ConditionalOnMissingBean(DiscoveryClientRouteLocator.class)
    public DiscoveryClientRouteLocator discoveryRouteLocator() {
        return new DiscoveryClientRouteLocator(this.server.getServletPrefix(), this.discovery, this.zuulProperties,
                this.serviceRouteMapper);
    }
    //以下是过滤器，也就是之前zuul提到的实现的ZuulFilter接口
    // pre filters
    //路由之前
    @Bean
    public PreDecorationFilter preDecorationFilter(RouteLocator routeLocator, ProxyRequestHelper proxyRequestHelper) {
        return new PreDecorationFilter(routeLocator, this.server.getServletPrefix(), this.zuulProperties,
                proxyRequestHelper);
    }

    // route filters
    // 路由时
    @Bean
    public RibbonRoutingFilter ribbonRoutingFilter(ProxyRequestHelper helper,
            RibbonCommandFactory<?> ribbonCommandFactory) {
        RibbonRoutingFilter filter = new RibbonRoutingFilter(helper, ribbonCommandFactory, this.requestCustomizers);
        return filter;
    }

    @Bean
    @ConditionalOnMissingBean(SimpleHostRoutingFilter.class)
    public SimpleHostRoutingFilter simpleHostRoutingFilter(ProxyRequestHelper helper, ZuulProperties zuulProperties) {
        return new SimpleHostRoutingFilter(helper, zuulProperties);
    }

    @Bean
    public ApplicationListener<ApplicationEvent> zuulDiscoveryRefreshRoutesListener() {
        return new ZuulDiscoveryRefreshListener();
    }

    @Bean
    @ConditionalOnMissingBean(ServiceRouteMapper.class)
    public ServiceRouteMapper serviceRouteMapper() {
        return new SimpleServiceRouteMapper();
    }

    @Configuration
    @ConditionalOnMissingClass("org.springframework.boot.actuate.endpoint.Endpoint")
    protected static class NoActuatorConfiguration {

        @Bean
        public ProxyRequestHelper proxyRequestHelper(ZuulProperties zuulProperties) {
            ProxyRequestHelper helper = new ProxyRequestHelper();
            helper.setIgnoredHeaders(zuulProperties.getIgnoredHeaders());
            helper.setTraceRequestBody(zuulProperties.isTraceRequestBody());
            return helper;
        }

    }

    @Configuration
    @ConditionalOnClass(Endpoint.class)
    protected static class RoutesEndpointConfiguration {

        @Autowired(required = false)
        private TraceRepository traces;

        @Bean
        public RoutesEndpoint zuulEndpoint(RouteLocator routeLocator) {
            return new RoutesEndpoint(routeLocator);
        }

        @Bean
        public RoutesMvcEndpoint zuulMvcEndpoint(RouteLocator routeLocator, RoutesEndpoint endpoint) {
            return new RoutesMvcEndpoint(endpoint, routeLocator);
        }

        @Bean
        public ProxyRequestHelper proxyRequestHelper(ZuulProperties zuulProperties) {
            TraceProxyRequestHelper helper = new TraceProxyRequestHelper();
            if (this.traces != null) {
                helper.setTraces(this.traces);
            }
            helper.setIgnoredHeaders(zuulProperties.getIgnoredHeaders());
            helper.setTraceRequestBody(zuulProperties.isTraceRequestBody());
            return helper;
        }
    }

    private static class ZuulDiscoveryRefreshListener implements ApplicationListener<ApplicationEvent> {

        private HeartbeatMonitor monitor = new HeartbeatMonitor();

        @Autowired
        private ZuulHandlerMapping zuulHandlerMapping;

        @Override
        public void onApplicationEvent(ApplicationEvent event) {
            if (event instanceof InstanceRegisteredEvent) {
                reset();
            }
            else if (event instanceof ParentHeartbeatEvent) {
                ParentHeartbeatEvent e = (ParentHeartbeatEvent) event;
                resetIfNeeded(e.getValue());
            }
            else if (event instanceof HeartbeatEvent) {
                HeartbeatEvent e = (HeartbeatEvent) event;
                resetIfNeeded(e.getValue());
            }

        }

        private void resetIfNeeded(Object value) {
            if (this.monitor.update(value)) {
                reset();
            }
        }

        private void reset() {
            this.zuulHandlerMapping.setDirty(true);
        }
    }
}

通过@Import注解可以找到几个类： 
RibbonCommandFactoryConfiguration.RestClientRibbonConfiguration、RibbonCommandFactoryConfiguration.OkHttpRibbonConfiguration、RibbonCommandFactoryConfiguration.HttpClientRibbonConfiguration 
我们知道zuul提供网关能力，通过上面这几个类就能分析到，它内部其实也是通过接口请求，找到每个服务提供的接口地址。 
进入RibbonCommandFactoryConfiguration类：

public class RibbonCommandFactoryConfiguration {
    //以下提供了3个不同的请求模式
    @Configuration
    @ConditionalOnRibbonRestClient
    protected static class RestClientRibbonConfiguration {

        @Autowired(required = false)
        private Set<ZuulFallbackProvider> zuulFallbackProviders = Collections.emptySet();

        @Bean
        @ConditionalOnMissingBean
        public RibbonCommandFactory<?> ribbonCommandFactory(
                SpringClientFactory clientFactory, ZuulProperties zuulProperties) {
            return new RestClientRibbonCommandFactory(clientFactory, zuulProperties,
                    zuulFallbackProviders);
        }
    }

    @Configuration
    @ConditionalOnRibbonOkHttpClient
    @ConditionalOnClass(name = "okhttp3.OkHttpClient")
    protected static class OkHttpRibbonConfiguration {

        @Autowired(required = false)
        private Set<ZuulFallbackProvider> zuulFallbackProviders = Collections.emptySet();

        @Bean
        @ConditionalOnMissingBean
        public RibbonCommandFactory<?> ribbonCommandFactory(
                SpringClientFactory clientFactory, ZuulProperties zuulProperties) {
            return new OkHttpRibbonCommandFactory(clientFactory, zuulProperties,
                    zuulFallbackProviders);
        }
    }

    @Configuration
    @ConditionalOnRibbonHttpClient
    protected static class HttpClientRibbonConfiguration {

        @Autowired(required = false)
        private Set<ZuulFallbackProvider> zuulFallbackProviders = Collections.emptySet();

        @Bean
        @ConditionalOnMissingBean
        public RibbonCommandFactory<?> ribbonCommandFactory(
                SpringClientFactory clientFactory, ZuulProperties zuulProperties) {
            return new HttpClientRibbonCommandFactory(clientFactory, zuulProperties, zuulFallbackProviders);
        }
    }

    @Target({ ElementType.TYPE, ElementType.METHOD })
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Conditional(OnRibbonHttpClientCondition.class)
    @interface ConditionalOnRibbonHttpClient { }

    private static class OnRibbonHttpClientCondition extends AnyNestedCondition {
        public OnRibbonHttpClientCondition() {
            super(ConfigurationPhase.PARSE_CONFIGURATION);
        }

        @Deprecated //remove in Edgware"
        @ConditionalOnProperty(name = "zuul.ribbon.httpclient.enabled", matchIfMissing = true)
        static class ZuulProperty {}

        @ConditionalOnProperty(name = "ribbon.httpclient.enabled", matchIfMissing = true)
        static class RibbonProperty {}
    }

    @Target({ ElementType.TYPE, ElementType.METHOD })
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Conditional(OnRibbonOkHttpClientCondition.class)
    @interface ConditionalOnRibbonOkHttpClient { }

    private static class OnRibbonOkHttpClientCondition extends AnyNestedCondition {
        public OnRibbonOkHttpClientCondition() {
            super(ConfigurationPhase.PARSE_CONFIGURATION);
        }

        @Deprecated //remove in Edgware"
        @ConditionalOnProperty("zuul.ribbon.okhttp.enabled")
        static class ZuulProperty {}

        @ConditionalOnProperty("ribbon.okhttp.enabled")
        static class RibbonProperty {}
    }

    @Target({ ElementType.TYPE, ElementType.METHOD })
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Conditional(OnRibbonRestClientCondition.class)
    @interface ConditionalOnRibbonRestClient { }

    private static class OnRibbonRestClientCondition extends AnyNestedCondition {
        public OnRibbonRestClientCondition() {
            super(ConfigurationPhase.PARSE_CONFIGURATION);
        }

        @Deprecated //remove in Edgware"
        @ConditionalOnProperty("zuul.ribbon.restclient.enabled")
        static class ZuulProperty {}

        @ConditionalOnProperty("ribbon.restclient.enabled")
        static class RibbonProperty {}
    }
}

总结

前面带领大家分析了一小段源码，SpringCloud很庞大，不可能一一分析，文本的主要目的就是教大家如何分析源码，从何处下手，以便大家可以按照这种思路继续跟踪下去。