zookeeper之注册中心

科技 08-29 来源：陈三岁98

zookeeper注册中心实现原理

服务注册：

springboot项目启动时，自定义监听器ApplicationListener去监听web服务启动事件
web server启动成功，则触发事件回调方法
回调方法中，在zookeeper指定节点下创建临时节点，临时节点的值保存当前项目启动的 ip + port
如果某个服务宕机，服务断开一定时间(默认30s)临时节点会自动删除

服务发现：

springboot项目启动时，会从zookeeper指定节点获取对应服务的所有可用url列表(可以缓存此url列表)
然后根据负载均衡算法，将请求负载到url列表中的某一个server上
利用spring初始化器扩展机制创建zookeeper节点监听，当节点列表发生变更，则更新url列表缓存

服务注册大体流程

服务注册，等对应的service的容器启动成功，针对微服务项目，一般是spring boot内置的tomcat启动成功，这个服务才可以使用，这个时候才可以将服务注册到zookeeper中。

那么如何知道tomcat容器启动成功了呢？

通过spring的事件监听机制，当tomcat启动成功会发布一个事件，我们可以监听这个事件，当tomcat启动成功做出相应。

Spring事件监听机制

1、手写注册中心

1.1 服务注册

服务注册原理

自定义监听器ApplicationListener去监听springboot项目的内置容器启动事件
一旦springboot项目内置的tomcat启动成功，会触发监听器回调方法
在回调方法中，创建临时节点在zookeeper指定的节点下，当前项目启动的 ip:port 即为节点名称

创建springboot项目order-service

(1) pom.xml


    org.springframework.boot
    spring-boot-starter-parent
    2.6.8
    



    
        org.springframework.boot
        spring-boot-starter
    
    
        org.springframework.boot
        spring-boot-starter-web
    
    
        org.springframework.boot
        spring-boot-starter-test
        test
    

    
        org.apache.curator
        curator-recipes
        5.2.1

(2) 配置文件application.properties

server.ip=192.168.9.1
server.port=9090

# 自定义的配置信息
zk.service-name=order-service
zk.server=192.168.1.104:2181

(3) 创建监听器 ApplicationListener

监听spring web服务器已经初始化完成事件 WebServerInitializedEvent

public class ZkApplicationListener implements ApplicationListener {
    @Override
    public void onApplicationEvent(WebServerInitializedEvent event) {
        System.out.println("事件监听机制的回调...");

        // 获取app.properties配置属性
        Environment environment = event.getApplicationContext().getEnvironment();
        String serviceName = environment.getProperty("zk.service-name");
        String ip = environment.getProperty("server.ip");
        String port = environment.getProperty("server.port");
        String zkServer = environment.getProperty("zk.server");
        // 服务注册
        ServiceRegistry zookeeperServiceRegistry = new ZookeeperServiceRegistry(serviceName,ip,port,zkServer);
        zookeeperServiceRegistry.register();
    }
}

(4) SPI配置spring.factories

# Application Listeners
org.springframework.context.ApplicationListener=\
  com.zk.serviceregistry.orderservice.listener.ZkApplicationListener

(5) 注册服务到zookeeper

// spring cloud 团队提供了服务注册的接口
public interface ServiceRegistry {
    void register();
}

public class ZookeeperServiceRegistry implements ServiceRegistry {

    private CuratorFramework curatorFramework;
    private final String ip;
    private final String port;
    private final String serviceName;
    private final String basePath = "/zk-registry";

    public ZookeeperServiceRegistry(String serviceName, String ip, String port, String zkServer) {
        this.serviceName = serviceName;
        this.ip = ip;
        this.port = port;
        this.curatorFramework = CuratorFrameworkFactory
                .builder()
                .connectionTimeoutMs(20000)
                .connectString(zkServer)
                .retryPolicy(new ExponentialBackoffRetry(1000, 3))
                .build();
        curatorFramework.start();
    }

    @Override
    public void register() {
        // 服务名称
        String serviceNamePath = basePath + "/" + serviceName;
        try {
            if (curatorFramework.checkExists().forPath(serviceNamePath) == null) {
                // 创建持久化的节点，作为服务名称
                this.curatorFramework.create().creatingParentsIfNeeded().withMode(CreateMode.PERSISTENT).forPath(serviceNamePath);
            }
            String urlNode = curatorFramework.create().withMode(CreateMode.EPHEMERAL).forPath(serviceNamePath + "/" + ip + ":" + port);
            System.out.println("服务 " + urlNode + " 成功注册到zookeeper server...");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

(6) 启动服务测试

会发现服务注册已经生效，日志中打印127.0.0.1:9090已经注册到zookeeper server

查看zookeeper，发现创建了新的节点

启动多个服务192.168.9.1:9091，192.168.9.1:9092，192.168.9.1:9093，192.168.9.1:9094，新的服务ip:port也会被依次注册到zookeeper中

停掉某个服务比如192.168.9.1:9094去模拟某个服务宕机的情况，当zookeeper server在一定时间内（默认30s）没有收到来自192.168.9.1:9094服务的反馈时，就会认为此服务已经挂了，会将此服务从zookeeper节点中删除

1.2 服务发现

服务发现原理

项目启动时自动获取zookeeper中配置的需要调用的服务order-service的所有可用url列表
利用zookeeper临时节点特性，如果某个服务节点宕机，那么对应临时节点会在一定时间后自动删除
访问服务user-service时，根据负载均衡算法从可用的服务url列表中获取某个节点url

创建springboot项目user-service

(1) pom.xml


    org.springframework.boot
    spring-boot-starter-parent
    2.6.8
    



    
        org.springframework.boot
        spring-boot-starter-web
    
    
        org.springframework.boot
        spring-boot-starter-test
        test
    

    
        org.apache.curator
        curator-recipes
        5.2.1

(2) 配置文件application.properties

server.port=9999
zk.server=192.168.1.104:2181

(3) SPI配置spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  com.zk.servicediscovery.userservice.config.ZookeeperDiscoveryAutoConfiguration

(4) 自动配置，项目启动时去执行服务发现

@Configuration
public class ZookeeperDiscoveryAutoConfiguration {
    @Resource
    private Environment environment;

    @Bean
    public ServiceDiscoveryImpl serviceDiscovery(){
        return new ServiceDiscoveryImpl(environment.getProperty("zk.server"));
    }
}

(5) 服务发现与监听

public interface ServiceDiscovery {
    // 服务发现：获取所有子节点(所有可用的服务url列表)
    List discovery(String serviceName);
    // 注册监听：当子节点发生变更(代表有新服务添加或者有服务宕机)，则会触发监听，更新服务url列表
    void registerWatch(String serviceNamePath);
}

public class ServiceDiscoveryImpl implements ServiceDiscovery {
    private final CuratorFramework curatorFramework;
    private final String basePath = "/zk-registry";

    public ServiceDiscoveryImpl(String zkServer) {
        this.curatorFramework = CuratorFrameworkFactory
                .builder()
                .connectionTimeoutMs(20000)
                .connectString(zkServer)
                .retryPolicy(new ExponentialBackoffRetry(1000, 3))
                .build();
        curatorFramework.start();
    }

    @Override
    public List discovery(String serviceName) {
        // /zk-registry/order-service
        String serviceNamePath = basePath + "/" + serviceName;
        try {
            if (this.curatorFramework.checkExists().forPath(serviceNamePath) != null) {
                return this.curatorFramework.getChildren().forPath(serviceNamePath);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }

    public void registerWatch(String serviceNamePath) {
        // 永久的监听，当/zk-registry/order-service下的子节点变动，则更新
        CuratorCache curatorCache = CuratorCache.build(curatorFramework, serviceNamePath);
        CuratorCacheListener listener = CuratorCacheListener.builder().forPathChildrenCache(serviceNamePath, curatorFramework, new PathChildrenCacheListener() {
            @Override
            public void childEvent(CuratorFramework client, PathChildrenCacheEvent event) throws Exception {
                // 拉模式
                System.out.println("最新的urls为: " + curatorFramework.getChildren().forPath(serviceNamePath));
            }
        }).build();
        curatorCache.listenable().addListener(listener);
        curatorCache.start();
    }
}

(6) 随机访问某个服务节点：模拟负载均衡

public interface LoadBalance {
    String select(List urls);
}

public class RandomLoadBalance implements LoadBalance{
    @Override
    public String select(List urls) {
        int len=urls.size();
        Random random=new Random();
        return urls.get(random.nextInt(len));
    }
}

(7) UserController模拟请求

@RestController
@RequestMapping("/user")
public class UserController {
    @Autowired
    private ServiceDiscovery serviceDiscovery;

    @RequestMapping("/discovery")
    public void discovery() throws IOException {
        List urls= this.serviceDiscovery.discovery("order-service");
        LoadBalance loadBalance=new RandomLoadBalance();
        String url = loadBalance.select(urls);
        System.out.println("获取可用的order-service服务节点路径为: "+url);
        String response = new RestTemplate().getForObject("http://" + url + "/order/query", String.class);
        System.out.println("order-service response: "+response);
        // 添加对节点order-service的监听
        this.serviceDiscovery.registerWatch("/zk-registry/order-service");
    }
}

(8) 测试

访问 http://192.168.9.1:9999/user/discovery 测试

停掉order-service某个服务节点，不需要重启，再次访问user-service

2、Spring Cloud Zookeeper实现注册中心

2.1 示例代码

创建spring-cloud-zookeeper的spring boot项目，Spring Boot版本为2.6.8

(1) pom.xml


    org.springframework.boot
    spring-boot-starter-parent
    2.6.8
     



    
        org.springframework.boot
        spring-boot-starter-web
    
    
        org.springframework.boot
        spring-boot-starter-test
        test
    

    
    
        org.springframework.cloud
        spring-cloud-starter-zookeeper-config
    

    
    
        org.springframework.cloud
        spring-cloud-starter-bootstrap
    

    
    
        org.springframework.cloud
        spring-cloud-starter-zookeeper-discovery
    





    
        
        
            org.springframework.cloud
            spring-cloud-dependencies
            2021.0.3
            import
            pom

(2) 配置文件

application.properties

server.port=9091
spring.cloud.zookeeper.connect-string=192.168.1.104:2181
spring.cloud.zookeeper.discovery.root=/services/registries
spring.application.name=spring-cloud-zookeeper

或者application.yml

spring:
  cloud:
    zookeeper:
      connect-string: 192.168.1.104:2181
      discovery:
        root: /services/registries
  application:
    name: spring-cloud-zookeeper
server:
  port: 9091

(3) bootstrap.yaml

spring:
  profiles:
    active: dev
  application:
    name: spring-cloud-zookeeper    # 找哪一个ZNode节点  spring-cloud-zookeeper-dev
  cloud:
    zookeeper:
      config:
        root: config    # 相当于 /zk-config/spring-cloud-zookeeper-dev
        profile-separator: "-"
        enabled: true
      connect-string: 192.168.1.104:2181

(4) 启动Spring Boot项目，观察Zookeeper Server上的数据

可以发现zookeeper server上自动创建了对应的节点

(5) 服务发现代码

@RestController
public class SpringCloudZkDiscoveryController {
    // 1.注入服务发现客户端接口
    @Autowired
    private DiscoveryClient discoveryClient;

    @RequestMapping("/sc-zk-discovery")
    public List serviceUrl() {
        // 2.调用getInstances方法可获得所有可用实例
        List instances = discoveryClient.getInstances("spring-cloud-zookeeper");
        String url = instances.get(0).getUri().toString();
        System.out.println("url=" + url);
        return discoveryClient.getInstances("spring-cloud-zookeeper");
    }
}

访问测试

2.2 Spring Cloud Zookeeper注册中心实现原理

(1) 监听器AbstractAutoServiceRegistration监听web容器启动

类似于我们手动实现服务注册，Spring Cloud也自定义了一个监听器 AbstractAutoServiceRegistration 去监听 web服务器启动事件 WebServerInitializedEvent

org.springframework.cloud.client.serviceregistry.AbstractAutoServiceRegistration#onApplicationEvent(WebServerInitializedEvent)源代码片段：

public abstract class AbstractAutoServiceRegistration implements AutoServiceRegistration, ApplicationContextAware, ApplicationListener {

    private final ServiceRegistry serviceRegistry;
    // .................

    public void onApplicationEvent(WebServerInitializedEvent event) {
        this.bind(event);
    }

    @Deprecated
    public void bind(WebServerInitializedEvent event) {
        ApplicationContext context = event.getApplicationContext();
        if (!(context instanceof ConfigurableWebServerApplicationContext) || !"management".equals(((ConfigurableWebServerApplicationContext)context).getServerNamespace())) {
            this.port.compareAndSet(0, event.getWebServer().getPort());
            this.start();
        }
    }

    public void start() {
        if (!this.isEnabled()) {
        } else {
            if (!this.running.get()) {
                this.context.publishEvent(new InstancePreRegisteredEvent(this, this.getRegistration()));
                this.register();
                if (this.shouldRegisterManagement()) {
                    this.registerManagement();
                }

                this.context.publishEvent(new InstanceRegisteredEvent(this, this.getConfiguration()));
                this.running.compareAndSet(false, true);
            }
        }
    }
}

(2) 服务注册与发现

getRegistration() 获取具体注册实现类

org.springframework.cloud.zookeeper.serviceregistry.ZookeeperAutoServiceRegistration

serviceRegistry.register(registration) 具体服务注册实现

public class ZookeeperServiceRegistry implements ServiceRegistry, SmartInitializingSingleton, Closeable {
    protected CuratorFramework curator;
    protected ZookeeperDiscoveryProperties properties;
    private ServiceDiscovery serviceDiscovery;

    public void register(ZookeeperRegistration registration) {
        try {
            this.getServiceDiscovery().registerService(registration.getServiceInstance());
        } catch (Exception var3) {
            ReflectionUtils.rethrowRuntimeException(var3);
        }
    }
}

服务发现实现类：创建zookeeper节点，创建节点监听

public class ServiceDiscoveryImpl implements ServiceDiscovery {
    public void registerService(ServiceInstance service) throws Exception {
        ServiceDiscoveryImpl.Entry newEntry = new ServiceDiscoveryImpl.Entry(service);
        ServiceDiscoveryImpl.Entry oldEntry = (ServiceDiscoveryImpl.Entry)this.services.putIfAbsent(service.getId(), newEntry);
        ServiceDiscoveryImpl.Entry useEntry = oldEntry != null ? oldEntry : newEntry;
        synchronized(useEntry) {
            if (useEntry == newEntry) {
                // 创建节点监听
                useEntry.cache = this.makeNodeCache(service);
            }
            // 创建zookeeper节点
            this.internalRegisterService(service);
        }
    }
    // 创建节点监听
    private CuratorCacheBridge makeNodeCache(ServiceInstance instance) {
        if (!this.watchInstances) {
            return null;
        } else {
            CuratorCacheBridge cache = CuratorCache.bridgeBuilder(this.client, this.pathForInstance(instance.getName(), instance.getId())).withOptions(new Options[]{Options.SINGLE_NODE_CACHE}).withDataNotCached().build();
            CuratorCacheListener listener = CuratorCacheListener.builder().afterInitialized().forAll((__, ___, data) -> {
                if (data != null) {
                    try {
                        ServiceInstance newInstance = this.serializer.deserialize(data.getData());
                        ServiceDiscoveryImpl.Entry entry = (ServiceDiscoveryImpl.Entry)this.services.get(newInstance.getId());
                        if (entry != null) {
                            synchronized(entry) {
                                entry.service = newInstance;
                            }
                        }
                    } catch (Exception var10) {
                        this.log.debug("Could not deserialize: " + data.getPath());
                    }
                } else {
                    this.log.warn("Instance data has been deleted for: " + instance);
                }

            }).build();
            cache.listenable().addListener(listener);
            cache.start();
            return cache;
        }
    }

    // 创建zookeeper节点
    @VisibleForTesting
    protected void internalRegisterService(ServiceInstance service) throws Exception {
        byte[] bytes = this.serializer.serialize(service);
        String path = this.pathForInstance(service.getName(), service.getId());
        int MAX_TRIES = true;
        boolean isDone = false;

        for(int i = 0; !isDone && i < 2; ++i) {
            try {
                CreateMode mode;
                switch(service.getServiceType()) {
                case DYNAMIC:
                    mode = CreateMode.EPHEMERAL;
                    break;
                case DYNAMIC_SEQUENTIAL:
                    mode = CreateMode.EPHEMERAL_SEQUENTIAL;
                    break;
                default:
                    mode = CreateMode.PERSISTENT;
                }
                ((ACLBackgroundPathAndBytesable)this.client.create().creatingParentContainersIfNeeded().withMode(mode)).forPath(path, bytes);
                isDone = true;
            } catch (NodeExistsException var8) {
                this.client.delete().forPath(path);
            }
        }
    }

}

(3) spring容器启动事件

SpringBoot项目启动 -> webServer启动 -> 监听器监听服务启动事件执行流程：

SpringApplication.run(args) -> refreshContext(context) -> refresh(context) -> ServletWebServerApplicationContext.refresh() -> AbstractApplicationContext.refresh() -> finishRefresh() -> DefaultLifecycleProcessor.onRefresh() -> startBeans(true) -> DefaultLifecycleProcessor$LifecycleGroup.start() -> doStart() -> WebServerStartStopLifecycle.start() -> AbstractApplicationContext.publishEvent(new ServletWebServerInitializedEvent(this.webServer, this.applicationContext)) -> SimpleApplicationEventMulticaster.multicastEvent(applicationEvent, eventType) -> invokeListener(listener, event) -> doInvokeListener(listener, event) -> listener.onApplicationEvent(event);

堆栈信息：

onApplicationEvent:12, ZkApplicationListener (com.zk.serviceregistry.orderservice.listener)
doInvokeListener:176, SimpleApplicationEventMulticaster (org.springframework.context.event)
invokeListener:169, SimpleApplicationEventMulticaster (org.springframework.context.event)
multicastEvent:143, SimpleApplicationEventMulticaster (org.springframework.context.event)
publishEvent:421, AbstractApplicationContext (org.springframework.context.support)
publishEvent:378, AbstractApplicationContext (org.springframework.context.support)
start:46, WebServerStartStopLifecycle (org.springframework.boot.web.servlet.context)
doStart:178, DefaultLifecycleProcessor (org.springframework.context.support)
access$200:54, DefaultLifecycleProcessor (org.springframework.context.support)
start:356, DefaultLifecycleProcessor$LifecycleGroup (org.springframework.context.support)
accept:-1, 1643565953 (org.springframework.context.support.DefaultLifecycleProcessor$Lambda$541)
forEach:75, Iterable (java.lang)
startBeans:155, DefaultLifecycleProcessor (org.springframework.context.support)
onRefresh:123, DefaultLifecycleProcessor (org.springframework.context.support)
finishRefresh:935, AbstractApplicationContext (org.springframework.context.support)
refresh:586, AbstractApplicationContext (org.springframework.context.support)
refresh:145, ServletWebServerApplicationContext (org.springframework.boot.web.servlet.context)
refresh:745, SpringApplication (org.springframework.boot)
refreshContext:420, SpringApplication (org.springframework.boot)
run:307, SpringApplication (org.springframework.boot)
run:1317, SpringApplication (org.springframework.boot)
run:1306, SpringApplication (org.springframework.boot)
main:10, OrderServiceApplication (com.zk.serviceregistry.orderservice)

1) SpringApplicaton.run()

public class SpringApplication {
    public ConfigurableApplicationContext run(String... args) {
        long startTime = System.nanoTime();
        DefaultBootstrapContext bootstrapContext = this.createBootstrapContext();
        ConfigurableApplicationContext context = null;
        this.configureHeadlessProperty();
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        listeners.starting(bootstrapContext, this.mainApplicationClass);

        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, bootstrapContext, applicationArguments);
            this.configureIgnoreBeanInfo(environment);
            Banner printedBanner = this.printBanner(environment);
            context = this.createApplicationContext();
            context.setApplicationStartup(this.applicationStartup);
            this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
            this.refreshContext(context);                // 1
            this.afterRefresh(context, applicationArguments);
            Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), timeTakenToStartup);
            }

            listeners.started(context, timeTakenToStartup);
            this.callRunners(context, applicationArguments);
        } catch (Throwable var12) {
            this.handleRunFailure(context, var12, listeners);
            throw new IllegalStateException(var12);
        }

        try {
            Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);
            listeners.ready(context, timeTakenToReady);
            return context;
        } catch (Throwable var11) {
            this.handleRunFailure(context, var11, (SpringApplicationRunListeners)null);
            throw new IllegalStateException(var11);
        }
    }
}

2) refreshContext(context)

private void refreshContext(ConfigurableApplicationContext context) {
        if (this.registerShutdownHook) {
            shutdownHook.registerApplicationContext(context);
        }
        this.refresh(context);                // 2
    }

3) AbstractApplicationContext.refresh()

public void refresh() throws BeansException, IllegalStateException {
        synchronized(this.startupShutdownMonitor) {
            StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
            this.prepareRefresh();
            ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
            this.prepareBeanFactory(beanFactory);

            try {
                this.postProcessBeanFactory(beanFactory);
                StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
                this.invokeBeanFactoryPostProcessors(beanFactory);
                this.registerBeanPostProcessors(beanFactory);
                beanPostProcess.end();
                this.initMessageSource();
                this.initApplicationEventMulticaster();
                this.onRefresh();
                this.registerListeners();
                this.finishBeanFactoryInitialization(beanFactory);
                this.finishRefresh();                 // 3
            } catch (BeansException var10) {
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var10);
                }

                this.destroyBeans();
                this.cancelRefresh(var10);
                throw var10;
            } finally {
                this.resetCommonCaches();
                contextRefresh.end();
            }

        }
    }

4) finishRefresh()

protected void finishRefresh() {
        this.clearResourceCaches();
        this.initLifecycleProcessor();
        this.getLifecycleProcessor().onRefresh();                // 4
        this.publishEvent((ApplicationEvent)(new ContextRefreshedEvent(this)));
        if (!NativeDetector.inNativeImage()) {
            LiveBeansView.registerApplicationContext(this);
        }
    }

5) DefaultLifecycleProcessor.onRefresh()

public void onRefresh() {
        this.startBeans(true);                // 5
        this.running = true;
    }

6) startBeans(true)

private void startBeans(boolean autoStartupOnly) {
        Map lifecycleBeans = this.getLifecycleBeans();
        Map phases = new TreeMap();
        lifecycleBeans.forEach((beanName, bean) -> {
            if (!autoStartupOnly || bean instanceof SmartLifecycle && ((SmartLifecycle)bean).isAutoStartup()) {
                int phase = this.getPhase(bean);
                ((DefaultLifecycleProcessor.LifecycleGroup)phases.computeIfAbsent(phase, (p) -> {
                    return new DefaultLifecycleProcessor.LifecycleGroup(phase, this.timeoutPerShutdownPhase, lifecycleBeans, autoStartupOnly);
                })).add(beanName, bean);
            }
        });
        if (!phases.isEmpty()) {
            phases.values().forEach(DefaultLifecycleProcessor.LifecycleGroup::start);                // 6
        }
    }

7) DefaultLifecycleProcessor$LifecycleGroup.start()

public void start() {
            if (!this.members.isEmpty()) {
                if (DefaultLifecycleProcessor.this.logger.isDebugEnabled()) {
                    DefaultLifecycleProcessor.this.logger.debug("Starting beans in phase " + this.phase);
                }
                Collections.sort(this.members);
                Iterator var1 = this.members.iterator();
                while(var1.hasNext()) {
                    DefaultLifecycleProcessor.LifecycleGroupMember member = (DefaultLifecycleProcessor.LifecycleGroupMember)var1.next();
                    DefaultLifecycleProcessor.this.doStart(this.lifecycleBeans, member.name, this.autoStartupOnly);                // 7
                }
            }
        }

8) DefaultLifecycleProcessor.doStart()

private void doStart(Map lifecycleBeans, String beanName, boolean autoStartupOnly) {
        Lifecycle bean = (Lifecycle)lifecycleBeans.remove(beanName);
        if (bean != null && bean != this) {
            String[] dependenciesForBean = this.getBeanFactory().getDependenciesForBean(beanName);
            String[] var6 = dependenciesForBean;
            int var7 = dependenciesForBean.length;
            for(int var8 = 0; var8 < var7; ++var8) {
                String dependency = var6[var8];
                this.doStart(lifecycleBeans, dependency, autoStartupOnly);
            }

            if (!bean.isRunning() && (!autoStartupOnly || !(bean instanceof SmartLifecycle) || ((SmartLifecycle)bean).isAutoStartup())) {
                if (this.logger.isTraceEnabled()) {
                    this.logger.trace("Starting bean '" + beanName + "' of type [" + bean.getClass().getName() + "]");
                }

                try {
                    bean.start();                // 8
                } catch (Throwable var10) {
                    throw new ApplicationContextException("Failed to start bean '" + beanName + "'", var10);
                }
            }
        }
    }

9) 发布web服务启动完成事件

事件 ServletWebServerInitializedEvent extends WebServerInitializedEvent

public void start() {
        this.webServer.start();
        this.running = true;
        this.applicationContext.publishEvent(new ServletWebServerInitializedEvent(this.webServer, this.applicationContext));                // 9
    }

10) 发布事件 AbstractApplicationContext.publishEvent()

protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
        if (this.earlyApplicationEvents != null) {
            this.earlyApplicationEvents.add(applicationEvent);
        } else {
            this.getApplicationEventMulticaster().multicastEvent((ApplicationEvent)applicationEvent, eventType);                // 10
        }

        if (this.parent != null) {
            if (this.parent instanceof AbstractApplicationContext) {
                ((AbstractApplicationContext)this.parent).publishEvent(event, eventType);
            } else {
                this.parent.publishEvent(event);
            }
        }
    }

11) 广播器发布事件 SimpleApplicationEventMulticaster.multicastEvent(event, eventType)

public void multicastEvent(final ApplicationEvent event, @Nullable ResolvableType eventType) {
        ResolvableType type = eventType != null ? eventType : this.resolveDefaultEventType(event);
        Executor executor = this.getTaskExecutor();
        Iterator var5 = this.getApplicationListeners(event, type).iterator();
        while(var5.hasNext()) {
            ApplicationListener<?> listener = (ApplicationListener)var5.next();
            if (executor != null) {
                executor.execute(() -> {
                    this.invokeListener(listener, event);
                });
            } else {
                this.invokeListener(listener, event);                // 11
            }
        }
    }

12) 调用监听器 invokeListener(listener, event)

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
        ErrorHandler errorHandler = this.getErrorHandler();
        if (errorHandler != null) {
            try {
                this.doInvokeListener(listener, event);
            } catch (Throwable var5) {
                errorHandler.handleError(var5);
            }
        } else {
            this.doInvokeListener(listener, event);                // 12
        }
    }

    private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
        try {
            listener.onApplicationEvent(event);                // 13
        } catch (ClassCastException var6) {}
    }

13) 调用监听器事件回调方法

public abstract class AbstractAutoServiceRegistration implements AutoServiceRegistration, ApplicationContextAware, ApplicationListener {
    public void onApplicationEvent(WebServerInitializedEvent event) {
        // TODO ..................
    }
}