一、背景

        在我们在做application启动的时候，我们经常需要做一些初始化的操作，这个时候我们尝尝纠结于他们的执行顺序，其实我们真正应该去关心的是我们的需求，它应该什么时候被执行才是正确的。

二、源码分析

         为了不陈述大量的源码，只贴出了springboot项目在启动时，运行的run方法并分别在这几种方式调用的入口处加了注释

public ConfigurableApplicationContext run(String... args) {long startTime = System.nanoTime();DefaultBootstrapContext bootstrapContext = createBootstrapContext();ConfigurableApplicationContext context = null;configureHeadlessProperty();SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting(bootstrapContext, this.mainApplicationClass);try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, bootstrapContext, applicationArguments);configureIgnoreBeanInfo(environment);Banner printedBanner = printBanner(environment);context = createApplicationContext();context.setApplicationStartup(this.applicationStartup);prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);//@PostConstruct和InitializingBean初始化位置refreshContext(context);afterRefresh(context, applicationArguments);Duration timeTakenToStartup = Duration.ofNanos(System.nanoTime() - startTime);if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), timeTakenToStartup);}//@EventListener(ApplicationStartedEvent.class)listeners.started(context, timeTakenToStartup);//ApplicationRunner和CommandLineRunner初始化位置callRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, listeners);throw new IllegalStateException(ex);}try {Duration timeTakenToReady = Duration.ofNanos(System.nanoTime() - startTime);//@EventListener(ApplicationReadyEvent.class)listeners.ready(context, timeTakenToReady);}catch (Throwable ex) {handleRunFailure(context, ex, null);throw new IllegalStateException(ex);}return context;}

可以清楚的看到@PostConstruct和InitializingBean是在refreshContext的地方调用，代码的执行过程要早于@EventListener(ApplicationStartedEvent.class)早于ApplicationRunner/CommandLineRunner早于@EventListener(ApplicationReadyEvent.class)

并且默认是单线程执行，所以得出初步结论

1. @PostConstruct和InitializingBean
2. @EventListener(ApplicationStartedEvent.class)
3. ApplicationRunner/CommandLineRunner  
4. @EventListener(ApplicationReadyEvent.class)

再来看@PostConstruct和InitializingBean的触发时机

protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {invokeAwareMethods(beanName, bean);return null;}, getAccessControlContext());}else {invokeAwareMethods(beanName, bean);}Object wrappedBean = bean;if (mbd == null || !mbd.isSynthetic()) {//@PostConstruct初始化wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);}try {//InitializingBean初始化invokeInitMethods(beanName, wrappedBean, mbd);}catch (Throwable ex) {throw new BeanCreationException((mbd != null ? mbd.getResourceDescription() : null),beanName, "Invocation of init method failed", ex);}if (mbd == null || !mbd.isSynthetic()) {wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);}return wrappedBean;}

 可以清楚的看到在initializeBean方法中，@PostConstruct和InitializingBean是被依次调用的，并且是在同一线程执行，所以得出第二步结论

@PostConstruct > InitializingBean

 所以综合来看

1. @PostConstruct
2. InitializingBean
3. @EventListener(ApplicationStartedEvent.class)
4. ApplicationRunner/CommandLineRunner  
5. @EventListener(ApplicationReadyEvent.class)

三、实践

同时我们也必须了解，spring bean的创建过程或者说生命周期，以及结合上述的初始化方法一起来看

spring 多个bean之间的执行顺序画了个草图，把多个bean的构建拆分来看的话，其实每个bean的全部周期不是原子的

所以在bean的初始化过程中如果依赖其他的bean来做初始化，这时候我们不仅要考虑执行顺序的问题，还要考虑依赖的bean是否已初始化完成，哪个部分已初始化完成，才能评估出是否对当前bean的初始化带来影响。

这里还会有一些问题，比如多种定义bean的途径，是如何扫描进入容器的？又是怎样的顺序？

如何控制这个过程

网上很多文章都是人云亦云，写的比较简单建议大家还是自己阅读源码以免出错

protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)throws IOException {if (configClass.getMetadata().isAnnotated(Component.class.getName())) {// Recursively process any member (nested) classes firstprocessMemberClasses(configClass, sourceClass, filter);}// Process any @PropertySource annotationsfor (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(), PropertySources.class,org.springframework.context.annotation.PropertySource.class)) {if (this.environment instanceof ConfigurableEnvironment) {processPropertySource(propertySource);}else {logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +"]. Reason: Environment must implement ConfigurableEnvironment");}}// Process any @ComponentScan annotationsSet<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);if (!componentScans.isEmpty() &&!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {for (AnnotationAttributes componentScan : componentScans) {// The config class is annotated with @ComponentScan -> perform the scan immediatelySet<BeanDefinitionHolder> scannedBeanDefinitions =this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());// Check the set of scanned definitions for any further config classes and parse recursively if neededfor (BeanDefinitionHolder holder : scannedBeanDefinitions) {BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();if (bdCand == null) {bdCand = holder.getBeanDefinition();}if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {parse(bdCand.getBeanClassName(), holder.getBeanName());}}}}// Process any @Import annotationsprocessImports(configClass, sourceClass, getImports(sourceClass), filter, true);// Process any @ImportResource annotationsAnnotationAttributes importResource =AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);if (importResource != null) {String[] resources = importResource.getStringArray("locations");Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");for (String resource : resources) {String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);configClass.addImportedResource(resolvedResource, readerClass);}}// Process individual @Bean methodsSet<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);for (MethodMetadata methodMetadata : beanMethods) {configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));}// Process default methods on interfacesprocessInterfaces(configClass, sourceClass);// Process superclass, if anyif (sourceClass.getMetadata().hasSuperClass()) {String superclass = sourceClass.getMetadata().getSuperClassName();if (superclass != null && !superclass.startsWith("java") &&!this.knownSuperclasses.containsKey(superclass)) {this.knownSuperclasses.put(superclass, configClass);// Superclass found, return its annotation metadata and recursereturn sourceClass.getSuperClass();}}// No superclass -> processing is completereturn null;}