Spring Cloud启动-4-应用监听器ApplicationListener之BootstrapApplicationListener

在org.springframework.context.ApplicationListener众多实现者有一个非常重要的实现就是
org.springframework.cloud.bootstrap.BootstrapApplicationListener
可以看到，ApplicationListener是定义在spring-context包中，属于经典上下文范畴，BootstrapApplicationListener是在spring-cloud-context中，属于cloud上线文范畴。

在classpath中出现spring-cloud-context.jar包时，它在spring.factories文件里写了BootstrapApplicationListener，所以在构建SpringApplication对象时就扫描出这个监听器了。然后在run()方法中发送事件给监听器，监听器执行回调。

官方解释：
这个listener会prepare SpringApplication(例如填充他的Environment等)，通过代理给在独立的bootstrap context里的ApplicationContextInitializer beans。bootstrap context 是SpringApplication从spring.factories中BootstrapConfiguration中创建的，从bootstrap.properties(or yml)而不是普通的application.properties(or yml)初始化的。

BootstrapApplicationListener执行后会创建一个id=bootstrap的上下文，这个上下文不是一个独立的父上下文，不是装载应用bean的上下文。

public class BootstrapApplicationListenerimplements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered {@Overridepublic void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {ConfigurableEnvironment environment = event.getEnvironment();if (!environment.getProperty("spring.cloud.bootstrap.enabled", Boolean.class,true)) {return;}// don't listen to events in a bootstrap contextif (environment.getPropertySources().contains(BOOTSTRAP_PROPERTY_SOURCE_NAME)) {return;}ConfigurableApplicationContext context = null;String configName = environment.resolvePlaceholders("${spring.cloud.bootstrap.name:bootstrap}");for (ApplicationContextInitializer<?> initializer : event.getSpringApplication().getInitializers()) {if (initializer instanceof ParentContextApplicationContextInitializer) {context = findBootstrapContext((ParentContextApplicationContextInitializer) initializer,configName);}}if (context == null) {context = bootstrapServiceContext(environment, event.getSpringApplication(),configName);event.getSpringApplication().addListeners(new CloseContextOnFailureApplicationListener(context));}apply(context, event.getSpringApplication(), environment);}}

整个回调方法通过开关控制

spring.cloud.bootstrap.enabled

org.springframework.cloud.bootstrap.BootstrapApplicationListener#onApplicationEvent()方法做的事情是

其中核心的方法是：
org.springframework.cloud.bootstrap.BootstrapApplicationListener#bootstrapServiceContext()
这个方法会创建一个bootstrapEnvironment，一个bootstrapApplicationContext，这个applicationContext可以认为是一个父上下文。

private ConfigurableApplicationContext bootstrapServiceContext(ConfigurableEnvironment environment, final SpringApplication application,String configName) {StandardEnvironment bootstrapEnvironment = new StandardEnvironment();MutablePropertySources bootstrapProperties = bootstrapEnvironment.getPropertySources();for (PropertySource<?> source : bootstrapProperties) {bootstrapProperties.remove(source.getName());}String configLocation = environment.resolvePlaceholders("${spring.cloud.bootstrap.location:}");String configAdditionalLocation = environment.resolvePlaceholders("${spring.cloud.bootstrap.additional-location:}");Map<String, Object> bootstrapMap = new HashMap<>();bootstrapMap.put("spring.config.name", configName);// if an app (or test) uses spring.main.web-application-type=reactive, bootstrap// will fail// force the environment to use none, because if though it is set below in the// builder// the environment overrides itbootstrapMap.put("spring.main.web-application-type", "none");if (StringUtils.hasText(configLocation)) {bootstrapMap.put("spring.config.location", configLocation);}if (StringUtils.hasText(configAdditionalLocation)) {bootstrapMap.put("spring.config.additional-location",configAdditionalLocation);}bootstrapProperties.addFirst(new MapPropertySource(BOOTSTRAP_PROPERTY_SOURCE_NAME, bootstrapMap));for (PropertySource<?> source : environment.getPropertySources()) {if (source instanceof StubPropertySource) {continue;}bootstrapProperties.addLast(source);}// TODO: is it possible or sensible to share a ResourceLoader?SpringApplicationBuilder builder = new SpringApplicationBuilder().profiles(environment.getActiveProfiles()).bannerMode(Mode.OFF).environment(bootstrapEnvironment)// Don't use the default properties in this builder.registerShutdownHook(false).logStartupInfo(false).web(WebApplicationType.NONE);final SpringApplication builderApplication = builder.application();if (builderApplication.getMainApplicationClass() == null) {// gh_425:// SpringApplication cannot deduce the MainApplicationClass here// if it is booted from SpringBootServletInitializer due to the// absense of the "main" method in stackTraces.// But luckily this method's second parameter "application" here// carries the real MainApplicationClass which has been explicitly// set by SpringBootServletInitializer itself already.builder.main(application.getMainApplicationClass());}if (environment.getPropertySources().contains("refreshArgs")) {// If we are doing a context refresh, really we only want to refresh the// Environment, and there are some toxic listeners (like the// LoggingApplicationListener) that affect global static state, so we need a// way to switch those off.builderApplication.setListeners(filterListeners(builderApplication.getListeners()));}builder.sources(BootstrapImportSelectorConfiguration.class);final ConfigurableApplicationContext context = builder.run();// gh-214 using spring.application.name=bootstrap to set the context id via// `ContextIdApplicationContextInitializer` prevents apps from getting the actual// spring.application.name// during the bootstrap phase.context.setId("bootstrap");// Make the bootstrap context a parent of the app contextaddAncestorInitializer(application, context);// It only has properties in it now that we don't want in the parent so remove// it (and it will be added back later)bootstrapProperties.remove(BOOTSTRAP_PROPERTY_SOURCE_NAME);mergeDefaultProperties(environment.getPropertySources(), bootstrapProperties);return context;
}

比较关键的点是这里手动引入了一个导入配置类：
org.springframework.cloud.bootstrap.BootstrapImportSelectorConfiguration

@Configuration(proxyBeanMethods = false)
@Import(BootstrapImportSelector.class)
public class BootstrapImportSelectorConfiguration {}

进而导入：
org.springframework.cloud.bootstrap.BootstrapImportSelector
BootstrapImportSelector的主要作用就是从spring.factories文件中读取：
org.springframework.cloud.bootstrap.BootstrapConfiguration配置的类

public class BootstrapImportSelector implements EnvironmentAware, DeferredImportSelector {private Environment environment;private MetadataReaderFactory metadataReaderFactory = new CachingMetadataReaderFactory();@Overridepublic void setEnvironment(Environment environment) {this.environment = environment;}@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) {ClassLoader classLoader = Thread.currentThread().getContextClassLoader();// Use names and ensure unique to protect against duplicatesList<String> names = new ArrayList<>(SpringFactoriesLoader.loadFactoryNames(BootstrapConfiguration.class, classLoader));names.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(this.environment.getProperty("spring.cloud.bootstrap.sources", ""))));List<OrderedAnnotatedElement> elements = new ArrayList<>();for (String name : names) {try {elements.add(new OrderedAnnotatedElement(this.metadataReaderFactory, name));}catch (IOException e) {continue;}}AnnotationAwareOrderComparator.sort(elements);String[] classNames = elements.stream().map(e -> e.name).toArray(String[]::new);return classNames;}class OrderedAnnotatedElement implements AnnotatedElement {private final String name;private Order order = null;private Integer value;OrderedAnnotatedElement(MetadataReaderFactory metadataReaderFactory, String name)throws IOException {MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(name);AnnotationMetadata metadata = metadataReader.getAnnotationMetadata();Map<String, Object> attributes = metadata.getAnnotationAttributes(Order.class.getName());this.name = name;if (attributes != null && attributes.containsKey("value")) {this.value = (Integer) attributes.get("value");this.order = new Order() {@Overridepublic Class<? extends Annotation> annotationType() {return Order.class;}@Overridepublic int value() {return OrderedAnnotatedElement.this.value;}};}}@Override@SuppressWarnings("unchecked")public <T extends Annotation> T getAnnotation(Class<T> annotationClass) {if (annotationClass == Order.class) {return (T) this.order;}return null;}@Overridepublic Annotation[] getAnnotations() {return this.order == null ? new Annotation[0]: new Annotation[] { this.order };}@Overridepublic Annotation[] getDeclaredAnnotations() {return getAnnotations();}@Overridepublic String toString() {return new ToStringCreator(this).append("name", this.name).append("value", this.value).toString();}}}

# Bootstrap components
org.springframework.cloud.bootstrap.BootstrapConfiguration=\
org.springframework.cloud.bootstrap.config.PropertySourceBootstrapConfiguration,\
org.springframework.cloud.bootstrap.encrypt.EncryptionBootstrapConfiguration,\
org.springframework.cloud.autoconfigure.ConfigurationPropertiesRebinderAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.cloud.util.random.CachedRandomPropertySourceAutoConfiguration

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