Nacos 配置中心之环境准备
Nacos 配置中心的工作流程是怎么样的呢？首先启动 SpringBoot 项目，在启动项目之后，需要把远程服务器的配置文件加载到 Spring 容器中
@SpringBootApplicationpublic class SpringCloudNacosConfigApplication {
public static void main(String[] args) { ConfigurableApplicationContext context= SpringApplication.run(SpringCloudNacosConfigApplication.class, args); String info=context.getEnvironment().getProperty("info"); System.out.println(info); }}
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重点在于 Environment 的获取，那么如何从远处服务器上的配置加载到 Environment？
我们看一下 SpringBoot 启动的时候，SpringApplication.run()方法做了哪些环境准备
public ConfigurableApplicationContext run(String... args) { StopWatch stopWatch = new StopWatch(); stopWatch.start(); ConfigurableApplicationContext context = null; Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>(); configureHeadlessProperty(); SpringApplicationRunListeners listeners = getRunListeners(args); listeners.starting(); try { ApplicationArguments applicationArguments = new DefaultApplicationArguments( args); //环境准备 ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments); configureIgnoreBeanInfo(environment); Banner printedBanner = printBanner(environment); context = createApplicationContext(); exceptionReporters = getSpringFactoriesInstances( SpringBootExceptionReporter.class, new Class[] { ConfigurableApplicationContext.class }, context); prepareContext(context, environment, listeners, applicationArguments, printedBanner); refreshContext(context); afterRefresh(context, applicationArguments); stopWatch.stop(); if (this.logStartupInfo) { new StartupInfoLogger(this.mainApplicationClass) .logStarted(getApplicationLog(), stopWatch); } listeners.started(context); callRunners(context, applicationArguments); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, listeners); throw new IllegalStateException(ex); }
try { listeners.running(context); } catch (Throwable ex) { handleRunFailure(context, ex, exceptionReporters, null); throw new IllegalStateException(ex); } return context;}
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获取监听并启动监听
根据传入的参数创建 applicationArguments 对象
调用 prepareEnvironment 方法，进行环境准备
打印 banner，准备上下文，刷新上下文，然后执行刷新之后操作等等
租后返回 context 上下文
我们看看 prepareEnvironment 方法做了什么
prepareEnvironment()方法
private ConfigurableEnvironment prepareEnvironment( SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments) { // Create and configure the environment ConfigurableEnvironment environment = getOrCreateEnvironment(); configureEnvironment(environment, applicationArguments.getSourceArgs()); listeners.environmentPrepared(environment); bindToSpringApplication(environment); if (!this.isCustomEnvironment) { environment = new EnvironmentConverter(getClassLoader()) .convertEnvironmentIfNecessary(environment, deduceEnvironmentClass()); } ConfigurationPropertySources.attach(environment); return environment;}
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获取或则创建环境，在 listeners.environmentPrepared(environment);中发布一个 ApplicationEnvironmentPreparedEvent 事件，所有定义这个的 Listener 监听器都会监听到这个事件。
BootstrapApplicationListener 就会收到该事件并进行处理
@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 context if (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);}
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在 bootstrapServiceContext 方法中调用 builder.sources(BootstrapImportSelectorConfiguration.class)，进行自动装配，这里就不贴具体的代码了，我们看一下 BootstrapImportSelectorConfiguration 这个类做了什么
BootstrapImportSelectorConfiguration 类
BootstrapImportSelectorConfiguration 是配置类
@Configuration@Import(BootstrapImportSelector.class)public class BootstrapImportSelectorConfiguration {
}
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用 @Import 导入 BootstrapImportSelector 实现自动装配
BootstrapImportSelector 的 selectImports 方法：
@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) { ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); // Use names and ensure unique to protect against duplicates List<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;}
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利用 Spring 的 SPI 机制查找 META-INF/spring.factories 扩展点
key 是 BootstrapConfiguration
在 spring-cloud-context.jar 中的 spring.factories：PropertySourceBootstrapConfiguration
# Bootstrap componentsorg.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
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在 spring-cloud-context.jar 中的 spring.factories：
NacosConfigBootstrapConfiguration
org.springframework.cloud.bootstrap.BootstrapConfiguration=\com.alibaba.cloud.nacos.NacosConfigBootstrapConfigurationorg.springframework.boot.autoconfigure.EnableAutoConfiguration=\com.alibaba.cloud.nacos.NacosConfigAutoConfiguration,\com.alibaba.cloud.nacos.endpoint.NacosConfigEndpointAutoConfigurationorg.springframework.boot.diagnostics.FailureAnalyzer=\com.alibaba.cloud.nacos.diagnostics.analyzer.NacosConnectionFailureAnalyzer
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水落石出了，这里自动装配了 NacosConfigBootstrapConfiguration 配置类
总结
这就是 Nacos 配置中心的环境准备方面，从 SpringBoot 项目启动类的环境准备的方法入手，通过 BootstrapApplicationListener 类的监听方法，调用了 builder.sources(BootstrapImportSelectorConfiguration.class)加载了 BootstrapImportSelectorConfiguration 类，它的 selectImports()方法中利用 Spring 的 SPI 扩展机制记载了 key 为 BootstrapConfiguration 定义的一些 nacos 中类，从而对 nacos 的启动环境做了准备。
其实 nacos 最重要的功能就是注册中心和配置中心，作为 nacos 的使用者，我们要掌握好 nacos 的流程和工作原理，才能定位到问题，并在合适的业务场景中进行对 nacos 服务的功能优化和利用好 nacos，这篇文章仅仅是对 nacos 作为配置中心功能的环境准备方面的结束，后续还会进一步分析 nacos 配置文件的流程和工作原理。如果你觉得这篇文章对你有帮助的话，欢迎给我留言点赞评论转发！
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