在SpringCloud工程中，可以使用@RefreshScope+@Value实现配置文件内容变更后的动态刷新。

在SpringBoot工程中，可以使用@NacosValue来实现配置文件内容变更后的动态刷新。

@NacosValue的使用

引入依赖：

<dependency>
	<groupId>com.alibaba.boot</groupId>
	<artifactId>nacos-config-spring-boot-starter</artifactId>
	<version>0.2.12</version>
</dependency>

配置文件增加配置：

nacos:
  config:
    server-addr: 127.0.0.1:8848
    bootstrap:
      enable: true
      log:
        enable: true
    data-id: order-service
    type: yaml
    auto-refresh: true # 开启自动刷新

@NacosValue的例子：

package com.morris.order.controller;

import com.alibaba.nacos.api.config.annotation.NacosValue;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("order")
public class NacosValueController {

    @NacosValue(value = "${user.age}", autoRefreshed = true)
    private Integer age2;

    @GetMapping("age2")
    public Integer getAge2() {
        return age2;
    }

}

注意在SpringCloud项目中不能使用@NacosValue注解，虽然这个注解存在，但是其源码的实现不存在。

@NacosValue源码分析

NacosConfigEnvironmentProcessor

NacosConfigEnvironmentProcessor实现了EnvironmentPostProcessor，会在SpringBoot项目启动时调用postProcessEnvironment()方法，去Nacos配置中心拉取配置。

需要在配置文件中开启预加载nacos.config.bootstrap.enable=true。

com.alibaba.boot.nacos.config.autoconfigure.NacosConfigEnvironmentProcessor#postProcessEnvironment

public void postProcessEnvironment(ConfigurableEnvironment environment,
								   SpringApplication application) {
	application.addInitializers(new NacosConfigApplicationContextInitializer(this));
	nacosConfigProperties = NacosConfigPropertiesUtils
		.buildNacosConfigProperties(environment);
	if (enable()) {
		// nacos.config.bootstrap.enable=true
		System.out.println(
			"[Nacos Config Boot] : The preload log configuration is enabled");
		loadConfig(environment);
		NacosConfigLoader nacosConfigLoader = NacosConfigLoaderFactory.getSingleton(nacosConfigProperties, environment, builder);
		LogAutoFreshProcess.build(environment, nacosConfigProperties, nacosConfigLoader, builder).process();
	}
}

com.alibaba.boot.nacos.config.autoconfigure.NacosConfigEnvironmentProcessor#loadConfig

private void loadConfig(ConfigurableEnvironment environment) {
	NacosConfigLoader configLoader = new NacosConfigLoader(nacosConfigProperties,
														   environment, builder);
	configLoader.loadConfig();
	// set defer NacosPropertySource
	deferPropertySources.addAll(configLoader.getNacosPropertySources());
}

从配置中心拉取配置加入到Environment。
com.alibaba.boot.nacos.config.util.NacosConfigLoader#loadConfig

public void loadConfig() {
	MutablePropertySources mutablePropertySources = environment.getPropertySources();
	List<NacosPropertySource> sources = reqGlobalNacosConfig(globalProperties,
															 nacosConfigProperties.getType());
	for (NacosConfigProperties.Config config : nacosConfigProperties.getExtConfig()) {
		List<NacosPropertySource> elements = reqSubNacosConfig(config,
															   globalProperties, config.getType());
		sources.addAll(elements);
	}
	if (nacosConfigProperties.isRemoteFirst()) {
		// 默认为false，默认本地的配置优先级高
		for (ListIterator<NacosPropertySource> itr = sources.listIterator(sources.size()); itr.hasPrevious();) {
			mutablePropertySources.addAfter(
				StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, itr.previous());
		}
	} else {
		for (NacosPropertySource propertySource : sources) {
			mutablePropertySources.addLast(propertySource);
		}
	}
}

NacosConfigApplicationContextInitializer

NacosConfigApplicationContextInitializer注册负责监听Nacos配置，实现自动刷新配置

com.alibaba.boot.nacos.config.autoconfigure.NacosConfigApplicationContextInitializer#initialize

public void initialize(ConfigurableApplicationContext context) {
	singleton.setApplicationContext(context);
	environment = context.getEnvironment();
	nacosConfigProperties = NacosConfigPropertiesUtils
		.buildNacosConfigProperties(environment);
	final NacosConfigLoader configLoader = NacosConfigLoaderFactory.getSingleton(
		nacosConfigProperties, environment, builder);
	if (!enable()) {
		logger.info("[Nacos Config Boot] : The preload configuration is not enabled");
	}
	else {

		// If it opens the log level loading directly will cache
		// DeferNacosPropertySource release

		if (processor.enable()) {
			// nacos.config.bootstrap.enable=true
			processor.publishDeferService(context);
			// 添加监听器
			configLoader
				.addListenerIfAutoRefreshed(processor.getDeferPropertySources());
		}
		else {
			configLoader.loadConfig();
			configLoader.addListenerIfAutoRefreshed();
		}
	}

	final ConfigurableListableBeanFactory factory = context.getBeanFactory();
	if (!factory
		.containsSingleton(NacosBeanUtils.GLOBAL_NACOS_PROPERTIES_BEAN_NAME)) {
		factory.registerSingleton(NacosBeanUtils.GLOBAL_NACOS_PROPERTIES_BEAN_NAME,
								  configLoader.getGlobalProperties());
	}
}

遍历所有的Nacos配置文件，每个配置文件都添加一个监听器。
com.alibaba.boot.nacos.config.util.NacosConfigLoader#addListenerIfAutoRefreshed(java.util.List<com.alibaba.boot.nacos.config.util.NacosConfigLoader.DeferNacosPropertySource>)

public void addListenerIfAutoRefreshed(
	final List<DeferNacosPropertySource> deferNacosPropertySources) {
	for (DeferNacosPropertySource deferNacosPropertySource : deferNacosPropertySources) {
		NacosPropertySourcePostProcessor.addListenerIfAutoRefreshed(
			deferNacosPropertySource.getNacosPropertySource(),
			deferNacosPropertySource.getProperties(),
			deferNacosPropertySource.getEnvironment());
	}
}

要实现配置的自动刷新，需要在配置文件中开启nacos.config.auto-refresh=true。
com.alibaba.nacos.spring.core.env.NacosPropertySourcePostProcessor#addListenerIfAutoRefreshed

public static void addListenerIfAutoRefreshed(
	final NacosPropertySource nacosPropertySource, final Properties properties,
	final ConfigurableEnvironment environment) {

	// nacos.config.auto-refresh=true
	if (!nacosPropertySource.isAutoRefreshed()) { // Disable Auto-Refreshed
		return;
	}

	final String dataId = nacosPropertySource.getDataId();
	final String groupId = nacosPropertySource.getGroupId();
	final String type = nacosPropertySource.getType();
	final NacosServiceFactory nacosServiceFactory = getNacosServiceFactoryBean(
		beanFactory);

	try {

		ConfigService configService = nacosServiceFactory
			.createConfigService(properties);

		Listener listener = new AbstractListener() {

			@Override
			public void receiveConfigInfo(String config) {
				String name = nacosPropertySource.getName();
				NacosPropertySource newNacosPropertySource = new NacosPropertySource(
					dataId, groupId, name, config, type);
				newNacosPropertySource.copy(nacosPropertySource);
				MutablePropertySources propertySources = environment
					.getPropertySources();
				// replace NacosPropertySource
				// 监听事件的处理，直接替换Environment中的配置
				propertySources.replace(name, newNacosPropertySource);
			}
		};

		if (configService instanceof EventPublishingConfigService) {
			// 通过EventPublishingConfigService进行代理
			((EventPublishingConfigService) configService).addListener(dataId,
																	   groupId, type, listener);
		}
		else {
			configService.addListener(dataId, groupId, listener);
		}

	}
	catch (NacosException e) {
		throw new RuntimeException(
			"ConfigService can't add Listener with properties : " + properties,
			e);
	}
}

通过EventPublishingConfigService添加监听器，里面对监听器做了一层包装。
com.alibaba.nacos.spring.context.event.config.EventPublishingConfigService#addListener(java.lang.String, java.lang.String, java.lang.String, com.alibaba.nacos.api.config.listener.Listener)

public void addListener(String dataId, String group, String type, Listener listener)
	throws NacosException {
	Listener listenerAdapter = new DelegatingEventPublishingListener(configService,
																	 dataId, group, type, applicationEventPublisher, executor, listener);
	addListener(dataId, group, listenerAdapter);
}

当配置中心的配置变更后，首先会回调DelegatingEventPublishingListener的receiveConfigInfo()，这里会调用被代理的监听器的receiveConfigInfo()，还会发布NacosConfigReceivedEvent事件。
com.alibaba.nacos.spring.context.event.config.DelegatingEventPublishingListener#receiveConfigInfo

public void receiveConfigInfo(String content) {
	// 调用被代理的监听器的receiveConfigInfo()
	onReceived(content);
	// 发布NacosConfigReceivedEvent事件
	publishEvent(content);
}

private void publishEvent(String content) {
	NacosConfigReceivedEvent event = new NacosConfigReceivedEvent(configService,
																  dataId, groupId, content, configType);
	applicationEventPublisher.publishEvent(event);
}

private void onReceived(String content) {
	delegate.receiveConfigInfo(content);
}

NacosValueAnnotationBeanPostProcessor

postProcessPropertyValues()

NacosValueAnnotationBeanPostProcessor实现了InstantiationAwareBeanPostProcessorAdapter，其postProcessBeforeInitialization()方法会在Bean实例化之后和初始化之前执行，对Bean上加了@NacosValue注解的属性进行设置。

com.alibaba.spring.beans.factory.annotation.AbstractAnnotationBeanPostProcessor#postProcessPropertyValues

public PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {
	// 查找@NacosVaLue注解的属性和方法
	InjectionMetadata metadata = this.findInjectionMetadata(beanName, bean.getClass(), pvs);

	try {
		// 为属性设置值
		metadata.inject(bean, beanName, pvs);
		return pvs;
	} catch (BeanCreationException var7) {
		throw var7;
	} catch (Throwable var8) {
		throw new BeanCreationException(beanName, "Injection of @" + this.getAnnotationType().getSimpleName() + " dependencies is failed", var8);
	}
}

postProcessBeforeInitialization()

NacosValueAnnotationBeanPostProcessor实现了BeanPostProcessor，其postProcessBeforeInitialization()方法会在Bean实例化之后和初始化之前执行，主要负责收集带有@NacosValue注解且需要自动刷新的属性。

如果属性需要自动刷新，需要设置@NacosValue的autoRefreshed为true。

com.alibaba.nacos.spring.context.annotation.config.NacosValueAnnotationBeanPostProcessor#postProcessBeforeInitialization

public Object postProcessBeforeInitialization(Object bean, final String beanName)
	throws BeansException {

	// 对Bean所有的属性进行处理
	doWithFields(bean, beanName);

	// 对Bean所有的方法进行处理
	doWithMethods(bean, beanName);

	return super.postProcessBeforeInitialization(bean, beanName);
}

private void doWithFields(final Object bean, final String beanName) {
	ReflectionUtils.doWithFields(bean.getClass(),
								 new ReflectionUtils.FieldCallback() {
									 @Override
									 public void doWith(Field field) throws IllegalArgumentException {
										 NacosValue annotation = getAnnotation(field, NacosValue.class);
										 doWithAnnotation(beanName, bean, annotation, field.getModifiers(),
														  null, field);
									 }
								 });
}

private void doWithAnnotation(String beanName, Object bean, NacosValue annotation,
							  int modifiers, Method method, Field field) {
	if (annotation != null) {
		if (Modifier.isStatic(modifiers)) {
			return;
		}

		if (annotation.autoRefreshed()) {
			String placeholder = resolvePlaceholder(annotation.value());

			if (placeholder == null) {
				return;
			}

			NacosValueTarget nacosValueTarget = new NacosValueTarget(bean, beanName,
																	 method, field, annotation.value());
			// 将带有@NacosValue注解的属性加入到一个Map中
			put2ListMap(placeholderNacosValueTargetMap, placeholder,
						nacosValueTarget);
		}
	}
}

监听NacosConfigReceivedEvent

收到NacosConfigReceivedEvent事件时，通过反射调用上面收集好的属性和方法从而实现自动刷新。

com.alibaba.nacos.spring.context.annotation.config.NacosValueAnnotationBeanPostProcessor#onApplicationEvent

public void onApplicationEvent(NacosConfigReceivedEvent event) {
	// In to this event receiver, the environment has been updated the
	// latest configuration information, pull directly from the environment
	// fix issue #142
	for (Map.Entry<String, List<NacosValueTarget>> entry : placeholderNacosValueTargetMap
		 .entrySet()) {
		String key = environment.resolvePlaceholders(entry.getKey());
		String newValue = environment.getProperty(key);

		if (newValue == null) {
			continue;
		}
		List<NacosValueTarget> beanPropertyList = entry.getValue();
		for (NacosValueTarget target : beanPropertyList) {
			String md5String = MD5Utils.md5Hex(newValue, "UTF-8");
			boolean isUpdate = !target.lastMD5.equals(md5String);
			if (isUpdate) {
				target.updateLastMD5(md5String);
				Object evaluatedValue = resolveNotifyValue(target.nacosValueExpr, key, newValue);
				if (target.method == null) {
					setField(target, evaluatedValue);
				}
				else {
					setMethod(target, evaluatedValue);
				}
			}
		}
	}
}