记一次解决Nacos-SpringBoot不支持@ConditionalOnProperty

错误介绍

@ConditionalOnProperty配置生效问题

错误原因分析

在使用Nacos-Config-SpringBoot时，有使用者反馈说无法支持@ConditionalOnProperty注解，经过研究Nacos-Config-SpringBoot、Nacos-Spring-Context源码以及调试跟踪SpringBoot的初始化流程，最终发现问题所在——@ConditionalOnProperty的解析时间与Nacos-Spring-Context相关Bean的注册以及工作时间存在先后问题（其本质原因就是Bean的加载顺序）

@ConditionalOnProperty解析时间

要想知道@ConditionalOnProperty注解何时被Spring解析，首先要看另外一个类——ConfigurationClassPostProcessor，这个类实现了BeanFactoryPostProcessor接口，该接口是在所有的BeanDefinition加载之后执行的，可以在此接口中对bean的定义进行修改。即Spring允许BeanFactoryPostProcessor在容器实例化任何其他bean之前读取配置元数据，并根据其进行修改，就比如@ConditionalOnProperty注解常用来根据配置文件的相关配置控制是否需要创建相应的bean。

核心代码

ConfigurationClassPostProcessor执行、装载时机

refreshContext(context);

@Override
protected final void refreshBeanFactory() throws BeansException {
		...
				loadBeanDefinitions(beanFactory);
				synchronized (this.beanFactoryMonitor) {
						this.beanFactory = beanFactory;
				}
		}
		catch (IOException ex) {
			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
		}
}

处理@Configuration等注解的处理代码

public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
	List<BeanDefinitionHolder> configCandidates = new ArrayList();
	String[] candidateNames = registry.getBeanDefinitionNames();
	String[] var4 = candidateNames;
	int var5 = candidateNames.length;

	...
	ConfigurationClassParser parser = new ConfigurationClassParser(
		this.metadataReaderFactory, this.problemReporter, this.environment,
		this.resourceLoader, this.componentScanBeanNameGenerator, registry);

	Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
	Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
	do {
		parser.parse(candidates);
		parser.validate();
		Set<ConfigurationClass> configClasses = new LinkedHashSet(parser.getConfigurationClasses());
		configClasses.removeAll(alreadyParsed);
		if (this.reader == null) {
			this.reader = new ConfigurationClassBeanDefinitionReader(registry, this.sourceExtractor, this.resourceLoader, this.environment, this.importBeanNameGenerator, parser.getImportRegistry());
		}
		...
}

在这里我们可以看到一个ConfigurationClassParser对象，进入他的构造函数看下

public ConfigurationClassParser(MetadataReaderFactory metadataReaderFactory,
			ProblemReporter problemReporter, Environment environment, ResourceLoader resourceLoader,
			BeanNameGenerator componentScanBeanNameGenerator, BeanDefinitionRegistry registry) {

	this.metadataReaderFactory = metadataReaderFactory;
	this.problemReporter = problemReporter;
	this.environment = environment;
	this.resourceLoader = resourceLoader;
	this.registry = registry;
	this.componentScanParser = new ComponentScanAnnotationParser(
				environment, resourceLoader, componentScanBeanNameGenerator, registry);
	this.conditionEvaluator = new ConditionEvaluator(registry, environment, resourceLoader);
}

重点关注这个ConditionEvaluator类，看看官方对这个类作用的介绍

Internal class used to evaluate {@link Conditional} annotations.

那么就清楚了，@ConditionalOnXXX注解的解析都是由这个类去做的，然后通过ConfigurationClassParser类的parse方法执行后，就会将满足@ConditionalOnXXX注解的Bean进行创建

在代码parser.getConfigurationClasses()执行后，会根据Environment中的配置元数据以及@ConditionalOnProperty中的设置，对bean进行过滤操作，不满条件的bean不会被转载进Spring Container中

people.enable=false时

people.enable=true时

（提醒：之所以这里nacos的配置能够与SpringBoot的@ConditionalOnProperty，是因为我这进行了修改，初步解决了此问题）

而Nacos-Spring-Context中相关的bean，都是在这之后才被解析、装载进入Spring Container的，为什么这么说？这里直接拉出官方对于ConfigurationClassPostProcessor的注释

{@link BeanFactoryPostProcessor} used for bootstrapping processing of
{@link Configuration @Configuration} classes.

Registered by default when using {@code context:annotation-config/} or
{@code context:component-scan/}. Otherwise, may be declared manually as
with any other BeanFactoryPostProcessor.
This post processor is priority-ordered as it is important that any
{@link Bean} methods declared in {@code @Configuration} classes have
their corresponding bean definitions registered before any other
{@link BeanFactoryPostProcessor} executes.

大致意思就是，ConfigurationClassPostProcessor的优先级是所有BeanFactoryPostProcessor实现中最高的，他必须在其他的BeanFactoryPostProcessor之前执行，因为其他的BeanFactoryPostProcessor需要ConfigurationClassPostProcessor进行解析装载进Spring Container。

解决方案

清楚该ISSUE为什么会出现的原因之后，那么相应的解决方案就很快的出来了。之前说到ConfigurationClassPostProcessor它会去解析@ConditionalOnProperty，并且它的执行先于其他的BeanFactoryPostProcessor实现；那么，还有什么比它先执行呢？

ApplicationContextInitializer

prepareContext(context, environment, listeners, applicationArguments, printedBanner);

可以看出，ApplicationContextInitializer是在prepareContext执行的，而ConfigurationClassPostProcessor是在refreshContext执行的，因此，我们只需要将配置拉取的代码提前到ApplicationContextInitializer中即可

@Override
public void initialize(ConfigurableApplicationContext context) {
    environment = context.getEnvironment();
    if (isEnable()) {
        CompositePropertySource compositePropertySource = new CompositePropertySource(NacosConfigConstants.NACOS_BOOTSTRAP_PROPERTY_APPLICATION);
        CacheableEventPublishingNacosServiceFactory singleton = CacheableEventPublishingNacosServiceFactory.getSingleton();
        singleton.setApplicationContext(context);
        String[] dataIds;
        String[] groupIds;
        String[] namespaces;
        try {
            dataIds = environment.getProperty(NacosConfigConstants.NACOS_CONFIG_DATA_ID, String[].class, new String[]{});
            groupIds = environment.getProperty(NacosConfigConstants.NACOS_CONFIG_GROUP_ID, String[].class, new String[dataIds.length]);
            namespaces = environment.getProperty(NacosProperties.NAMESPACE, String[].class, new String[dataIds.length]);

            for (int i = 0; i < dataIds.length; i ++) {
                Properties buildInfo = properties(namespaces[i]);
                ConfigService configService = singleton.createConfigService(buildInfo);

                String group = StringUtils.isEmpty(groupIds[i]) ? Constants.DEFAULT_GROUP : groupIds[i];
                String config = configService.getConfig(dataIds[i], group, 1000);
                if (config == null) {
                    logger.error("nacos-config-spring-boot : get config failed");
                    continue;
                }
                String name = buildDefaultPropertySourceName(dataIds[i], groupIds[i], buildInfo);
                NacosPropertySource nacosPropertySource = new NacosPropertySource(name, config);
                compositePropertySource.addPropertySource(nacosPropertySource);
            }
            environment.getPropertySources().addFirst(compositePropertySource);
        } catch (NacosException e) {
            logger.error(e.getErrMsg());
        }
    }
}