扩展EnvironmentPostProcessor从数据库加载配置项&spring.factories

应用未接入配置中心时，一些配置项(如oss存储配置信息、邮件服务配置信息等)需要从其它数据源获取，下面以从数据库获取配置信息为例。

@Value

既然需要通过从数据库中读取配置信息，那么先了解一下@Value的工作原理：

1. SpringBoot应用启动

   org.springframework.boot.SpringApplication#run(java.lang.Class<?>, java.lang.String...) =>

   org.springframework.boot.SpringApplication#run(java.lang.String...) =>

2. 刷新Spring容器

   org.springframework.boot.SpringApplication#refreshContext =>

   org.springframework.context.support.AbstractApplicationContext#refresh

3. 实例化Bean

   org.springframework.context.support.AbstractApplicationContext#finishBeanFactoryInitialization =>

   org.springframework.beans.factory.config.ConfigurableListableBeanFactory#preInstantiateSingletons（实例化非懒加载的单例Bean) =>

   org.springframework.beans.factory.support.AbstractBeanFactory#getBean(java.lang.String) =>

   org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean =>

   org.springframework.beans.factory.support.AbstractBeanFactory#createBean =>

   org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean

4. 属性填充

   org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean =>

   protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
   	if (bw == null) {
   		if (mbd.hasPropertyValues()) {
   			throw new BeanCreationException(
   					mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
   		}
   		else {
   			// Skip property population phase for null instance.
   			return;
   		}
   	}
      
   	if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
   		for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
   			//实例化后
   			if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
   				return;
   			}
   		}
   	}
      
   	PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
      
   	//@Bean(autowire = Autowire.BY_NAME)
   	int resolvedAutowireMode = mbd.getResolvedAutowireMode();
   	if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
   		MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
   		// Add property values based on autowire by name if applicable.
   		if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
   			autowireByName(beanName, mbd, bw, newPvs);
   		}
   		// Add property values based on autowire by type if applicable.
   		if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
   			autowireByType(beanName, mbd, bw, newPvs);
   		}
   		pvs = newPvs;
   	}
      
   	boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
   	boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
      
   	PropertyDescriptor[] filteredPds = null;
   	if (hasInstAwareBpps) {
   		if (pvs == null) {
   			pvs = mbd.getPropertyValues();
   		}
   		for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
   			//依赖注入入口
   			//@Autowired @Value：AutowiredAnnotationBeanPostProcessor @Resource：CommonAnnotationBeanPostProcessor
   			PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
   			if (pvsToUse == null) {
   				if (filteredPds == null) {
   					filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
   				}
   				pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
   				if (pvsToUse == null) {
   					return;
   				}
   			}
   			pvs = pvsToUse;
   		}
   	}
   	if (needsDepCheck) {
   		if (filteredPds == null) {
   			filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
   		}
   		checkDependencies(beanName, mbd, filteredPds, pvs);
   	}
      
   	if (pvs != null) {
   		// MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition()会覆盖@Autowired
   		applyPropertyValues(beanName, mbd, bw, pvs);
   	}
   }

   org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessProperties =>

   org.springframework.beans.factory.annotation.InjectionMetadata#inject =>

   public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
   		Collection<InjectedElement> checkedElements = this.checkedElements;
   		Collection<InjectedElement> elementsToIterate =
   				(checkedElements != null ? checkedElements : this.injectedElements);
   		if (!elementsToIterate.isEmpty()) {
   			for (InjectedElement element : elementsToIterate) {
   				//遍历每个注入点进行依赖注入
   				//Autowired：AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement.inject
   				//AutowiredAnnotationBeanPostProcessor.AutowiredMethodElement.inject
   				//Resource：InjectionMetadata.InjectedElement.inject
   				element.inject(target, beanName, pvs);
   			}
   		}
   	}

   org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#resolveFieldValue =>

   org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveDependency =>

   org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency

5. 处理@Value

   org.springframework.beans.factory.annotation.QualifierAnnotationAutowireCandidateResolver#findValue

   org.springframework.beans.factory.support.AbstractBeanFactory#resolveEmbeddedValue =>

   

   org.springframework.context.support.PropertySourcesPlaceholderConfigurer#processProperties(org.springframework.beans.factory.config.ConfigurableListableBeanFactory, org.springframework.core.env.ConfigurablePropertyResolver) (把所有的配置文件都变成了一个个propertysource对象，同时把environment对象也包装成了一个propertysource对象，并且一个个propertysource对象存储在了MutablePropertySources中。)

大致上来看，@Value解析，分为以下几步：

1. org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessMergedBeanDefinition =>

   org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#findAutowiringMetadata =>

   org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#buildAutowiringMetadata

   收集标注了@Value等注解的字段；

2. org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor#postProcessProperties开始注入值；

3. 注入过程中，从DefaultListableBeanFactory中遍历所有的embeddedValueResolver，这些embeddedValueResolvers是StringValueResolver，调用其resolveStringValue方法；

4. 其中有一个PropertySourcesPlaceholderConfigurer类构造的StringValueResolver，调用它的resolveStringValue，最终从一个PropertySourcesPropertyResolver的propertySources中遍历所有的propertySource，其中就有Environment的propertySources，匹配到值后返回。

EnvironmentPostProcessor

从上面可以得出结论，在容器refresh之前，从数据库读取信息封装为MapPropertySource塞入Environment#Property即可。

主要实现类：

public class DatabaseEnvironmentPostProcessor implements EnvironmentPostProcessor {

  @Override
  public void postProcessEnvironment(
      ConfigurableEnvironment environment, SpringApplication application) {
    if (environment.getPropertySources().contains("databasePropertySources")) {
      return;
    }
    // 命令行参数
    boolean commandLineArgs = environment.getPropertySources().contains("commandLineArgs");
    if (commandLineArgs) {
      environment
          .getPropertySources()
          .addBefore("commandLineArgs", loadConfigurationFromDatabase(environment));
    } else {
      if (environment.getProperty("spring.datasource.url") != null) {
        environment.getPropertySources().addFirst(loadConfigurationFromDatabase(environment));
        // 设置激活的Profile
        String activeProfile = environment.getProperty("spring.profiles.active", "prd");
        environment.addActiveProfile(activeProfile);
      }
    }
  }

  private PropertySource loadConfigurationFromDatabase(ConfigurableEnvironment environment) {
    String url = environment.getProperty("spring.datasource.url");
    String username = environment.getProperty("spring.datasource.username");
    String password = environment.getProperty("spring.datasource.password");
    String driverClassName = environment.getProperty("spring.datasource.druid.driver-class-name");
    Map<String, ?> configs =
        new DatabasePropertySourceLoader(url, username, password, driverClassName).load();
    PropertySource propertySource =
        new MapPropertySource("databasePropertySources", (Map<String, Object>) configs);
    return propertySource;
  }
}

spring.factories

org.springframework.boot.env.EnvironmentPostProcessor=\
cn.imokkkk.env.DatabaseEnvironmentPostProcessor

spring.factories

Spring Factories是一种类似于Java SPI的机制，在resources/META-INF/spring.factories文件中配置接口的实现类名称(接口名称=实现类)，然后在程序中读取该配置文件并实例化，是spring-boot-starter-xxx的实现基础。

为了实现从数据库读取配置信息的需求，显然需要在容器refresh之前完成从数据库读取并添加到环境变量中。

调用链路

1. 从spring.factories加载ApplicationListener对应的监听器，并启动

   org.springframework.boot.SpringApplication#run(java.lang.String...) =>

   org.springframework.boot.SpringApplication#getRunListenersorg.springframework.boot.SpringApplication#run(java.lang.String...) => org.springframework.boot.SpringApplication#getSpringFactoriesInstances(java.lang.Class<T>, java.lang.Class<?>[], java.lang.Object...) => org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames

2. 发布ApplicationEnvironmentPreparedEvent

   org.springframework.boot.SpringApplication#prepareEnvironment =>

   org.springframework.boot.context.event.EventPublishingRunListener#environmentPrepared

3. 监听到事件并执行

   org.springframework.context.event.SimpleApplicationEventMulticaster#invokeListener =>

   org.springframework.boot.context.config.ConfigFileApplicationListener#onApplicationEvent =>

   org.springframework.boot.context.config.ConfigFileApplicationListener#onApplicationEnvironmentPreparedEvent =>

   cn.imokkkk.env.DatabaseEnvironmentPostProcessor#postProcessEnvironment

sql语句：

>CREATE TABLE `app_config`  (
 `id` bigint(20) NOT NULL AUTO_INCREMENT,
 `config_key` varchar(50) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
 `config_value` varchar(200) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
 `remark` varchar(200) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
 `is_halt` char(1) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NULL DEFAULT NULL,
 PRIMARY KEY (`id`) USING BTREE
>) ENGINE = InnoDB AUTO_INCREMENT = 1 CHARACTER SET = utf8mb4 COLLATE = utf8mb4_general_ci;