springcloud情操陶冶-bootstrapContext(二)

承接前文監聽器對bootstrapContext創建的引導，筆者了解到其主要入口類為BootstrapImportSelectorConfiguration。本文將基於此類進行簡單的分析

BootstrapImportSelectorConfiguration

簡單的配置類，看下源碼

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

嗯，引入了延遲加載類BootstrapImportSelector，那筆者就繼續往下看下此會延遲加載哪些類，直接去觀察其主方法

	@Override
	public 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(
				environment.getProperty("spring.cloud.bootstrap.sources", ""))));

		List<OrderedAnnotatedElement> elements = new ArrayList<>();
		for (String name : names) {
			try {
				elements.add(new OrderedAnnotatedElement(metadataReaderFactory, name));
			} catch (IOException e) {
				continue;
			}
		}
		AnnotationAwareOrderComparator.sort(elements);

		String[] classNames = elements.stream()
				.map(e -> e.name)
				.toArray(String[]::new);

		return classNames;
	}

上述的代碼很簡單，其會去加載classpath路徑下所有spring.factories文件中以org.springframework.cloud.bootstrap.BootstrapConfiguration作為Key的所有類；
同時springcloud也支持通過設置spring.cloud.bootstrap.sources屬性來加載指定類

筆者就先以springcloud context板塊內的spring.factories作為分析的源頭

# 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

在分析上述的源碼之前，筆者必須得清楚現在bootstrapContext加載的配置文件默認為bootstrap.properties抑或是bootstrap.yml，其屬性已經被加入至相應的Environment對象中。基於此我們再往下走，以免犯糊塗

PropertySourceBootstrapConfiguration

配置源的加載，此Configuration主要用於確定是否外部加載的配置屬性復寫Spring內含的環境變量。注意其是ApplicationContextInitializer接口的實現類，前文已經提到，bootstrapContext上的此接口的bean類都會被注冊至子級的SpringApplication對象上。
直接看下主要的代碼片段把

	@Override
	public void initialize(ConfigurableApplicationContext applicationContext) {
		CompositePropertySource composite = new CompositePropertySource(
				BOOTSTRAP_PROPERTY_SOURCE_NAME);
		AnnotationAwareOrderComparator.sort(this.propertySourceLocators);
		boolean empty = true;
		// 此處為子級的環境變量對象
		ConfigurableEnvironment environment = applicationContext.getEnvironment();
		// 通過PropertySourceLocator接口去加載外部配置
		for (PropertySourceLocator locator : this.propertySourceLocators) {
			PropertySource<?> source = null;
			source = locator.locate(environment);
			if (source == null) {
				continue;
			}
			logger.info("Located property source: " + source);
			composite.addPropertySource(source);
			empty = false;
		}
		if (!empty) {
			MutablePropertySources propertySources = environment.getPropertySources();
			String logConfig = environment.resolvePlaceholders("${logging.config:}");
			LogFile logFile = LogFile.get(environment);
			if (propertySources.contains(BOOTSTRAP_PROPERTY_SOURCE_NAME)) {
				propertySources.remove(BOOTSTRAP_PROPERTY_SOURCE_NAME);
			}
			// 確定屬性讀取的先后順序
			insertPropertySources(propertySources, composite);
			// reinitialize log
			reinitializeLoggingSystem(environment, logConfig, logFile);
			setLogLevels(applicationContext, environment);
			// active profiles process
			handleIncludedProfiles(environment);
		}
	}

上述的代碼就涉及兩點，一個是通過PropertySourceLocator接口加載外部配置；一個是用於解析以spring.cloud.config為開頭的PropertySourceBootstrapProperties屬性，默認情況下，外部配置比內部變量有更高的優先級。具體的用戶可自行分析

備注：PropertiesSourceBootstrapProperties中的屬性變量可通過PropertySourceLocator接口配置

PropertyPlaceholderAutoConfiguration

和spring常見的解析文件一樣的操作，具體就不分析了

@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
public class PropertyPlaceholderAutoConfiguration {

	// 配置文件屬性讀取常用類
	@Bean
	@ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
	public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
		return new PropertySourcesPlaceholderConfigurer();
	}

}

ConfigurationPropertiesRebinderAutoConfiguration

通過命名便會發現其跟刷新屬性的功能有關，先優先看下其類結構

@Configuration
@ConditionalOnBean(ConfigurationPropertiesBindingPostProcessor.class)
public class ConfigurationPropertiesRebinderAutoConfiguration
		implements ApplicationContextAware, SmartInitializingSingleton {
}

上述代碼表示其依據於當前類環境存在ConfigurationPropertiesBindingPostProcessorBean對象才會被應用，仔細查閱了下，發現只要有使用到@EnableConfigurationProperties注解即就會被注冊。看來此配置跟ConfigurationProperties注解也有一定的關聯性。

本文就羅列筆者比較關注的幾個地方

---

1.ConfigurationPropertiesRebinder對象的創建

	@Bean
	@ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
	public ConfigurationPropertiesRebinder configurationPropertiesRebinder(
			ConfigurationPropertiesBeans beans) {
		ConfigurationPropertiesRebinder rebinder = new ConfigurationPropertiesRebinder(
				beans);
		return rebinder;
	}

此類讀者可以自行翻閱代碼，可以發現其暴露了JMX接口以及監聽了springcloud context自定義的EnvironmentChangeEvent事件。看來其主要用來刷新ApplicationContext上的beans(含@ConfigurationProperties注解)對象集合

---

2.ConfigurationPropertiesBeans對象的創建

	@Bean
	@ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
	public ConfigurationPropertiesBeans configurationPropertiesBeans() {
		// 
		ConfigurationBeanFactoryMetadata metaData = this.context.getBean(
				ConfigurationBeanFactoryMetadata.BEAN_NAME,
						ConfigurationBeanFactoryMetadata.class);
		ConfigurationPropertiesBeans beans = new ConfigurationPropertiesBeans();
		beans.setBeanMetaDataStore(metaData);
		return beans;
	}

配合ConfigurationProperties注解的表演，其會緩存ApplicationContext上的所有含有ConfigurationProperties注解的bean。與第一點所提的ConfigurationPropertiesRebinder對象搭配使用

---

3.實例化結束后刷新父級ApplicationContext上的屬性

	@Override
	public void afterSingletonsInstantiated() {
		// refresh parent application context
		if (this.context.getParent() != null) {
			// TODO: make this optional? (E.g. when creating child contexts that prefer to
			// be isolated.)
			ConfigurationPropertiesRebinder rebinder = context
					.getBean(ConfigurationPropertiesRebinder.class);
			for (String name : context.getParent().getBeanDefinitionNames()) {
				rebinder.rebind(name);
			}
		}
	}

EncryptionBootstrapConfiguration

與屬性讀取的加解密有關，跟JDK的keystore也有一定的關聯，具體就不去解析了。讀者可自行分析

Bean加載問題

此處本文插入這個Bean的加載問題，因為筆者發現SpringApplication會被調用兩次，那么ApplicationContext實例也會被創建兩次。那么基於@Configuration修飾過的自定義的Bean是不是也會被加載兩次呢？？

經過在cloud環境下編寫了一個簡單的Bean

package com.example.clouddemo;

import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.annotation.Configuration;

/**
 * @author nanco
 * -------------
 * -------------
 * @create 19/8/20
 */
@Configuration
public class TestApplication implements ApplicationContextAware {

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        System.out.println("application parent context id: " + applicationContext.getParent().getId());
        System.out.println("application context id: " + applicationContext.getId());
    }
}

且在application.properties文件中指定spring.application.name=springChild以及bootstrap.properties文件中也指定spring.application.name=springBootstrap

運行main方法之后打印的關鍵信息如下

application parent context id: bootstrap
application context id: springBootstrap-1

經過在org.springframework.boot.context.ContextIdApplicationContextInitializer類上進行斷點調試發現只有用戶級ApplicationContext被創建的過程中會實例化用戶自定義Bean。也就是說bootstrapContext並不會去實例化用戶自定義的Bean，這樣就很安全。

那么為何如此呢？其實很簡單，因為bootstrapContext指定的source類只有BootstrapImportSelectorConfiguration，並沒有用戶編寫的啟動類，也就無法影響用戶級別Context的Bean加載實例化了~~~並且該類上無@EnableAutoConfiguration注解，表明其也不會去處理spring.factories文件中@EnableAutoConfiguration注解key對應的配置集合。

小結

1.針對springcloud context模塊下的以BootstrapConfiguration作為Key的自動配置類，除了PropertySourceBootstrapConfiguration自動類的應用范圍在子級ApplicationContext，其它三個均有作用於父級ApplicationContext。

2.關於外部源文件的屬性，默認情況下其有更高的優先級於本地系統以及環境變量。當然用戶也可以通過修改spring.cloud.config.allowOverride/spring.cloud.config.overrideSystemProperties/spring.cloud.config.overrideNone屬性來進行優先級更改，通過此，用戶需要復寫PropertySourceLocator接口來進行配置

package com.example.cloud.external.resource;

import org.springframework.cloud.bootstrap.config.PropertySourceLocator;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;
import org.springframework.core.env.MapPropertySource;
import org.springframework.core.env.PropertySource;

import java.util.HashMap;
import java.util.Map;

/**
 * @author nanco
 * -------------
 * cloud-demo
 * -------------
 * @create 2019/1/15 19:40
 * @descrption
 **/
@Configuration
public class ExternalPropertySourceLocator implements PropertySourceLocator {

    private static final String EXTERNAL_KEY = "external";

    @Override
    public PropertySource<?> locate(Environment environment) {
        Map<String, Object> externalMap = new HashMap<>();

        // user custom property
        externalMap.put("username", "nanco");
        externalMap.put("password", "nanco123");
        externalMap.put("mail", "[e-mail]");

        // application property
        externalMap.put("spring.cloud.config.allowOverride", true);
        externalMap.put("spring.cloud.config.overrideSystemProperties", false); //system擁有更高的優先級
        externalMap.put("spring.cloud.config.overrideNone", false);

        return new MapPropertySource(EXTERNAL_KEY, externalMap);
    }
}

最后將上述的類放置在META-INF/spring.factories文件中便可以生效了

# Bootstrap components
org.springframework.cloud.bootstrap.BootstrapConfiguration=\
com.example.cloud.external.resource.ExternalPropertySourceLocator

3.關於針對@ConfigurationProperties注解的Beans對象的刷新操作，本文只講解了JMX方式去調用，如果與第三方插件結合，應該會有更多的形式。

4.針對監聽器環節的分析到本章暫時告一段落，下一篇便分析cloud-context板塊spring.factories文件中以org.springframework.boot.autoconfigure.EnableAutoConfiguration作為Key的類集合。
熟悉的氣息再次降臨，迫不及待ing....