@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Repeatable(PropertySources.class)
public @interface PropertySource {
	//資源名稱，為空則根據資源的描述符生成
	String name() default "";
	/**
	 *資源路徑
	 *classpath:application.properties
	 *file:/
	 */
	String[] value();
	//是否忽略資源不存在的情況，如果不忽略，當資源不存在時報錯
	boolean ignoreResourceNotFound() default false;
	//指定資源文件的編碼格式
	String encoding() default "";
	//資源工廠
	Class<? extends PropertySourceFactory> factory() default PropertySourceFactory.class;
}

public int getOrder() {
	return Ordered.LOWEST_PRECEDENCE;  // within PriorityOrdered
}

new ConfigurationClass(metadata, beanName)

SourceClass sourceClass = asSourceClass(configClass, filter);

for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
		sourceClass.getMetadata(), PropertySources.class,
		org.springframework.context.annotation.PropertySource.class)) {
	if (this.environment instanceof ConfigurableEnvironment) {
		//核心邏輯
		processPropertySource(propertySource);
	}
	else {
		logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
				"]. Reason: Environment must implement ConfigurableEnvironment");
	}
}

private void processPropertySource(AnnotationAttributes propertySource) throws IOException {
	String name = propertySource.getString("name");
	if (!StringUtils.hasLength(name)) {
		name = null;
	}
	String encoding = propertySource.getString("encoding");
	if (!StringUtils.hasLength(encoding)) {
		encoding = null;
	}
	//獲取配置文件路徑
	String[] locations = propertySource.getStringArray("value");
	Assert.isTrue(locations.length > 0, "At least one @PropertySource(value) location is required");
	boolean ignoreResourceNotFound = propertySource.getBoolean("ignoreResourceNotFound");

	Class<? extends PropertySourceFactory> factoryClass = propertySource.getClass("factory");
	PropertySourceFactory factory = (factoryClass == PropertySourceFactory.class ?
			DEFAULT_PROPERTY_SOURCE_FACTORY : BeanUtils.instantiateClass(factoryClass));

	for (String location : locations) {
		try {
			//替換占位符
			String resolvedLocation = this.environment.resolveRequiredPlaceholders(location);
			//流的方式加載配置文件并封裝成Resource對象
			Resource resource = this.resourceLoader.getResource(resolvedLocation);
			//加載Resource中的配置屬性封裝成Properties對象中，并創(chuàng)建PropertySource對象加入到Environment對象中
			addPropertySource(factory.createPropertySource(name, new EncodedResource(resource, encoding)));
		}
		catch (IllegalArgumentException | FileNotFoundException | UnknownHostException ex) {
			// Placeholders not resolvable or resource not found when trying to open it
			if (ignoreResourceNotFound) {
				if (logger.isInfoEnabled()) {
					logger.info("Properties location [" + location + "] not resolvable: " + ex.getMessage());
				}
			}
			else {
				throw ex;
			}
		}
	}
}

public PropertySource<?> createPropertySource(@Nullable String name, EncodedResource resource) throws IOException {
	return (name != null ? new ResourcePropertySource(name, resource) : new ResourcePropertySource(resource));
}

private void addPropertySource(PropertySource<?> propertySource) {
	String name = propertySource.getName();
	//獲取Environment對象中的MutablePropertySources
	MutablePropertySources propertySources = ((ConfigurableEnvironment) this.environment).getPropertySources();

	//如果已經存在了該配置文件的PropertySource則合并久的
	if (this.propertySourceNames.contains(name)) {
		// We've already added a version, we need to extend it
		PropertySource<?> existing = propertySources.get(name);
		if (existing != null) {
			PropertySource<?> newSource = (propertySource instanceof ResourcePropertySource ?
					((ResourcePropertySource) propertySource).withResourceName() : propertySource);
			//合并二次后的類型
			if (existing instanceof CompositePropertySource) {
				((CompositePropertySource) existing).addFirstPropertySource(newSource);
			}
			else {
				if (existing instanceof ResourcePropertySource) {
					existing = ((ResourcePropertySource) existing).withResourceName();
				}
				//其實就是CompositePropertySource里面有一個Set，Set里面裝了新和舊的PropertySource對象
				CompositePropertySource composite = new CompositePropertySource(name);
				composite.addPropertySource(newSource);
				composite.addPropertySource(existing);
				propertySources.replace(name, composite);
			}
			return;
		}
	}

	if (this.propertySourceNames.isEmpty()) {
		propertySources.addLast(propertySource);
	}
	else {
		//用于計算插入的位置index
		String firstProcessed = this.propertySourceNames.get(this.propertySourceNames.size() - 1);
		//吧propertySource對象存入MutablePropertySources的list中
		propertySources.addBefore(firstProcessed, propertySource);
	}
	this.propertySourceNames.add(name);
}

public Object getProperty(String name) {
	for (PropertySource<?> propertySource : this.propertySources) {
		Object candidate = propertySource.getProperty(name);
		if (candidate != null) {
			return candidate;
		}
	}
	return null;
}