springboot中@Value的工作原理說明

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {       
...
if (hasInstAwareBpps) {
	// 遍歷所有InstantiationAwareBeanPostProcessor實例設置屬性字段值。
	for (BeanPostProcessor bp : getBeanPostProcessors()) {
		// AutowiredAnnotationBeanPostProcessor會進入此分支
		if (bp instanceof InstantiationAwareBeanPostProcessor) {
			InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
			pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
		//上行代碼執行後,bw.getWrappedInstance()就得到瞭@Value註解裝配屬性後的bean瞭
			if (pvs == null) {
				return;
			}
		}
	}
}
...
}

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()) {
	    // 依次註入屬性
		for (InjectedElement element : elementsToIterate) {
			if (logger.isDebugEnabled()) {
				logger.debug("Processing injected element of bean '" + beanName + "': " + element);
			}
			element.inject(target, beanName, pvs);
		}
	}
}

/**
 *  一個參數示例 value = "${company.ceo}"
 *
 */
protected String parseStringValue(
		String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {
	StringBuilder result = new StringBuilder(value);
	// this.placeholderPrefix = "${"
	int startIndex = value.indexOf(this.placeholderPrefix);
	while (startIndex != -1) {
		// 占位符的結束位置，以value = "${company.ceo}"為例,endIndex=13
		int endIndex = findPlaceholderEndIndex(result, startIndex);
		if (endIndex != -1) {
			// 獲取{}裡的真正屬性名稱，此例為"company.ceo"
			String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
			String originalPlaceholder = placeholder;
			if (!visitedPlaceholders.add(originalPlaceholder)) {
				throw new IllegalArgumentException(
						"Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
			}
			// Recursive invocation, parsing placeholders contained in the placeholder key.
			// 遞歸調用本方法，因為屬性鍵中可能仍然有占位符
			placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
			// Now obtain the value for the fully resolved key...
			// 獲取屬性鍵placeholder對應的屬性值
			String propVal = placeholderResolver.resolvePlaceholder(placeholder);
			// 此處邏輯是當company.ceo=${bi:li}時,company.ceo最終被li所替代的原因
			// 所以配置文件中，最好不要出現類似${}的東西，因為它本身就會被spring框架所解析
			if (propVal == null && this.valueSeparator != null) {
				int separatorIndex = placeholder.indexOf(this.valueSeparator);
				if (separatorIndex != -1) {
					String actualPlaceholder = placeholder.substring(0, separatorIndex);
					String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
					propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
					if (propVal == null) {
						propVal = defaultValue;
					}
				}
			}
			if (propVal != null) {
				// Recursive invocation, parsing placeholders contained in the
				// previously resolved placeholder value.
				propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
				// 將${company.ceo}替換為li
				result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
				if (logger.isTraceEnabled()) {
					logger.trace("Resolved placeholder '" + placeholder + "'");
				}
				startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
			}
			else if (this.ignoreUnresolvablePlaceholders) {
				// Proceed with unprocessed value.
				startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
			}
			else {
				throw new IllegalArgumentException("Could not resolve placeholder '" +
						placeholder + "'" + " in value \"" + value + "\"");
			}
			visitedPlaceholders.remove(originalPlaceholder);
		}
		else {
			startIndex = -1;
		}
	}
	return result.toString();
}