Spring PropertySourcesPropertyResolver

	@Nullable
	private final PropertySources propertySources;

@Nullable
protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
   if (this.propertySources != null) {
      // 循环
      for (PropertySource<?> propertySource : this.propertySources) {
         if (logger.isTraceEnabled()) {
            logger.trace("Searching for key '" + key + "' in PropertySource '" +
                  propertySource.getName() + "'");
         }
         // 获取对象结果
         Object value = propertySource.getProperty(key);
         if (value != null) {
            // 是否需要处理嵌套
            // 是否是 string 类型
            if (resolveNestedPlaceholders && value instanceof String) {
               // 嵌套获取数据
               value = resolveNestedPlaceholders((String) value);
            }
            // 日志
            logKeyFound(key, propertySource, value);
            // 类型转换
            return convertValueIfNecessary(value, targetValueType);
         }
      }
   }
   if (logger.isTraceEnabled()) {
      logger.trace("Could not find key '" + key + "' in any property source");
   }
   return null;
}

protected String resolveNestedPlaceholders(String value) {
   return (this.ignoreUnresolvableNestedPlaceholders ?
         resolvePlaceholders(value) : resolveRequiredPlaceholders(value));
}

@SuppressWarnings("unchecked")
@Nullable
protected <T> T convertValueIfNecessary(Object value, @Nullable Class<T> targetType) {
   if (targetType == null) {
      // 类型强制转换
      return (T) value;
   }
   ConversionService conversionServiceToUse = this.conversionService;
   if (conversionServiceToUse == null) {
      // Avoid initialization of shared DefaultConversionService if
      // no standard type conversion is needed in the first place...
      if (ClassUtils.isAssignableValue(targetType, value)) {
         return (T) value;
      }
      // 默认的转换接口
      conversionServiceToUse = DefaultConversionService.getSharedInstance();
   }
   // 通过默认接口进行转换
   return conversionServiceToUse.convert(value, targetType);
}