@Value和@Autowired这两个注解都是由AutowiredAnnotationBeanPostProcessor来处理的，这两个注解被处理的地方也是一样的，就是在一个bean被new出来之后，要填充属性的populateBean方法里。会调用 AutoWiredAnnotationBeanPostProcessor.postProcessPropertyValues。不同的地方就是处理的逻辑

    1. public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
    2.             Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
    4.         InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
    5.         try {
    6.             Object shortcut = descriptor.resolveShortcut(this);
    7.             if (shortcut != null) {
    8.                 return shortcut;
    9.             }
    11.             Class<?> type = descriptor.getDependencyType();
    12.             //${driverClassName}
    13.             Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
    14.             if (value != null) {
    15.                 if (value instanceof String) {
    16.                     // 解析占位符号
    17.                     String strVal = resolveEmbeddedValue((String) value);
    18.                     BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
    19.                     value = evaluateBeanDefinitionString(strVal, bd);
    20.                 }
    21.                 TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
    22.                 return (descriptor.getField() != null ?
    23.                         converter.convertIfNecessary(value, type, descriptor.getField()) :
    24.                         converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
    25.             }

    解析占位符

    1. public String resolveEmbeddedValue(String value) {
    2.         if (value == null) {
    3.             return null;
    4.         }
    5.         String result = value;
    6.         //[org.springframework.beans.factory.config.
    7.         // PropertyPlaceholderConfigurer$PlaceholderResolvingStringValueResolver@17d88132]
    8.         for (StringValueResolver resolver : this.embeddedValueResolvers) {
    9.             //这里就取到了真实的值
    10.             result = resolver.resolveStringValue(result);
    11.             if (result == null) {
    12.                 return null;
    13.             }
    14.         }
    15.         return result;
    16.     }

    总结
    如果一个bean里面的某个字段有注释@Value 

    1. @Component
    2. public class CommonPo {
    4.     @Value("${driverClassName}")
    5.     private String driver;

    注释的值如果是默认的占位符标志，就会调用 PropertyPlaceholderConfigurer 里的方法获取配置文件里的值