15-依赖注入-@Value原理(5)

@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

public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
            Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
        InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
        try {
            Object shortcut = descriptor.resolveShortcut(this);
            if (shortcut != null) {
                return shortcut;
            }
            Class<?> type = descriptor.getDependencyType();
            //${driverClassName}
            Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
            if (value != null) {
                if (value instanceof String) {
                    // 解析占位符号
                    String strVal = resolveEmbeddedValue((String) value);
                    BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
                    value = evaluateBeanDefinitionString(strVal, bd);
                }
                TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
                return (descriptor.getField() != null ?
                        converter.convertIfNecessary(value, type, descriptor.getField()) :
                        converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
            }

解析占位符

public String resolveEmbeddedValue(String value) {
        if (value == null) {
            return null;
        }
        String result = value;
        //[org.springframework.beans.factory.config.
        // PropertyPlaceholderConfigurer$PlaceholderResolvingStringValueResolver@17d88132]
        for (StringValueResolver resolver : this.embeddedValueResolvers) {
            //这里就取到了真实的值
            result = resolver.resolveStringValue(result);
            if (result == null) {
                return null;
            }
        }
        return result;
    }

总结
如果一个bean里面的某个字段有注释@Value

@Component
public class CommonPo {
    @Value("${driverClassName}")
    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