Spring 之@Value注解原理

　　@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

public PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {

        InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
　　　　　//findAutowiringMetadata表示找到该bean中所有的@Value和@Autowired注解，然后组成InjectionMetadata

try {
            metadata.inject(bean, beanName, pvs);
        }
        catch (BeanCreationException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
        }
        return pvs;
    }

metadata.inject还是会遍历处理每一个 AutowiredFieldElement 或者 AutowiredMethodElement

protected void inject(Object bean, String beanName, PropertyValues pvs) throws Throwable {
            Field field = (Field) this.member;
            Object value;
            if (this.cached) {
                value = resolvedCachedArgument(beanName, this.cachedFieldValue);
            }
            else {
                DependencyDescriptor desc = new DependencyDescriptor(field, this.required);
                desc.setContainingClass(bean.getClass());
                Set<String> autowiredBeanNames = new LinkedHashSet<String>(1);
                TypeConverter typeConverter = beanFactory.getTypeConverter();
                try {
                    value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
                }
                catch (BeansException ex) {
                    throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);
                }

　　继续

　　DefaultListableBeanFactory.resolveDependency

public Object resolveDependency(DependencyDescriptor descriptor, String requestingBeanName,
            Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

        descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
        if (javaUtilOptionalClass == descriptor.getDependencyType()) {
            return new OptionalDependencyFactory().createOptionalDependency(descriptor, requestingBeanName);
        }
        else if (ObjectFactory.class == descriptor.getDependencyType() ||
                ObjectProvider.class == descriptor.getDependencyType()) {
            return new DependencyObjectProvider(descriptor, requestingBeanName);
        }
        else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
            return new Jsr330ProviderFactory().createDependencyProvider(descriptor, requestingBeanName);
        }
        else {
            Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
                    descriptor, requestingBeanName);
            if (result == null) {
                result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
            }
            return result;
        }
    }

　　

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();
            Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);//${driverClassName}
            if (value != null) {
                if (value instanceof String) {
                    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()));
            }

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