@Required 注释应用于 bean 属性的 setter 方法,它表明受影响的 bean 属性在配置时必须放在 XML 配置文件中,否则容器就会抛出一个 BeanInitializationException 异常。
RequiredAnnotationBeanPostProcessor就是用来解析@Required的,当一个bean实例经过SmartInstantiationAwareBeanPostProcessor的postProcessPropertyValues()方法时,检测bean中属性对应的setter方法被标记@Required但是用于赋值的PropertyValues对象不包含此属性就会抛出异常。如果不想对某个bean进行检测,可以将此bean的bean definition的org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor.skipRequiredCheck属性设置为false。此属性默认在ConfigurationClassBeanDefinitionReader#loadBeanDefinitionsForBeanMethod()方法中设置为true的。
@Override
public PropertyValues postProcessPropertyValues(
PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {
if (!this.validatedBeanNames.contains(beanName)) {
if (!shouldSkip(this.beanFactory, beanName)) {
List<String> invalidProperties = new ArrayList<>();
for (PropertyDescriptor pd : pds) {
if (isRequiredProperty(pd) && !pvs.contains(pd.getName())) {
invalidProperties.add(pd.getName());
}
}
if (!invalidProperties.isEmpty()) {
throw new BeanInitializationException(buildExceptionMessage(invalidProperties, beanName));
}
}
this.validatedBeanNames.add(beanName);
}
return pvs;
}
protected boolean shouldSkip(@Nullable ConfigurableListableBeanFactory beanFactory, String beanName) {
if (beanFactory == null || !beanFactory.containsBeanDefinition(beanName)) {
return false;
}
BeanDefinition beanDefinition = beanFactory.getBeanDefinition(beanName);
if (beanDefinition.getFactoryBeanName() != null) {
return true;
}
Object value = beanDefinition.getAttribute(SKIP_REQUIRED_CHECK_ATTRIBUTE);
return (value != null && (Boolean.TRUE.equals(value) || Boolean.valueOf(value.toString())));
}
protected boolean isRequiredProperty(PropertyDescriptor propertyDescriptor) {
Method setter = propertyDescriptor.getWriteMethod();
return (setter != null && AnnotationUtils.getAnnotation(setter, getRequiredAnnotationType()) != null);
}
private String buildExceptionMessage(List<String> invalidProperties, String beanName) {
int size = invalidProperties.size();
StringBuilder sb = new StringBuilder();
sb.append(size == 1 ? "Property" : "Properties");
for (int i = 0; i < size; i++) {
String propertyName = invalidProperties.get(i);
if (i > 0) {
if (i == (size - 1)) {
sb.append(" and");
}
else {
sb.append(",");
}
}
sb.append(" '").append(propertyName).append("'");
}
sb.append(size == 1 ? " is" : " are");
sb.append(" required for bean '").append(beanName).append("'");
return sb.toString();
}