2021-11-28 Spring源码中的deference是什

// BeanFactory接口
/**
Used to dereference a FactoryBean instance and distinguish it from beans created by the FactoryBean. For example, if the bean named myJndiObject is a FactoryBean, getting &myJndiObject will return the factory, not the instance returned by the factory.
*/
String FACTORY_BEAN_PREFIX = "&";

看来说的就是这个&。继续看AbstractBeanFactory类中的getObjectForBeanInstance方法：

protected Object getObjectForBeanInstance(
            Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {

    // Don't let calling code try to dereference the factory if the bean isn't a factory.
    // 1，name带&，beanInstance是FactoryBean，说明这是要获取FactoryBean本身的实例
    if (BeanFactoryUtils.isFactoryDereference(name)) {
        if (beanInstance instanceof NullBean) {
            return beanInstance;
        }
        if (!(beanInstance instanceof FactoryBean)) {
            throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
        }
        if (mbd != null) {
            mbd.isFactoryBean = true;
        }
        return beanInstance;
    }

    // Now we have the bean instance, which may be a normal bean or a FactoryBean.
    // If it's a FactoryBean, we use it to create a bean instance, unless the
    // caller actually wants a reference to the factory.
    // 2，name不带&，beanInstance不是FactoryBean，说明是一个普通的bean
    if (!(beanInstance instanceof FactoryBean)) {
        return beanInstance;
    }

    // 3，name不带&，beanInstance是FactoryBean，说明需要FactoryBean去创建name的实例
    Object object = null;
    if (mbd != null) {
        mbd.isFactoryBean = true;
    }
    else {
        object = getCachedObjectForFactoryBean(beanName);
    }
    if (object == null) {
        // Return bean instance from factory.
        FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
        // Caches object obtained from FactoryBean if it is a singleton.
        if (mbd == null && containsBeanDefinition(beanName)) {
            mbd = getMergedLocalBeanDefinition(beanName);
        }
        boolean synthetic = (mbd != null && mbd.isSynthetic());
        object = getObjectFromFactoryBean(factory, beanName, !synthetic);
    }
    return object;
}

进到BeanFactoryUtils的isFactoryDereference方法里：

/**
Return whether the given name is a factory dereference (beginning with the factory dereference prefix).
Params:
name – the name of the bean
Returns:
whether the given name is a factory dereference
See Also:
BeanFactory.FACTORY_BEAN_PREFIX
*/
public static boolean isFactoryDereference(@Nullable String name) {
    return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
}

从这个方法也进一步证实，deference说的就是这个&。