6、什么是FactoryBean

答：

FactoryBean接口源码：

public interface FactoryBean<T> {

    /**
     * The name of an attribute that can be
     * {@link org.springframework.core.AttributeAccessor#setAttribute set} on a
     * {@link org.springframework.beans.factory.config.BeanDefinition} so that
     * factory beans can signal their object type when it can't be deduced from
     * the factory bean class.
     * @since 5.2
     */
    String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";


    /**
     * Return an instance (possibly shared or independent) of the object
     * managed by this factory.
     * <p>As with a {@link BeanFactory}, this allows support for both the
     * Singleton and Prototype design pattern.
     * <p>If this FactoryBean is not fully initialized yet at the time of
     * the call (for example because it is involved in a circular reference),
     * throw a corresponding {@link FactoryBeanNotInitializedException}.
     * <p>As of Spring 2.0, FactoryBeans are allowed to return {@code null}
     * objects. The factory will consider this as normal value to be used; it
     * will not throw a FactoryBeanNotInitializedException in this case anymore.
     * FactoryBean implementations are encouraged to throw
     * FactoryBeanNotInitializedException themselves now, as appropriate.
     * @return an instance of the bean (can be {@code null})
     * @throws Exception in case of creation errors
     * @see FactoryBeanNotInitializedException
     */
    @Nullable
    T getObject() throws Exception;

    /**
     * Return the type of object that this FactoryBean creates,
     * or {@code null} if not known in advance.
     * <p>This allows one to check for specific types of beans without
     * instantiating objects, for example on autowiring.
     * <p>In the case of implementations that are creating a singleton object,
     * this method should try to avoid singleton creation as far as possible;
     * it should rather estimate the type in advance.
     * For prototypes, returning a meaningful type here is advisable too.
     * <p>This method can be called <i>before</i> this FactoryBean has
     * been fully initialized. It must not rely on state created during
     * initialization; of course, it can still use such state if available.
     * <p><b>NOTE:</b> Autowiring will simply ignore FactoryBeans that return
     * {@code null} here. Therefore it is highly recommended to implement
     * this method properly, using the current state of the FactoryBean.
     * @return the type of object that this FactoryBean creates,
     * or {@code null} if not known at the time of the call
     * @see ListableBeanFactory#getBeansOfType
     */
    @Nullable
    Class<?> getObjectType();

    /**
     * Is the object managed by this factory a singleton? That is,
     * will {@link #getObject()} always return the same object
     * (a reference that can be cached)?
     * <p><b>NOTE:</b> If a FactoryBean indicates to hold a singleton object,
     * the object returned from {@code getObject()} might get cached
     * by the owning BeanFactory. Hence, do not return {@code true}
     * unless the FactoryBean always exposes the same reference.
     * <p>The singleton status of the FactoryBean itself will generally
     * be provided by the owning BeanFactory; usually, it has to be
     * defined as singleton there.
     * <p><b>NOTE:</b> This method returning {@code false} does not
     * necessarily indicate that returned objects are independent instances.
     * An implementation of the extended {@link SmartFactoryBean} interface
     * may explicitly indicate independent instances through its
     * {@link SmartFactoryBean#isPrototype()} method. Plain {@link FactoryBean}
     * implementations which do not implement this extended interface are
     * simply assumed to always return independent instances if the
     * {@code isSingleton()} implementation returns {@code false}.
     * <p>The default implementation returns {@code true}, since a
     * {@code FactoryBean} typically manages a singleton instance.
     * @return whether the exposed object is a singleton
     * @see #getObject()
     * @see SmartFactoryBean#isPrototype()
     */
    default boolean isSingleton() {
        return true;
    }

}