Is Spring's @Lazy initialization thread-safe?

Question

Hopefully a simple question to answer. I couldn't find any absolute clear answer on this. Can anyone please help with a credible source.

I'm going by the general perception that lazy loading is dangerous in a multithreaded application without using a proper synchronized block. I'm wondering if this is applicable for Spring or not.

Thanks.

Answer 1

I dont know anywhere Spring claims for safe publication of the beans managed by IoC container.

However, quick glance through the bean factory/application context code detects that there are a lot of crucial places guarded by synchronization (eg registering/unregistering bean definition, post-processing phase etc). See many of the methods in DefaultSingletonBeanRegistry has synchronized blocks.

About lazy initialized beans - i see that inside of DefaultSingletonBeanRegistry, construction is guarded by a lock around singletonObjects instance variable. in addition, singletonObjects itself is a concurrent hash map.

It uses double-checked locking (org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton()), so the bean is safely published.

In addition regarding thread safety,i remember spring had bugs previously in initial release and is improved with versions , And note that :

1.singleton beans (lazy and non-lazy) are always fully instantiated (including init methods) and safely published. therefore their construction is thread-safe - if another thread does see this bean, it would never see a half-initialized singleton with default values for some fields

2.prototype beans construction is not thread-safe, but usually prototype beans are not shared between threads, and if they have to be - you are on your own to make sure you publish them to other threads safely this behavior is intended by the framework