Client-side locking

Question

Following is an excerpt from JCIP

The author says that in order to make the above code thread-safe we have to use client-side locking.

To make this approach work, we have to use the same lock that the List uses by using client-side locking or external locking. Client-side locking entails guarding client code that uses some object X with the lock X uses to guard its own state. In order to use client-side locking, you must know what lock X uses.

Why can't we simply make the List< E > object private in the very first place to make the ListHelper class thread-safe?

Answer 1

In that case where each instance of ListHelper would contain its own list you could make that list private and just synchronize on the ListHelper instance. I guess this is a somewhat constructed example to make a point with as little code as possible. IMO the name ListHelper would imply that I could pass an external list which could clearly be reused by multiple ListHelper instances.

I'd say the point is: given the code as it is and without changing visibility of list (could break other code) you better synchronize on list than the current ListHelper instance.