Teardown 02 - Copy-on-Write Array List and Set

"Rather use java.util.concurrent than write your own"

Who has not heard this sage advice?

Of course we agree, and follow this advice ourselves. But what if the constructs in java.util.concurrent differ from what we need? Can we write our own? And what can we learn from studying how the java.util.concurrent classes work?

In our Teardown series we study the classes in java.util.concurrent line-by-line. We analyze every for-loop and if statement, asking why the authors wrote it this way. We dissect the concurrency controls, looking at how the authors apply it to the construct.

The 2nd teardown course looks at the CopyOnWriteArrayList and its close cousin, the CopyOnWriteArraySet. Most of the concurrency controls in this class are fairly easy, but we do have delve into releaseFence for visibility and some odd volatile writes for maintaining order. The class used to use the ReentrantLock for synchronization, but in Java 9 this was changed to synchronized. These classes are useful when we need a thread-safe list that gives us a snapshot iterator. A lot of care has been taken to avoid synchronization unless it is necessary. For example, in the remove(Object) method, we first try find the index of the given object in the list. If we cannot find it, then we immediately leave. We then synchronize and verify that the list has not changed. If it has, then we need to search again.

This mini-course consists of almost 4 hours of detailed instruction of how the CopyOnWrite collections work. Watch it either from start to finish, or jump around between sections as your curiosity is piqued. You will definitely learn something - or your money back!

(Make sure to also check out our CopyOnWriteArrayList/Set Teardown)