Java: Synchronizing on primitives?

Question

In our system, we have a method that will do some work when it\'s called with a certain ID:

public void doWork(long id) { /* ... */ }

Now, this

Answer 1

To start with:

1. You can't lock on a primitive and
2. Don't lock on a Long unless you're careful how you construct them. Long values created by autoboxing or Long.valueOf() in a certain range are guaranteed to be the same across the JVM which means other threads could be locking on the same exact Long object and giving you cross-talk. This can be a subtle concurrency bug (similar to locking on intern'ed strings).

You're talking here about a lock-striping setup. One end of the continuum is a single giant lock for all ids which will is easy and safe but not concurrent. The other end is a lock per id which is easy (to some degree) and safe and very concurrent but might require a large number of "lock-able objects" in memory (if you don't already have them). Somewhere in the middle is the idea of creating a lock for a range of ids - this lets you adjust concurrency based on your environment and make choices about tradeoffs between memory and concurrency.

ConcurrentHashMap can be used to achieve this as CHM is made up internally of segments (sub-maps) and there is one lock per segment. This gives you concurrency equal to the number of segments (which defaults to 16 but is configurable).

There are a bunch of other possible solutions for partitioning your ID space and creating sets of locks but you are right to be sensitive to the clean up and memory leak issues - taking care of that while maintaining concurrency is a tricky business. You'll need to use some kind of reference counting on each lock and manage the eviction of old locks carefully to avoid evicting a lock that's in the process of being locked. If you go this route, use ReentrantLock or ReentrantReadWriteLock (and not synchronized on objects) as that lets you explicitly manage the lock as an object and use the extra methods available on it.

There is also some stuff on this and a StripedMap example in Java Concurrency in Practice section 11.4.3.