Real life use and explanation of the AtomicLongFieldUpdate class

问题

Is anybody aware of any real life use of the class AtomicLongFieldUpdate? I have read the description but I have not quite grasped the meaning of it. Why do I want to know that? Curiosity and for OCPJP preparation.

Thanks in advance.

回答1:

You can think of a cost ladder for the following:

ordinary long: cheap, but unsafe for multi-threaded access
volatile long: more expensive, safe for multi-threaded access, atomic operations not possible
AtomicLong: most expensive, safe for multi-threaded access, atomic operations possible

(When I say 'unsafe' or 'not possible' I mean 'without an external mechanism like synchronization' of course.)

In the case where multi-threaded access is needed, but most operations are simple reads or writes, with only a few atomic operations needed, you can create one static instance of AtomicLongFieldUpdate and use this when atomic updates are needed. The memory/runtime overhead is then similar to a simple volatile variable, except for the atomic operations which are of the order of (or slightly more expensive than) the ordinary AtomicLong operations.

Here is a nice little tutorial.

回答2:

Is anybody aware of any real life use of the AtomicLongFieldUpdate class?

I've never used this class myself but in doing a get usage on my workspace I see a couple "real life" instances of its use:

com.google.common.util.concurrent.AtomicDouble uses it to atomically modify their internal volatile long field which stores the bits from a double using Number.doubleToRawLongBits(...). Pretty cool.
net.sf.ehcache.Element uses it to atomically update the hitCount field.

I have read the description but I have not quite grasped the meaning of it.

It basically provides the same functionality as AtomicLong but on a field local to another class. The memory load of the AtomicLongFieldUpdate is less than the AtomicLong in that you configure one instance of the update for each field so lower memory overhead but more CPU overhead (albeit maybe small) from the reflection.

The javadocs say:

This class is designed for use in atomic data structures in which several fields of the same node are independently subject to atomic updates.

Sure but then I'd just use multiple Atomic* fields. Just about the only reason why I'd use the class is if there was an existing class that I could not change that I wanted to increment atomically.

回答3:

The reason why you would use e.g. AtomicLongFieldUpdater in favor to AtomicLong is simply to reduce the heap cost. Internally both work pretty much the same on th compareAndSet level which both use sun.misc.Unsafe at the end.

Consider you have a certain class that is initialized 1000k times. With AtomicLong you'd create 1000k AtomicLongs. With AtomicLongFieldUpdater on the other hand, you'd create 1 CONSTANT AtomicLongFieldUpdater and 1000k long primitives which of course does not need so much heap space.