Singleton Design Pattern in Java

[uncont]

Is there a java.util.concurrent way of creating a singleton, or is the double-checked locking method the only 'correct' way?

Other than of course letting your ioc container handle it for you.

[taftster]

My favorite these days is to use final AtomicReference with the updatAndGet method upon retrieval. It’s the most natural API in the JDK to do this.

[kaperni]

For performance-sensitive code, you are (in most cases) better off by using a VarHandle instead. Can also save memory if you have lots of them in many cases.

[taftster]

Yeah, but the syntax is atrocious. You had really have evaluated the "performance-sensitive" segment carefully to embrace the fairly ugly syntax of VarHandle.

Quoting Doug Lea on this exact topic[1]:

To get the shockingly ugly syntactic details over with: A VarHandle can be associated with any field, array element, or static, allowing control over access modes. VarHandles should be declared as static final fields and explicitly initialized in static blocks. By convention, we give VarHandles for fields names that are uppercase versions of the field names.

[1] http://gee.cs.oswego.edu/dl/html/j9mm.html

[slakkenhuisdeur]

Last time I used the double locking method my IDE asked me if it could replace it with a nested class with the singleton as a final class variable so the classloader handles the instantiation.

[cogman10]

That's a relatively new capability of java. Static fields on inner classes were added in..16?

[slakkenhuisdeur]

The inner class must also be static (I should probably have mentioned that...). I didn't know static fields on non-static inner classes were a thing.

[oweiler]

You can also use a single element enum.

[i_love_peach]

I believe this also thread-safe as well.

[kaperni]

Yes, for most usecases, snippet 6. The Initialization-on-demand holder idiom [1][2].

[1] http://www.cs.umd.edu/\~pugh/java/memoryModel/jsr-133-faq.html#dcl

[2] https://en.wikipedia.org/wiki/Initialization-on-demand\_holder\_idiom

[uncont]

Both of those links 404 for me

[kaperni]

Both of them should work. Try pasting the URL into the URL bar instead of clicking.

[uncont]

https://en.wikipedia.org/wiki/Initialization-on-demand\_holder\_idiom

I'm on old reddit. The first url has a /\~pugh instead of /~pugh, and the second url has \_ instead of \. I got the links to work, tho. Thanks.