Fwd: questions about guava Suppliers

[Chris Povirk]

---------- Forwarded message ---------
From: Yang Song <heifr...@gmail.com>
Date: Fri, Sep 24, 2021 at 11:04 AM
Subject: questions about guava Suppliers
To: <cpo...@google.com>

Pardon me for a question about guava Suppliers.

I noticed there are two class in Suppliers, MemoizingSupplier and
NonSerializableMemoizingSupplier. The codes are similar, but the
delegate field is not the same, one is volatile and the other is not.

I cannot understand why. I think both can be non-volatile, or there is
something that i missed. Could you help explain a bit?

Thank you!

[Benjamin Manes]

My guess is that this might have been to protect against wrong lazily initialized static fields. While final and volatile fields have happens-before semantics, plain fields do not. Assigning to a plain static field after initialization would not insert a memory barrier, so the below logic was unfortunately. common pre-Java 5. In that case not only could the supplier be set twice, but another thread might not see a fully-formed instance (null delegate). Even though that practice was squashed for server-side Java many years prior, it was sadly still common to see in Android at the time of this change (2016). Otherwise, it may have merely been a desire to make clear and simple code since volatiles are generally very cheap compared to the mental energy to debug concurrent code. =)

static Supplier<X> supplier;

...

if (supplier == null) {

  supplier = Suppliers.memoize(X::new)

}

return supplier;

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