问题
What are the thread safety guarantees for Guava's ImmutableList.Builder? The javadocs don't say.
回答1:
While the Guava Immutable classes are threadsafe, their builders are not. For most applications, only one thread will interact with any particular Builder instance.
While the absence of thread-safety usually doesn't need to be documented, such Javadoc might make sense for the Immutable collection builders. People may be surprised that ImmutableList is threadsafe while ImmutableList.Builder isn't.
回答2:
If thread-safety is not mentioned in the javadocs, don't assume it!
More seriously, "no".
I would also prefer javadocs of ImmutableList and friends include such a -rather obvious, yes- remark (so you wouldn't have to assume it yourself), because the "obvious" is not always the case. Just the other day I was discussing scala.List, an immutable list, and some surprizing issues it may cause if exchanged between threads inappropriately (via a data race), which people didn't think about because they see the word "immutable" on the tin, plus they equate "immutable == thread-safe", so it pays off to be on the safe side even when documenting "obvious" thread-safety aspects.
回答3:
Agree with @Dimitris Andreou: definitely do not assume thread safety if its not documented as such. When you go to the effort of making a non-trivial class threadsafe, you want users to know it.
Beyond that, I think the most common use case for a builder will be thread-confined: ie as a local variable in some method. If you need multiple threads to build a List, is is really immutable yet?
If you have multiple threads feeding into a list, but want to snapshot it at some point and say "no more changes going forward, its immutable" then I'd write something that takes the elements from those threads and freezes the contents into a new ImmutableList when you know its ready.
来源：https://stackoverflow.com/questions/2870744/is-guavas-immutablelist-builder-thread-safe