Should I synchronize on a ReferenceQueue?
concurrency, java
Solution
If you look at `ReferenceQueue` you will see that it explicitly supports threading inside the platform, because it states that the `remove()` method will block until a new entry is available.
The `synchronized` you see in `WeakHashMap` is about ensuring that multiple threads accessing a `ReferenceQueue` are properly synchronized.
You might find this related bug interesting.
To answer your question, I think external synchronization of the `ReferenceQueue` is not required if you ensure it is only accessed by a single thread. I would not use (and can't think of a good reason) to use a single `ReferenceQueue` as a consumer from multiple threads.
Problem
I was taking a look at the source code for `WeakHashMap` and stumbled across this: ``` private final ReferenceQueue<Object> queue = new ReferenceQueue<>(); private void expungeStaleEntries() { for (Object x; (x = queue.poll()) != null; ) { synchronized (queue) { /* snip */ } } } ``` Why does this method synchronize on the `ReferenceQueue`? `WeakHashMap` itself does not make claims to be thread safe: Like most collection classes, this class is not synchronized. A synchronized WeakHashMap may be constructed using the Collections.synchronizedMap method. Which led me to believe that this implementation detail is to, somehow, ensure the thread safety of the `ReferenceQueue` itself (since the GC will be modifying it from its own `Thread`). However, the documentation for `ReferenceQueue` does not mention anything about any concurrency concerns, and taking a look at the source code for `ReferenceQueue` reveals that it doesn't even synchronize on itself (it uses an internal lock). Why is `WeakHashMap` synchronizing on its `ReferenceQueue`? Should I synchronize on a `ReferenceQueue` every time I use it?