Java blocking queues for concurrency beginners | Hacker News Reader

Java blocking queues for concurrency beginners

geek-programmer.com

75 pointsam_sandeepa20 comments

Not recommended reading... the author consistently misuses the term "blocking," among other sins of ignorance.

How so?

One thing that stands out is the article's description of a free (non-busy) thread in a thread pool as "blocked." This is IMHO a misuse of the term--in normal usage, a blocked thread is one that is waiting for a call to complete, whether that's IO, computation, trying to add an item to a full bounded queue configured to make its caller wait rather than fail, etc.

But you're drawing a distinction where none exists. When a thread in a thread pool is "free", what it's doing is waiting for the next item of work to arrive -- and in implementation terms, that means the thread is blocked in a "take" operation on a currently-empty work queue. So it's totally correct to use the term "blocked" in that situation.

It's a free resource in a pool, probably in interruptible wait() or poll(), but don't you think this is an implementation detail and separate topic of discussion from a thread carrying out program logic and stuck on, say, uninterruptible IO?

I honestly hope author goes more indepth in next part of the article. Most common use case that I have seen for BlockingQueues is to use them in conjunction with ExecutorService and ThreadPoolExecutor.

In that context you have to use/implement Task rejection policies if queue is full. Blocking operation Put on the queue is not used when submitting task to the service, it uses Offer method instead. It's a bit misleading.

A queue cares not where the threads that enqueue/dequeue elements on it , come from.

I just define a queue class which extends a blocking queue and overrides offer to call put and provide that to the executorservice.

Queues are one of my favorite tools in my programing tool belt.

In Python they make it easier to right thread-safe code. I have written a similar article for Python with a focus on thread safety: https://lemanchet.fr/articles/learning-python-3-threading-mo...

This sounds like a WIP implementation of channels[1]. Am I wrong?

[1] https://en.wikipedia.org/wiki/Channel_(programming)

It's not a WIP implementation it's the most common one. The blocking queues in Java behave very similarly to the golang channels for instance.

Is there a select() analogue? Maybe under another name?

The take() function is equivalent to a bare <- channel read. the poll(time, timeunit) function is equivalent to a select with a case and a read off the done channel of a context with a timeout. The poll() function is equivalent to a select with a case and a default condition.

To get the behavior of a select composed of several channels and no default case I'd have to dig into the exact behavior of the select statement in go, but it could be accomplished with a loop over if/else if/else statement where the else had a thread sleep/thread yield.

I actually wrote a dsl in Scala a few years back that actually used the keyword select for this operation before I'd ever encountered golang. I used "on" instead of case but the behaviors were nearly identical.

I assume the name came from the select syscall, maybe unconsciously in your case :)

Hopefully something comes along to fill this gap officially. Channels are my favorite concurrency primitive by a huge margin. The more languages that support them, the better.

It wasn't unintentional that I used select. The dsl actually abstracted over network operations first and I only later added queues to the abstraction.

No. And this is certainly an annoyance at times!

missed synchronous blocking queues i think, a thread queue with size 0

Not nice of Java to block the queues. Especially since we're talking about concurrency beginners. It's hard enough for them as it is.

Nice thing about Java's verbose naming is the name tells you the behavior. It also has a lot of internal documentation in the Java. Such documentation is easily accessible from their IDe or the web.

As the article points out, you can block the producers or consumers. So while queues and threading are advanced for an entirely new concurrency programmer, the are the most fundamental block of the concurrence developer.

That is up to the developer - there are non blocking implementations of the Queue interface: https://docs.oracle.com/javase/8/docs/api/java/util/Queue.ht... , LinkedList for example.

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