Unravel › Java concepts › Concurrency

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At a glance

1. 1ConcurrentHashMap locks only parts of the table, so threads rarely block each otherUse its atomic methods (putIfAbsent, computeIfAbsent, merge) for check-then-act logic.
2. 2BlockingQueue implements the producer/consumer pattern: put waits when full, take waits when emptyA bounded queue gives natural back-pressure.
3. 3CopyOnWriteArrayList copies the whole array on every change; it is good for rarely changed lists read very often (listener lists)

• Individually safe operations are not safe together: if (!map.containsKey(k)) map.put(k, v) is a race even on a ConcurrentHashMap; use putIfAbsent
• Since Java 8 ConcurrentHashMap uses compare-and-swap for empty buckets and locks only the head node of a bucket for updates

private final ConcurrentHashMap<String, LongAdder> hits = new ConcurrentHashMap<>();private final BlockingQueue<Job> queue = new ArrayBlockingQueue<>(100);public void count(String page) {    hits.computeIfAbsent(page, k -> new LongAdder()).increment();}public void submit(Job job) throws InterruptedException {    queue.put(job);          // waits when the queue is full}

1. line 1 uses ConcurrentHashMap, the core of Concurrent collections
2. line 2 uses BlockingQueue, the core of Concurrent collections
3. line 8 waits when the queue is full

How Unravel recognises it

When you paste code, Unravel looks for these signs of concurrent collections. Spotting them yourself is a good habit when you read unfamiliar code:

• Types: ConcurrentHashMap CopyOnWriteArrayList CopyOnWriteArraySet ConcurrentLinkedQueue BlockingQueue LinkedBlockingQueue ArrayBlockingQueue PriorityBlockingQueue ConcurrentSkipListMap ConcurrentLinkedDeque
• Method calls: synchronizedList() synchronizedMap()

Part of a learning path

• Concurrency fundamentals · previous: CompletableFuture

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