Unravel › Java concepts › Concurrency

At a glance

1. 1new Thread(task).start() begins a new thread that runs taskCalling run() directly does not start a thread; it just runs the code on the current one.
2. 2join() waits for a thread to finish, and Thread.sleep pauses the current threadBoth throw InterruptedException, which means "someone asked this thread to stop".
3. 3Threads share heap memoryIf two threads read and change the same variable without coordination, results depend on timing (a race condition).
4. 4Modern Java (21+) offers virtual threads: cheap threads for blocking code, created with Thread.ofVirtual() or Executors.newVirtualThreadPerTaskExecutor()

• When you catch InterruptedException, either rethrow it or call Thread.currentThread().interrupt() to restore the flag; swallowing it makes the thread impossible to stop cleanly
• A platform thread maps to an operating system thread with its own stack (about 1 MB by default)

Thread worker = new Thread(() -> {    try {        Thread.sleep(1000);    } catch (InterruptedException e) {        Thread.currentThread().interrupt();   // keep the stop request alive    }}, "report-worker");worker.start();worker.join();

1. line 1 uses Thread, the core of Threads and shared state
2. line 3 uses Thread, the core of Threads and shared state
3. line 5 keep the stop request alive
4. line 8 uses start, the core of Threads and shared state
5. line 9 uses join, the core of Threads and shared state

How Unravel recognises it

When you paste code, Unravel looks for these signs of threads and shared state. Spotting them yourself is a good habit when you read unfamiliar code:

• Types: Thread ThreadLocal
• Method calls: start() join() interrupt() sleep() isInterrupted() currentThread() startVirtualThread() ofVirtual()
• Created with new: Thread

Part of a learning path

• Concurrency fundamentals · next: synchronized and locks

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