Unravel › Java concepts › Concurrency

Learn these first

At a glance

1. 1Each thread has an interrupted flagt.interrupt() sets it; if the thread is blocked in sleep, wait or join, that call throws InterruptedException and clears the flag.
2. 2In a loop that does work, check while (!Thread.currentThread().isInterrupted()) { ..} and exit when asked.
3. 3If you catch InterruptedException and cannot rethrow, restore the flag: Thread.currentThread().interrupt(); so callers further up still see itNever leave an empty catch block.
4. 4Executors use it: shutdownNow() and future.cancel(true) interrupt the worker threads

• Swallowing the interrupt is a classic bug: the pool cannot shut down, the application hangs on exit, and a cancelled request keeps working
• Blocking I/O on plain sockets and synchronized waiting are not interruptible; use channels, timeouts, or lockInterruptibly()
• The flag lives in the JVM thread structure

public class Worker implements Runnable {  private final java.util.concurrent.BlockingQueue<String> queue;  Worker(java.util.concurrent.BlockingQueue<String> queue) { this.queue = queue; }  @Override public void run() {    try {      while (!Thread.currentThread().isInterrupted()) {        String job = queue.take();        System.out.println("job " + job);      }    } catch (InterruptedException e) {      Thread.currentThread().interrupt();    }  }}

1. line 6 uses currentThread, the core of Interruption and InterruptedException
2. line 10 uses InterruptedException, the core of Interruption and InterruptedException
3. line 11 uses currentThread, the core of Interruption and InterruptedException

How Unravel recognises it

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

• Types: InterruptedException
• Method calls: interrupted() isInterrupted() Thread.interrupted() Thread.currentThread() interrupt() sleep() join()

Part of a learning path

• Concurrency beyond the basics · previous: Runnable, Callable and Future · next: Latches, barriers and semaphores

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