Watcher Class — netty Architecture

Source Code

common/src/main/java/io/netty/util/ThreadDeathWatcher.java lines 146–228

    private static final class Watcher implements Runnable {

        private final List<Entry> watchees = new ArrayList<Entry>();

        @Override
        public void run() {
            for (;;) {
                fetchWatchees();
                notifyWatchees();

                // Try once again just in case notifyWatchees() triggered watch() or unwatch().
                fetchWatchees();
                notifyWatchees();

                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ignore) {
                    // Ignore the interrupt; do not terminate until all tasks are run.
                }

                if (watchees.isEmpty() && pendingEntries.isEmpty()) {

                    // Mark the current worker thread as stopped.
                    // The following CAS must always success and must be uncontended,
                    // because only one watcher thread should be running at the same time.
                    boolean stopped = started.compareAndSet(true, false);
                    assert stopped;

                    // Check if there are pending entries added by watch() while we do CAS above.
                    if (pendingEntries.isEmpty()) {
                        // A) watch() was not invoked and thus there's nothing to handle
                        //    -> safe to terminate because there's nothing left to do
                        // B) a new watcher thread started and handled them all
                        //    -> safe to terminate the new watcher thread will take care the rest
                        break;
                    }

                    // There are pending entries again, added by watch()
                    if (!started.compareAndSet(false, true)) {
                        // watch() started a new watcher thread and set 'started' to true.
                        // -> terminate this thread so that the new watcher reads from pendingEntries exclusively.
                        break;
                    }

                    // watch() added an entry, but this worker was faster to set 'started' to true.
                    // i.e. a new watcher thread was not started
                    // -> keep this thread alive to handle the newly added entries.
                }
            }
        }

        private void fetchWatchees() {
            for (;;) {
                Entry e = pendingEntries.poll();
                if (e == null) {
                    break;
                }

                if (e.isWatch) {
                    watchees.add(e);
                } else {
                    watchees.remove(e);
                }
            }
        }

        private void notifyWatchees() {
            List<Entry> watchees = this.watchees;
            for (int i = 0; i < watchees.size();) {
                Entry e = watchees.get(i);
                if (!e.thread.isAlive()) {
                    watchees.remove(i);
                    try {
                        e.task.run();
                    } catch (Throwable t) {
                        logger.warn("Thread death watcher task raised an exception:", t);
                    }
                } else {
                    i ++;
                }
            }