Java File WatchService

    String path = "/tmp";
    // 获取当前OS平台下的文件系统监控器
    WatchService watcher = FileSystems.getDefault().newWatchService();
    //将监控器注册给指定的文件节点,该方法会让监控器线程就绪并运行，调用完后监控器就开始监控

    /* 文件变化枚举类型
     * ENTRY_CREATE：创建
     * ENTRY_DELETE：删除
     * ENTRY_MODIFY：修改
     */
    Paths.get(path).register(watcher,
        StandardWatchEventKinds.ENTRY_CREATE,
        StandardWatchEventKinds.ENTRY_DELETE,
        StandardWatchEventKinds.ENTRY_MODIFY);

    while (true) {
        WatchKey key = watcher.take();
        // 获得WatchKey（监控池）中的具体监控信息,
        // !!! 一个文件变化动作可能会引发一系列的事件，因此WatchKey中保存着一个事件列表List<WatchEvent<?>> list
        for (WatchEvent<?> event: key.pollEvents()) {

            System.out.println(event.context() + " comes to " + event.kind());
        }
        // 完成一次监控就需要重置监控器一次.
        boolean valid = key.reset(); 
        if (!valid) {
            break;
        }
    }
// ---------- output --------------
// tmp.log comes to ENTRY_DELETE

abstract class AbstractWatchService implements WatchService {

    // signaled keys waiting to be dequeued
    private final LinkedBlockingDeque<WatchKey> pendingKeys =
        new LinkedBlockingDeque<WatchKey>();

    //watchservice 关闭标志key, 特殊无意义消息用于防止消费者阻塞
    private final WatchKey CLOSE_KEY =
        new AbstractWatchKey(null, null) {
            @Override
            public boolean isValid() {
                return true;
            }

            @Override
            public void cancel() {
            }
        };

    // used when closing watch service
    private volatile boolean closed;
    private final Object closeLock = new Object();

    protected AbstractWatchService() {
    }

    abstract WatchKey register(Path path,
                               WatchEvent.Kind<?>[] events,
                               WatchEvent.Modifier... modifers)
        throws IOException;

    // used by AbstractWatchKey to enqueue key
    final void enqueueKey(WatchKey key) {
        pendingKeys.offer(key);
    }

    /**
     * Throws ClosedWatchServiceException if watch service is closed
     */
    private void checkOpen() {
        if (closed)
            throw new ClosedWatchServiceException();
    }

    /**
     * Checks the key isn't the special CLOSE_KEY used to unblock threads when
     * the watch service is closed.
     */
    private void checkKey(WatchKey key) {
        if (key == CLOSE_KEY) {
            // 重新入队，防止有线程阻塞，直到不再有线程阻塞。
            enqueueKey(key);
        }
        checkOpen();
    }

    @Override
    public final WatchKey poll() {
        checkOpen();
        WatchKey key = pendingKeys.poll();
        checkKey(key);
        return key;
    }

    @Override
    public final WatchKey poll(long timeout, TimeUnit unit)
        throws InterruptedException
    {
        checkOpen();
        WatchKey key = pendingKeys.poll(timeout, unit);
        checkKey(key);
        return key;
    }

    @Override
    public final WatchKey take()
        throws InterruptedException
    {
        checkOpen();
        WatchKey key = pendingKeys.take();
        checkKey(key);
        return key;
    }

    /**
     * Tells whether or not this watch service is open.
     */
    final boolean isOpen() {
        return !closed;
    }

    /**
     * Retrieves the object upon which the close method synchronizes.
     */
    final Object closeLock() {
        return closeLock;
    }

    /**
     * Closes this watch service. This method is invoked by the close
     * method to perform the actual work of closing the watch service.
     */
    abstract void implClose() throws IOException;

    @Override
    public final void close()
        throws IOException
    {
        synchronized (closeLock) { 
            // 此处加锁，防止多次调用 implclose()方法
            if (closed)
                return;
            closed = true;

            implClose();

            // clear pending keys and queue special key to ensure that any
            // threads blocked in take/poll wakeup
            pendingKeys.clear();
            // 插入一条标志结束的key，防止调用线程一直阻塞
            pendingKeys.offer(CLOSE_KEY);
        }
    }
}

class PollingWatchService extends AbstractWatchService{
    // 注册 map，用于存放注册信息。
    private final Map<Object,PollingWatchKey> map =
        new HashMap<Object,PollingWatchKey>();

    // polling 定时线程池
    private final ScheduledExecutorService scheduledExecutor;

    PollingWatchService() {
        scheduledExecutor = Executors
            .newSingleThreadScheduledExecutor(new ThreadFactory() {
                 @Override
                 public Thread newThread(Runnable r) {
                     Thread t = new Thread(r);
                     t.setDaemon(true);
                     return t;
                 }});
    }

    @Override
    WatchKey register(final Path path, WatchEvent.Kind<?>[] events,WatchEvent.Modifier... modifiers)
         throws IOException{

        final Set<WatchEvent.Kind<?>> eventSet =
            new HashSet<WatchEvent.Kind<?>>(events.length);
        for (WatchEvent.Kind<?> event: events) {
            // standard events
            if (event == StandardWatchEventKinds.ENTRY_CREATE ||
                event == StandardWatchEventKinds.ENTRY_MODIFY ||
                event == StandardWatchEventKinds.ENTRY_DELETE){
                eventSet.add(event);
                continue; // 跳过无法识别的 event
            }
            // OVERFLOW is ignored
            if (event == StandardWatchEventKinds.OVERFLOW) {
                continue;
            }

            // null/unsupported
            if (event == null)
                throw new NullPointerException("An element in event set is 'null'");
            throw new UnsupportedOperationException(event.name());
        }
        if (eventSet.isEmpty())
            throw new IllegalArgumentException("No events to register");

        // 改变 polling 的时间间隔，敏感度调节(如果支持)
        SensitivityWatchEventModifier sensivity = SensitivityWatchEventModifier.MEDIUM;
        if (modifiers.length > 0) {
            for (WatchEvent.Modifier modifier: modifiers) {
                if (modifier == null)
                    throw new NullPointerException();
                if (modifier instanceof SensitivityWatchEventModifier) {
                    sensivity = (SensitivityWatchEventModifier)modifier;
                    continue;
                }
                throw new UnsupportedOperationException("Modifier not supported");
            }
        }

        // check if watch service is closed
        if (!isOpen())
            throw new ClosedWatchServiceException();

        // registration is done in privileged block as it requires the
        // attributes of the entries in the directory.
        try {
            final SensitivityWatchEventModifier s = sensivity;
            return AccessController.doPrivileged(
                new PrivilegedExceptionAction<PollingWatchKey>() {
                    @Override
                    public PollingWatchKey run() throws IOException {
                        return doPrivilegedRegister(path, eventSet, s);
                    }
                });
        } catch (PrivilegedActionException pae) {
            Throwable cause = pae.getCause();
            if (cause != null && cause instanceof IOException)
                throw (IOException)cause;
            throw new AssertionError(pae);
        }
    }

    // registers directory returning a new key if not already registered or
    // existing key if already registered
    private PollingWatchKey doPrivilegedRegister(Path path,
                                                 Set<? extends WatchEvent.Kind<?>> events,
                                                 SensitivityWatchEventModifier sensivity)
        throws IOException
    {
        // check file is a directory and get its file key if possible
        BasicFileAttributes attrs = Files.readAttributes(path, BasicFileAttributes.class);
        if (!attrs.isDirectory()) {
            throw new NotDirectoryException(path.toString());
        }
        Object fileKey = attrs.fileKey();
        if (fileKey == null)
            throw new AssertionError("File keys must be supported");

        // grab close lock to ensure that watch service cannot be closed
        synchronized (closeLock()) {
            if (!isOpen())
                throw new ClosedWatchServiceException();

            PollingWatchKey watchKey;
            synchronized (map) {
                watchKey = map.get(fileKey);
                if (watchKey == null) {
                    // new registration
                    watchKey = new PollingWatchKey(path, this, fileKey);
                    map.put(fileKey, watchKey);
                } else {
                    // update to existing registration
                    // 更新配置前，先禁用
                    watchKey.disable();
                }
            }
            // 此处开启 定时 polling 线程
            watchKey.enable(events, sensivity.sensitivityValueInSeconds());
            return watchKey;
        }
    }

    @Override
    void implClose() throws IOException {
        synchronized (map) {
            // 先 close 资源
            for (Map.Entry<Object,PollingWatchKey> entry: map.entrySet()) {
                PollingWatchKey watchKey = entry.getValue();
                watchKey.disable();
                watchKey.invalidate();
            }
            map.clear();
        }
        AccessController.doPrivileged(new PrivilegedAction<Void>() {
            @Override
            public Void run() {
                scheduledExecutor.shutdown();
                return null;
            }
         });
    }
}

abstract class AbstractWatchKey implements WatchKey {

    /**
     * 最大可以记录 event 事件数
     */
    static final int MAX_EVENT_LIST_SIZE    = 512;

    /**
     * 容量溢出标志事件
     */
    static final Event<Object> OVERFLOW_EVENT =
        new Event<Object>(StandardWatchEventKinds.OVERFLOW, null);

    /**
     * Possible key states
     */
    private static enum State { READY, SIGNALLED };

    // reference to watcher
    private final AbstractWatchService watcher;

    // reference to the original directory
    private final Path dir;

    // key state
    private State state;

    // pending events
    private List<WatchEvent<?>> events;

    // maps a context to the last event for the context (iff the last queued
    // event for the context is an ENTRY_MODIFY event).
    private Map<Object,WatchEvent<?>> lastModifyEvents;

    protected AbstractWatchKey(Path dir, AbstractWatchService watcher) {
        this.watcher = watcher;
        this.dir = dir;
        this.state = State.READY;
        this.events = new ArrayList<WatchEvent<?>>();
        this.lastModifyEvents = new HashMap<Object,WatchEvent<?>>();
    }

    final AbstractWatchService watcher() {
        return watcher;
    }

    /**
     * Return the original watchable (Path)
     */
    @Override
    public Path watchable() {
        return dir;
    }

    /**
     * Enqueues this key to the watch service
     */
    final void signal() {
        synchronized (this) {
            if (state == State.READY) { // 避免重复入队
                state = State.SIGNALLED;
                watcher.enqueueKey(this);
            }
        }
    }

    /**
     * 发送事件
     */
    @SuppressWarnings("unchecked")
    final void signalEvent(WatchEvent.Kind<?> kind, Object context) {
        boolean isModify = (kind == StandardWatchEventKinds.ENTRY_MODIFY);
        synchronized (this) {
            int size = events.size();
            if (size > 0) {
                // if the previous event is an OVERFLOW event or this is a
                // repeated event then we simply increment the counter
                // 合并相同事件
                WatchEvent<?> prev = events.get(size-1);
                if ((prev.kind() == StandardWatchEventKinds.OVERFLOW) ||
                    ((kind == prev.kind() &&
                     Objects.equals(context, prev.context()))))
                {
                    ((Event<?>)prev).increment();
                    return;
                }

                // if this is a modify event and the last entry for the context
                // is a modify event then we simply increment the count
                // 合并修改事件
                if (!lastModifyEvents.isEmpty()) {
                    if (isModify) {
                        WatchEvent<?> ev = lastModifyEvents.get(context);
                        if (ev != null) {
                            assert ev.kind() == StandardWatchEventKinds.ENTRY_MODIFY;
                            ((Event<?>)ev).increment();
                            return;
                        }
                    } else {
                        // not a modify event so remove from the map as the
                        // last event will no longer be a modify event.
                        // 如果不是修改事件，删除最近修改记录。
                        lastModifyEvents.remove(context);
                    }
                }

                // if the list has reached the limit then drop pending events
                // and queue an OVERFLOW event
                if (size >= MAX_EVENT_LIST_SIZE) {
                    kind = StandardWatchEventKinds.OVERFLOW;
                    isModify = false;
                    context = null;
                }
            }

            // non-repeated event
            Event<Object> ev =
                new Event<Object>((WatchEvent.Kind<Object>)kind, context);
            if (isModify) {
                lastModifyEvents.put(context, ev);
            } else if (kind == StandardWatchEventKinds.OVERFLOW) {
                // drop all pending events
                events.clear();
                lastModifyEvents.clear();
            }
            events.add(ev);
            signal(); // 此处watchkey入队
        }
    }

    @Override
    public final List<WatchEvent<?>> pollEvents() {
        synchronized (this) {
            List<WatchEvent<?>> result = events;
            events = new ArrayList<WatchEvent<?>>();
            lastModifyEvents.clear();
            return result;
        }
    }

    // 重置 watchkey
    @Override
    public final boolean reset() {
        synchronized (this) {
            if (state == State.SIGNALLED && isValid()) {
                if (events.isEmpty()) {
                    state = State.READY;
                } else {
                    // pending events so re-queue key
                    watcher.enqueueKey(this);
                }
            }
            return isValid();
        }
    }

    /**
     * WatchEvent implementation
     */
    private static class Event<T> implements WatchEvent<T> {
        private final WatchEvent.Kind<T> kind;
        private final T context;
        // ... ...

    }
}

private class PollingWatchKey extends AbstractWatchKey {
        private final Object fileKey;

        // current event set
        private Set<? extends WatchEvent.Kind<?>> events;

        // the result of the periodic task that causes this key to be polled
        private ScheduledFuture<?> poller;

        // indicates if the key is valid
        private volatile boolean valid;

        // used to detect files that have been deleted
        private int tickCount;

        // map of entries in directory
        private Map<Path,CacheEntry> entries;

        PollingWatchKey(Path dir, PollingWatchService watcher, Object fileKey)
            throws IOException
        {
            super(dir, watcher);
            this.fileKey = fileKey;
            this.valid = true;
            this.tickCount = 0;
            this.entries = new HashMap<Path,CacheEntry>();

            // get the initial entries in the directory
            try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir)) {
                for (Path entry: stream) {
                    // don't follow links
                    long lastModified =
                        Files.getLastModifiedTime(entry, LinkOption.NOFOLLOW_LINKS).toMillis();
                    entries.put(entry.getFileName(), new CacheEntry(lastModified, tickCount));
                }
            } catch (DirectoryIteratorException e) {
                throw e.getCause();
            }
        }

        Object fileKey() {
            return fileKey;
        }

        @Override
        public boolean isValid() {
            return valid;
        }

        void invalidate() {
            valid = false;
        }

        // 开启定时 polling
        void enable(Set<? extends WatchEvent.Kind<?>> events, long period) {
            synchronized (this) {
                // update the events
                this.events = events;

                // create the periodic task
                Runnable thunk = new Runnable() { public void run() { poll(); }};
                this.poller = scheduledExecutor
                    .scheduleAtFixedRate(thunk, period, period, TimeUnit.SECONDS);
            }
        }

        // disables periodic polling
        void disable() {
            synchronized (this) {
                if (poller != null)
                    poller.cancel(false);
            }
        }

        @Override
        public void cancel() {
            valid = false;
            synchronized (map) {
                map.remove(fileKey());
            }
            disable();
        }

        /**
         * Polls the directory to detect for new files, modified files, or
         * deleted files.
         */
        synchronized void poll() {
            if (!valid) {
                return;
            }

            // update tick, 轮询次数
            tickCount++;

            // open directory
            DirectoryStream<Path> stream = null;
            try {
                stream = Files.newDirectoryStream(watchable());
            } catch (IOException x) {
                // directory is no longer accessible so cancel key
                cancel();
                signal();
                return;
            }

            // iterate over all entries in directory
            try {
                for (Path entry: stream) {
                    long lastModified = 0L;
                    try {
                        lastModified =
                            Files.getLastModifiedTime(entry, LinkOption.NOFOLLOW_LINKS).toMillis();
                    } catch (IOException x) {
                        // unable to get attributes of entry. If file has just
                        // been deleted then we'll report it as deleted on the
                        // next poll
                        continue;
                    }

                    // lookup cache, 根据上次快照,产生文件变化事件
                    CacheEntry e = entries.get(entry.getFileName());
                    if (e == null) {
                        // new file found
                        entries.put(entry.getFileName(),
                                     new CacheEntry(lastModified, tickCount));

                        // queue ENTRY_CREATE if event enabled
                        if (events.contains(StandardWatchEventKinds.ENTRY_CREATE)) {
                            signalEvent(StandardWatchEventKinds.ENTRY_CREATE, entry.getFileName());
                            continue;
                        } else {
                            // if ENTRY_CREATE is not enabled and ENTRY_MODIFY is
                            // enabled then queue event to avoid missing out on
                            // modifications to the file immediately after it is
                            // created.
                            if (events.contains(StandardWatchEventKinds.ENTRY_MODIFY)) {
                                signalEvent(StandardWatchEventKinds.ENTRY_MODIFY, entry.getFileName());
                            }
                        }
                        continue;
                    }

                    // check if file has changed
                    if (e.lastModified != lastModified) {
                        if (events.contains(StandardWatchEventKinds.ENTRY_MODIFY)) {
                            signalEvent(StandardWatchEventKinds.ENTRY_MODIFY,
                                        entry.getFileName());
                        }
                    }
                    // entry in cache so update poll time
                    e.update(lastModified, tickCount);

                }
            } catch (DirectoryIteratorException e) {
                // ignore for now; if the directory is no longer accessible
                // then the key will be cancelled on the next poll
            } finally {

                // close directory stream
                try {
                    stream.close();
                } catch (IOException x) {
                    // ignore
                }
            }

            // iterate over cache to detect entries that have been deleted
            Iterator<Map.Entry<Path,CacheEntry>> i = entries.entrySet().iterator();
            while (i.hasNext()) {
                Map.Entry<Path,CacheEntry> mapEntry = i.next();
                CacheEntry entry = mapEntry.getValue();
                if (entry.lastTickCount() != tickCount) {
                    Path name = mapEntry.getKey();
                    // remove from map and queue delete event (if enabled)
                    i.remove();
                    if (events.contains(StandardWatchEventKinds.ENTRY_DELETE)) {
                        signalEvent(StandardWatchEventKinds.ENTRY_DELETE, name);
                    }
                }
            }
        }
    }


    /**
     * Entry in directory cache to record file last-modified-time and tick-count
     */
    private static class CacheEntry {
        private long lastModified;
        private int lastTickCount;

        CacheEntry(long lastModified, int lastTickCount) {
            this.lastModified = lastModified;
            this.lastTickCount = lastTickCount;
        }

        int lastTickCount() {
            return lastTickCount;
        }

        long lastModified() {
            return lastModified;
        }

        void update(long lastModified, int tickCount) {
            this.lastModified = lastModified;
            this.lastTickCount = tickCount;
        }
    }