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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.zookeeper.common;
import java.io.IOException;
import java.nio.file.ClosedWatchServiceException;
import java.nio.file.FileSystem;
import java.nio.file.Path;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.util.function.Consumer;
import org.apache.zookeeper.server.ZooKeeperThread;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* Instances of this class can be used to watch a directory for file changes. When a file is added to, deleted from,
* or is modified in the given directory, the callback provided by the user will be called from a background thread.
* Some things to keep in mind:
*
*
The callback should be thread-safe.
*
Changes that happen around the time the thread is started may be missed.
*
There is a delay between a file changing and the callback firing.
*
The watch is not recursive - changes to subdirectories will not trigger a callback.
*
*/
public final class FileChangeWatcher {
private static final Logger LOG = LoggerFactory.getLogger(FileChangeWatcher.class);
public enum State {
NEW, // object created but start() not called yet
STARTING, // start() called but background thread has not entered main loop
RUNNING, // background thread is running
STOPPING, // stop() called but background thread has not exited main loop
STOPPED // stop() called and background thread has exited, or background thread crashed
}
private final WatcherThread watcherThread;
private State state; // protected by synchronized(this)
/**
* Creates a watcher that watches dirPath and invokes callback on changes.
*
* @param dirPath the directory to watch.
* @param callback the callback to invoke with events. event.kind() will return the type of event,
* and event.context() will return the filename relative to dirPath.
* @throws IOException if there is an error creating the WatchService.
*/
public FileChangeWatcher(Path dirPath, Consumer> callback) throws IOException {
FileSystem fs = dirPath.getFileSystem();
WatchService watchService = fs.newWatchService();
LOG.debug("Registering with watch service: {}", dirPath);
dirPath.register(watchService, new WatchEvent.Kind[]{StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_DELETE, StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.OVERFLOW});
state = State.NEW;
this.watcherThread = new WatcherThread(watchService, callback);
this.watcherThread.setDaemon(true);
}
/**
* Returns the current {@link FileChangeWatcher.State}.
* @return the current state.
*/
public synchronized State getState() {
return state;
}
/**
* Blocks until the current state becomes desiredState.
* Currently only used by tests, thus package-private.
* @param desiredState the desired state.
* @throws InterruptedException if the current thread gets interrupted.
*/
synchronized void waitForState(State desiredState) throws InterruptedException {
while (this.state != desiredState) {
this.wait();
}
}
/**
* Sets the state to newState.
* @param newState the new state.
*/
private synchronized void setState(State newState) {
state = newState;
this.notifyAll();
}
/**
* Atomically sets the state to update if and only if the
* state is currently expected.
* @param expected the expected state.
* @param update the new state.
* @return true if the update succeeds, or false if the current state
* does not equal expected.
*/
private synchronized boolean compareAndSetState(State expected, State update) {
if (state == expected) {
setState(update);
return true;
} else {
return false;
}
}
/**
* Atomically sets the state to update if and only if the
* state is currently one of expectedStates.
* @param expectedStates the expected states.
* @param update the new state.
* @return true if the update succeeds, or false if the current state
* does not equal any of the expectedStates.
*/
private synchronized boolean compareAndSetState(State[] expectedStates, State update) {
for (State expected : expectedStates) {
if (state == expected) {
setState(update);
return true;
}
}
return false;
}
/**
* Tells the background thread to start. Does not wait for it to be running.
* Calling this method more than once has no effect.
*/
public void start() {
if (!compareAndSetState(State.NEW, State.STARTING)) {
// If previous state was not NEW, start() has already been called.
return;
}
this.watcherThread.start();
}
/**
* Tells the background thread to stop. Does not wait for it to exit.
*/
public void stop() {
if (compareAndSetState(new State[]{State.RUNNING, State.STARTING}, State.STOPPING)) {
watcherThread.interrupt();
}
}
/**
* Inner class that implements the watcher thread logic.
*/
private class WatcherThread extends ZooKeeperThread {
private static final String THREAD_NAME = "FileChangeWatcher";
final WatchService watchService;
final Consumer> callback;
WatcherThread(WatchService watchService, Consumer> callback) {
super(THREAD_NAME);
this.watchService = watchService;
this.callback = callback;
}
@Override
public void run() {
try {
LOG.info("{} thread started", getName());
if (!compareAndSetState(FileChangeWatcher.State.STARTING, FileChangeWatcher.State.RUNNING)) {
// stop() called shortly after start(), before
// this thread started running.
FileChangeWatcher.State state = FileChangeWatcher.this.getState();
if (state != FileChangeWatcher.State.STOPPING) {
throw new IllegalStateException("Unexpected state: " + state);
}
return;
}
runLoop();
} catch (Exception e) {
LOG.warn("Error in runLoop()", e);
throw e;
} finally {
try {
watchService.close();
} catch (IOException e) {
LOG.warn("Error closing watch service", e);
}
LOG.info("{} thread finished", getName());
FileChangeWatcher.this.setState(FileChangeWatcher.State.STOPPED);
}
}
private void runLoop() {
while (FileChangeWatcher.this.getState() == FileChangeWatcher.State.RUNNING) {
WatchKey key;
try {
key = watchService.take();
} catch (InterruptedException | ClosedWatchServiceException e) {
LOG.debug("{} was interrupted and is shutting down...", getName());
break;
}
for (WatchEvent event : key.pollEvents()) {
LOG.debug("Got file changed event: {} with context: {}", event.kind(), event.context());
try {
callback.accept(event);
} catch (Throwable e) {
LOG.error("Error from callback", e);
}
}
boolean isKeyValid = key.reset();
if (!isKeyValid) {
// This is likely a problem, it means that file reloading is broken, probably because the
// directory we are watching was deleted or otherwise became inaccessible (unmounted, permissions
// changed, ???).
// For now, we log an error and exit the watcher thread.
LOG.error("Watch key no longer valid, maybe the directory is inaccessible?");
break;
}
}
}
}
}
