Java tutorial
/** * 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.curator.framework.recipes.queue; import org.apache.curator.framework.CuratorFramework; import org.apache.curator.framework.EnsureContainers; import org.apache.curator.utils.PathUtils; import org.apache.curator.utils.ZKPaths; import org.apache.zookeeper.CreateMode; import org.apache.zookeeper.KeeperException; import org.apache.zookeeper.WatchedEvent; import org.apache.zookeeper.Watcher; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import java.util.Collections; import java.util.List; import java.util.NoSuchElementException; import java.util.concurrent.CountDownLatch; import java.util.concurrent.TimeUnit; /** * <p> * Drop in replacement for: org.apache.zookeeper.recipes.queue.DistributedQueue that is part of * the ZooKeeper distribution * </p> * * <p> * This class is data compatible with the ZK version. i.e. it uses the same naming scheme so * it can read from an existing queue * </p> */ public class SimpleDistributedQueue { private final Logger log = LoggerFactory.getLogger(getClass()); private final CuratorFramework client; private final String path; private final EnsureContainers ensureContainers; private final String PREFIX = "qn-"; /** * @param client the client * @param path path to store queue nodes */ public SimpleDistributedQueue(CuratorFramework client, String path) { this.client = client; this.path = PathUtils.validatePath(path); ensureContainers = new EnsureContainers(client, path); } /** * Return the head of the queue without modifying the queue. * * @return the data at the head of the queue. * @throws Exception errors * @throws NoSuchElementException if the queue is empty */ public byte[] element() throws Exception { byte[] bytes = internalElement(false, null); if (bytes == null) { throw new NoSuchElementException(); } return bytes; } /** * Attempts to remove the head of the queue and return it. * * @return The former head of the queue * @throws Exception errors * @throws NoSuchElementException if the queue is empty */ public byte[] remove() throws Exception { byte[] bytes = internalElement(true, null); if (bytes == null) { throw new NoSuchElementException(); } return bytes; } /** * Removes the head of the queue and returns it, blocks until it succeeds. * * @return The former head of the queue * @throws Exception errors */ public byte[] take() throws Exception { return internalPoll(0, null); } /** * Inserts data into queue. * * @param data the data * @return true if data was successfully added * @throws Exception errors */ public boolean offer(byte[] data) throws Exception { String thisPath = ZKPaths.makePath(path, PREFIX); client.create().creatingParentContainersIfNeeded().withMode(CreateMode.PERSISTENT_SEQUENTIAL) .forPath(thisPath, data); return true; } /** * Returns the data at the first element of the queue, or null if the queue is empty. * * @return data at the first element of the queue, or null. * @throws Exception errors */ public byte[] peek() throws Exception { try { return element(); } catch (NoSuchElementException e) { return null; } } /** * Retrieves and removes the head of this queue, waiting up to the * specified wait time if necessary for an element to become available. * * @param timeout how long to wait before giving up, in units of * <tt>unit</tt> * @param unit a <tt>TimeUnit</tt> determining how to interpret the * <tt>timeout</tt> parameter * @return the head of this queue, or <tt>null</tt> if the * specified waiting time elapses before an element is available * @throws Exception errors */ public byte[] poll(long timeout, TimeUnit unit) throws Exception { return internalPoll(timeout, unit); } /** * Attempts to remove the head of the queue and return it. Returns null if the queue is empty. * * @return Head of the queue or null. * @throws Exception errors */ public byte[] poll() throws Exception { try { return remove(); } catch (NoSuchElementException e) { return null; } } protected void ensurePath() throws Exception { ensureContainers.ensure(); } private byte[] internalPoll(long timeout, TimeUnit unit) throws Exception { ensurePath(); long startMs = System.currentTimeMillis(); boolean hasTimeout = (unit != null); long maxWaitMs = hasTimeout ? TimeUnit.MILLISECONDS.convert(timeout, unit) : Long.MAX_VALUE; for (;;) { final CountDownLatch latch = new CountDownLatch(1); Watcher watcher = new Watcher() { @Override public void process(WatchedEvent event) { latch.countDown(); } }; byte[] bytes = internalElement(true, watcher); if (bytes != null) { return bytes; } if (hasTimeout) { long elapsedMs = System.currentTimeMillis() - startMs; long thisWaitMs = maxWaitMs - elapsedMs; if (thisWaitMs <= 0) { return null; } latch.await(thisWaitMs, TimeUnit.MILLISECONDS); } else { latch.await(); } } } private byte[] internalElement(boolean removeIt, Watcher watcher) throws Exception { ensurePath(); List<String> nodes; try { nodes = (watcher != null) ? client.getChildren().usingWatcher(watcher).forPath(path) : client.getChildren().forPath(path); } catch (KeeperException.NoNodeException dummy) { return null; } Collections.sort(nodes); for (String node : nodes) { if (!node.startsWith(PREFIX)) { log.warn("Foreign node in queue path: " + node); continue; } String thisPath = ZKPaths.makePath(path, node); try { byte[] bytes = client.getData().forPath(thisPath); if (removeIt) { client.delete().forPath(thisPath); } return bytes; } catch (KeeperException.NoNodeException ignore) { //Another client removed the node first, try next } } return null; } }