Zookeeper（八）分布式队列

1. element  方法  获取对列头部第一个元素

 查找队列znode 下所有的子节点名称   使用TreeMap给顺序编号排序  返回第一个znode对应的值

    public byte[] element() throws NoSuchElementException, KeeperException, InterruptedException {        TreeMap<Long,String> orderedChildren;        while(true){            try{            // 返回对列中的全部元素name  这里使用到了TreeMap            // map的key为znode的顺序编号                  orderedChildren = orderedChildren(null);            }catch(KeeperException.NoNodeException e){                throw new NoSuchElementException();            }            if(orderedChildren.size() == 0 ) throw new NoSuchElementException();            for(String headNode : orderedChildren.values()){                if(headNode != null){                    try{                    // 返回对列头部第一个znode对应数据.                        return zookeeper.getData(dir+"/"+headNode, false, null);                    }catch(KeeperException.NoNodeException e){                    }                }            }        }    }

获取所有子节点名称

   private TreeMap<Long,String> orderedChildren(Watcher watcher) throws KeeperException, InterruptedException {        TreeMap<Long,String> orderedChildren = new TreeMap<Long,String>();        List<String> childNames = null;        try{        // 返回对列节点下   所有的子znode 名称            childNames = zookeeper.getChildren(dir, watcher);        }catch (KeeperException.NoNodeException e){            throw e;        }         // 所有子节点        for(String childName : childNames){            try{                //Check format                if(!childName.regionMatches(0, prefix, 0, prefix.length())){                    LOG.warn("Found child node with improper name: " + childName);                    continue;                }                String suffix = childName.substring(prefix.length());                // 顺序节点编号                Long childId = new Long(suffix);                orderedChildren.put(childId,childName);            }catch(NumberFormatException e){                LOG.warn("Found child node with improper format : " + childName + " " + e,e);            }        }        return orderedChildren;    }

2.   remove  返回对列头部的第一个元素的值  并删除该znode

    public byte[] remove() throws NoSuchElementException, KeeperException, InterruptedException {        TreeMap<Long,String> orderedChildren;        while(true){            try{            // 查找所有子节点  用TreeMap排序                orderedChildren = orderedChildren(null);            }catch(KeeperException.NoNodeException e){                throw new NoSuchElementException();            }            if(orderedChildren.size() == 0) throw new NoSuchElementException();            for(String headNode : orderedChildren.values()){                String path = dir +"/"+headNode;                try{                // 对列头部 第一个节点对应的数据                    byte[] data = zookeeper.getData(path, false, null);                    // 删除该节点                    zookeeper.delete(path, -1);                    return data;                }catch(KeeperException.NoNodeException e){                    // Another client deleted the node first.                }            }        }    }

3. take 检索并移除对列头部一个znode对应的值  如果对列为空  则一直等待

    public byte[] take() throws KeeperException, InterruptedException {        TreeMap<Long,String> orderedChildren;        // Same as for element.  Should refactor this.        while(true){            LatchChildWatcher childWatcher = new LatchChildWatcher();            try{            // 查找所有对列元素 并给对列主znode 设置监视器                orderedChildren = orderedChildren(childWatcher);            }catch(KeeperException.NoNodeException e){                zookeeper.create(dir, new byte[0], acl, CreateMode.PERSISTENT);                continue;            }            // 如果对列中不存在元素            // 对列节点下不存在子节点，  线程将一直等待. 使用到CountDownLatch            // 当客户端调用 offer方法 写入一个元素时  触发LatchChildWatcher监视器CountDownLatch 计数减1 为0时  当前线程获得运行机会            if(orderedChildren.size() == 0){                childWatcher.await();                continue;            }            for(String headNode : orderedChildren.values()){                String path = dir +"/"+headNode;                try{                // 返回对列头部第一个元素                    byte[] data = zookeeper.getData(path, false, null);                    // 删除该元素                    zookeeper.delete(path, -1);                    return data;                }catch(KeeperException.NoNodeException e){                    // Another client deleted the node first.                }            }        }    }

    private class LatchChildWatcher implements Watcher {        CountDownLatch latch;        public LatchChildWatcher(){            latch = new CountDownLatch(1);        }        public void process(WatchedEvent event){            LOG.debug("Watcher fired on path: " + event.getPath() + " state: " +                     event.getState() + " type " + event.getType());            latch.countDown();        }        public void await() throws InterruptedException {            latch.await();        }    }

4. offer  写入队列尾部  使用永久顺序节点

    public boolean offer(byte[] data) throws KeeperException, InterruptedException{        for(;;){            try{            // 写入对列    节点类型  永久顺序节点                zookeeper.create(dir+"/"+prefix, data, acl, CreateMode.PERSISTENT_SEQUENTIAL);                return true;            }catch(KeeperException.NoNodeException e){                zookeeper.create(dir, new byte[0], acl, CreateMode.PERSISTENT);            }        }    }

5.peek  返回对列头部  第一个znode对应的值

    public byte[] peek() throws KeeperException, InterruptedException{        try{        // 返回对列头部第一个znode的值            return element();        }catch(NoSuchElementException e){            return null;        }    }

6. 返回队列头部第一个znode对应的值  并删除该znode

    public byte[] poll() throws KeeperException, InterruptedException {        try{        // 返回对列znode下 第一个子节点值  并删除该节点            return remove();        }catch(NoSuchElementException e){            return null;        }    }