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;
        }
    }