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;
}
}