public interface DistributedLock {
 
   /**獲取鎖，如果沒有得到就等待*/
   public void acquire() throws Exception;
 
   /**
   * 獲取鎖，直到超時
   * @param time超時時間
   * @param unit time參數(shù)的單位
   * @return是否獲取到鎖
   * @throws Exception
   */
   public boolean acquire (long time, TimeUnit unit) throws Exception;
 
   /**
    * 釋放鎖
    * @throws Exception
    */
   public void release() throws Exception;
}

/**鎖接口的具體實現(xiàn)，主要借助于繼承的父類BaseDistributedLock來實現(xiàn)的接口方法
 * 該父類是基于Zookeeper實現(xiàn)分布式鎖的具體細節(jié)實現(xiàn)*/
public class SimpleDistributedLockMutex extends BaseDistributedLock implements DistributedLock {
  /*用于保存Zookeeper中實現(xiàn)分布式鎖的節(jié)點，如名稱為locker：/locker，
 *該節(jié)點應該是持久節(jié)點，在該節(jié)點下面創(chuàng)建臨時順序節(jié)點來實現(xiàn)分布式鎖 */
 private final String basePath;
 
 /*鎖名稱前綴，locker下創(chuàng)建的順序節(jié)點例如都以lock-開頭，這樣便于過濾無關節(jié)點
 *這樣創(chuàng)建后的節(jié)點類似：lock-00000001，lock-000000002*/
 private staticfinal String LOCK_NAME ="lock-";
 
 /*用于保存某個客戶端在locker下面創(chuàng)建成功的順序節(jié)點，用于后續(xù)相關操作使用（如判斷）*/
 private String ourLockPath;
 
 /**
 * 用于獲取鎖資源，通過父類的獲取鎖方法來獲取鎖
 * @param time獲取鎖的超時時間
 * @param unit time的時間單位
 * @return是否獲取到鎖
 * @throws Exception
 */
 private boolean internalLock (long time, TimeUnit unit) throws Exception {
  //如果ourLockPath不為空則認為獲取到了鎖，具體實現(xiàn)細節(jié)見attemptLock的實現(xiàn)
  ourLockPath = attemptLock(time, unit);
  return ourLockPath !=null;
 }
 
 /**
  * 傳入Zookeeper客戶端連接對象，和basePath
  * @param client Zookeeper客戶端連接對象
  * @param basePath basePath是一個持久節(jié)點
  */
 public SimpleDistributedLockMutex(ZkClientExt client, String basePath){
  /*調用父類的構造方法在Zookeeper中創(chuàng)建basePath節(jié)點，并且為basePath節(jié)點子節(jié)點設置前綴
  *同時保存basePath的引用給當前類屬性*/
  super(client,basePath,LOCK_NAME);
  this.basePath = basePath;
 }
 
 /**獲取鎖，直到超時，超時后拋出異常*/
 public void acquire() throws Exception {
  //-1表示不設置超時時間，超時由Zookeeper決定
  if (!internalLock(-1,null)){
   throw new IOException("連接丟失!在路徑:'"+basePath+"'下不能獲取鎖!");
  }
 }
 
 /**
 * 獲取鎖，帶有超時時間
 */
 public boolean acquire(long time, TimeUnit unit) throws Exception {
  return internalLock(time, unit);
 }
 
 /**釋放鎖*/
 public void release()throws Exception {
  releaseLock(ourLockPath);
 }
}

public class BaseDistributedLock {
 
 private final ZkClientExt client;
 private final String path;
 private final String basePath;
 private final String lockName;
 private static final Integer MAX_RETRY_COUNT = 10;
 
 public BaseDistributedLock(ZkClientExt client, String path, String lockName){
 this.client = client;
 this.basePath = path;
 this.path = path.concat("/").concat(lockName); 
 this.lockName = lockName;
 }
 
 private void deleteOurPath(String ourPath) throws Exception{
 client.delete(ourPath);
 }
 
 private String createLockNode(ZkClient client, String path) throws Exception{
 return client.createEphemeralSequential(path, null);
 }
 
 /**
 * 獲取鎖的核心方法
 * @param startMillis
 * @param millisToWait
 * @param ourPath
 * @return
 * @throws Exception
 */
 private boolean waitToLock(long startMillis, Long millisToWait, String ourPath) throws Exception{
 
 boolean haveTheLock = false;
 boolean doDelete = false;
 
 try{
  while ( !haveTheLock ) {
  //該方法實現(xiàn)獲取locker節(jié)點下的所有順序節(jié)點，并且從小到大排序 
  List<String> children = getSortedChildren();
  String sequenceNodeName = ourPath.substring(basePath.length()+1);
  
  //計算剛才客戶端創(chuàng)建的順序節(jié)點在locker的所有子節(jié)點中排序位置，如果是排序為0，則表示獲取到了鎖
  int ourIndex = children.indexOf(sequenceNodeName);
  
  /*如果在getSortedChildren中沒有找到之前創(chuàng)建的[臨時]順序節(jié)點，這表示可能由于網絡閃斷而導致
   *Zookeeper認為連接斷開而刪除了我們創(chuàng)建的節(jié)點，此時需要拋出異常，讓上一級去處理
   *上一級的做法是捕獲該異常，并且執(zhí)行重試指定的次數(shù) 見后面的 attemptLock方法 */
  if ( ourIndex<0 ){
   throw new ZkNoNodeException("節(jié)點沒有找到: " + sequenceNodeName);
  }

  //如果當前客戶端創(chuàng)建的節(jié)點在locker子節(jié)點列表中位置大于0，表示其它客戶端已經獲取了鎖
  //此時當前客戶端需要等待其它客戶端釋放鎖，
  boolean isGetTheLock = ourIndex == 0;
  
  //如何判斷其它客戶端是否已經釋放了鎖？從子節(jié)點列表中獲取到比自己次小的哪個節(jié)點，并對其建立監(jiān)聽
  String pathToWatch = isGetTheLock ? null : children.get(ourIndex - 1);

  if ( isGetTheLock ){
   haveTheLock = true;
  }else{
   //如果次小的節(jié)點被刪除了，則表示當前客戶端的節(jié)點應該是最小的了，所以使用CountDownLatch來實現(xiàn)等待
   String previousSequencePath = basePath .concat( "/" ) .concat( pathToWatch );
   final CountDownLatch latch = new CountDownLatch(1);
   final IZkDataListener previousListener = new IZkDataListener() {
   
   //次小節(jié)點刪除事件發(fā)生時，讓countDownLatch結束等待
   //此時還需要重新讓程序回到while，重新判斷一次！
   public void handleDataDeleted(String dataPath) throws Exception {
    latch.countDown();  
   }
   
   public void handleDataChange(String dataPath, Object data) throws Exception {
    // ignore     
   }
   };

   try{   
   //如果節(jié)點不存在會出現(xiàn)異常
   client.subscribeDataChanges(previousSequencePath, previousListener);
   
   if ( millisToWait != null ){
    millisToWait -= (System.currentTimeMillis() - startMillis);
    startMillis = System.currentTimeMillis();
    if ( millisToWait <= 0 ){
    doDelete = true; // timed out - delete our node
    break;
    }

    latch.await(millisToWait, TimeUnit.MICROSECONDS);
   }else{
    latch.await();
   }
   
   }catch ( ZkNoNodeException e ){
   //ignore
   }finally{
   client.unsubscribeDataChanges(previousSequencePath, previousListener);
   }
  }
  }
 }catch ( Exception e ){
  //發(fā)生異常需要刪除節(jié)點
  doDelete = true;
  throw e;
  
 }finally{
  //如果需要刪除節(jié)點
  if ( doDelete ){
  deleteOurPath(ourPath);
  }
 }
 return haveTheLock;
 } 
 
 private String getLockNodeNumber(String str, String lockName) {
 int index = str.lastIndexOf(lockName);
 if ( index >= 0 ){
  index += lockName.length();
  return index <= str.length() ? str.substring(index) : "";
 }
 return str;
 }
 
 private List<String> getSortedChildren() throws Exception {
 try{
  List<String> children = client.getChildren(basePath);
  Collections.sort(
  children,
  new Comparator<String>(){
   public int compare(String lhs, String rhs){
   return getLockNodeNumber(lhs, lockName).compareTo(getLockNodeNumber(rhs, lockName));
   }
  }
  );
  return children;
  
 }catch(ZkNoNodeException e){
  client.createPersistent(basePath, true);
  return getSortedChildren();
 }
 }
 
 
 protected void releaseLock(String lockPath) throws Exception{
 deleteOurPath(lockPath); 
 }
 
 
 protected String attemptLock(long time, TimeUnit unit) throws Exception{
 final long startMillis = System.currentTimeMillis();
 final Long millisToWait = (unit != null) ? unit.toMillis(time) : null;

 String  ourPath = null;
 boolean  hasTheLock = false;
 boolean  isDone = false;
 int  retryCount = 0;
 
 //網絡閃斷需要重試一試
 while ( !isDone ){
  isDone = true;

  try{
  //createLockNode用于在locker（basePath持久節(jié)點）下創(chuàng)建客戶端要獲取鎖的[臨時]順序節(jié)點
  ourPath = createLockNode(client, path);
  /**
   * 該方法用于判斷自己是否獲取到了鎖，即自己創(chuàng)建的順序節(jié)點在locker的所有子節(jié)點中是否最小
   * 如果沒有獲取到鎖，則等待其它客戶端鎖的釋放，并且稍后重試直到獲取到鎖或者超時
   */
  hasTheLock = waitToLock(startMillis, millisToWait, ourPath);
  
  }catch ( ZkNoNodeException e ){
  if ( retryCount++ < MAX_RETRY_COUNT ){
   isDone = false;
  }else{
   throw e;
  }
  }
 }
 
 if ( hasTheLock ){
  return ourPath;
 }
 
 return null；
｝