Zookeeper 分布式锁

我们常说的锁是单进程多线程锁，在多线程并发编程中，用于线程之间的数据同步，保护共享资源的访问。而分布式锁，指在分布式环境下，保护跨进程、跨主机、跨网络的共享资源，实现互斥访问，保证一致性。

架构图

左侧是zookeeper集群，locker是数据节点，node_1到node_n代表一系列的顺序节点。

右侧client_1至client_n代表客户端，Service代表需要互斥访问的服务。

总实现思路，是在获取锁的时候在locker节点下创建顺序节点，在释放锁的时候，把自己创建的节点删除。

流程图

类图

代码实现

public interface DistributedLock {
    
    /*
     * 获取锁，如果没有得到就等待
     */
    public void acquire() throws Exception;

    /*
     * 获取锁，直到超时
     */
    public boolean acquire(long time, TimeUnit unit) throws Exception;

    /*
     * 释放锁
     */
    public void release() throws Exception;


}
public class SimpleDistributedLockMutex extends BaseDistributedLock implements
        DistributedLock {
    
    //锁名称前缀,成功创建的顺序节点如lock-0000000000,lock-0000000001,...
    private static final String LOCK_NAME = "lock-";

    // zookeeper中locker节点的路径
    private final String basePath;

    // 获取锁以后自己创建的那个顺序节点的路径
    private String ourLockPath;
    
    private boolean internalLock(long time, TimeUnit unit) throws Exception {

        ourLockPath = attemptLock(time, unit);
        return ourLockPath != null;
        
    }
    
    public SimpleDistributedLockMutex(ZkClientExt client, String basePath){
                
        super(client,basePath,LOCK_NAME);
        this.basePath = basePath;
        
    }

    // 获取锁
    public void acquire() throws Exception {
        if ( !internalLock(-1, null) ) {
            throw new IOException("连接丢失!在路径:'"+basePath+"'下不能获取锁!");
        }
    }

    // 获取锁，可以超时
    public boolean acquire(long time, TimeUnit unit) throws Exception {

        return internalLock(time, unit);
    }

    // 释放锁
    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;
        
    }

    // 删除成功获取锁之后所创建的那个顺序节点
    private void deleteOurPath(String ourPath) throws Exception{
        client.delete(ourPath);
    }

    // 创建临时顺序节点
    private String createLockNode(ZkClient client,  String path) throws Exception{
        return client.createEphemeralSequential(path, null);
    }

    // 等待比自己次小的顺序节点的删除
    private boolean waitToLock(long startMillis, Long millisToWait, String ourPath) throws Exception{
        
        boolean  haveTheLock = false;
        boolean  doDelete = false;
        
        try {
 
            while ( !haveTheLock ) {
                // 获取/locker下的经过排序的子节点列表
                List<String> children = getSortedChildren();

                // 获取刚才自己创建的那个顺序节点名
                String sequenceNodeName = ourPath.substring(basePath.length()+1);

                // 判断自己排第几个
                int  ourIndex = children.indexOf(sequenceNodeName);
                if (ourIndex < 0){ // 网络抖动，获取到的子节点列表里可能已经没有自己了
                    throw new ZkNoNodeException("节点没有找到: " + sequenceNodeName);
                }

                // 如果是第一个，代表自己已经获得了锁
                boolean isGetTheLock = ourIndex == 0;

                // 如果自己没有获得锁，则要watch比我们次小的那个节点
                String  pathToWatch = isGetTheLock ? null : children.get(ourIndex - 1);

                if ( isGetTheLock ){
                    haveTheLock = true;
                    
                } else {

                    // 订阅比自己次小顺序节点的删除事件
                    String  previousSequencePath = basePath .concat( "/" ) .concat( pathToWatch );
                    final CountDownLatch    latch = new CountDownLatch(1);
                    final IZkDataListener previousListener = new IZkDataListener() {
                        
                        public void handleDataDeleted(String dataPath) throws Exception {
                            latch.countDown(); // 删除后结束latch上的await
                        }
                        
                        public void handleDataChange(String dataPath, Object data) throws Exception {
                            // ignore                                    
                        }
                    };

                    try {
                        //订阅次小顺序节点的删除事件，如果节点不存在会出现异常
                        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); // 在latch上await
                        } else {
                            latch.await(); // 在latch上await
                        }

                        // 结束latch上的等待后，继续while重新来过判断自己是否第一个顺序节点
                    }
                    catch ( ZkNoNodeException e ) {
                        //ignore
                    } finally {
                        client.unsubscribeDataChanges(previousSequencePath, previousListener);
                    }

                }
            }
        }
        catch ( Exception e ) {
            //发生异常需要删除节点
            doDelete = true;
            throw e;
        } finally {
            //如果需要删除节点
            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;
    }

    // 获取/locker下的经过排序的子节点列表
    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;
        
        //网络闪断需要重试一试
        while ( !isDone ) {
            isDone = true;

            try {
                // 在/locker下创建临时的顺序节点
                ourPath = createLockNode(client, path);
                // 判断自己是否获得了锁，如果没有获得那么等待直到获得锁或者超时
                hasTheLock = waitToLock(startMillis, millisToWait, ourPath);
            } catch ( ZkNoNodeException e ) { // 捕获这个异常
                if ( retryCount++ < MAX_RETRY_COUNT ) { // 重试指定次数
                    isDone = false;
                } else {
                    throw e;
                }
            }
        }
        if ( hasTheLock ) {
            return ourPath;
        }

        return null;
    }
    
    
}
public class TestDistributedLock {
    
    public static void main(String[] args) {
        
        final ZkClientExt zkClientExt1 = new ZkClientExt("192.168.1.105:2181", 5000, 5000, new BytesPushThroughSerializer());
        final SimpleDistributedLockMutex mutex1 = new SimpleDistributedLockMutex(zkClientExt1, "/Mutex");
        
        final ZkClientExt zkClientExt2 = new ZkClientExt("192.168.1.105:2181", 5000, 5000, new BytesPushThroughSerializer());
        final SimpleDistributedLockMutex mutex2 = new SimpleDistributedLockMutex(zkClientExt2, "/Mutex");
        
        try {
            mutex1.acquire();
            System.out.println("Client1 locked");
            Thread client2Thd = new Thread(new Runnable() {
                
                public void run() {
                    try {
                        mutex2.acquire();
                        System.out.println("Client2 locked");
                        mutex2.release();
                        System.out.println("Client2 released lock");
                        
                    } catch (Exception e) {
                        e.printStackTrace();
                    }                
                }
            });
            client2Thd.start();
            Thread.sleep(5000);
            mutex1.release();            
            System.out.println("Client1 released lock");
            
            client2Thd.join();
            
        } catch (Exception e) {

            e.printStackTrace();
        }
        
    }

}
public class ZkClientExt extends ZkClient {

    public ZkClientExt(String zkServers, int sessionTimeout, int connectionTimeout, ZkSerializer zkSerializer) {
        super(zkServers, sessionTimeout, connectionTimeout, zkSerializer);
    }

    @Override
    public void watchForData(final String path) {
        retryUntilConnected(new Callable<Object>() {

            public Object call() throws Exception {
                Stat stat = new Stat(); 
                _connection.readData(path, stat, true);
                return null;
            }

        });
    }   
    
}