分布式锁的三种实现方式
一、zookeeper
1、实现原理：
基于zookeeper瞬时有序节点实现的分布式锁，其主要逻辑如下（该图来自于IBM网站）。大致思想即为：每个客户端对某个功能加锁时，在zookeeper上的与该功能对应的指定节点的目录下，生成一个唯一的瞬时有序节点。判断是否获取锁的方式很简单，只需要判断有序节点中序号最小的一个。当释放锁的时候，只需将这个瞬时节点删除即可。同时，其可以避免服务宕机导致的锁无法释放，而产生的死锁问题。
2、优点
锁安全性高，zk可持久化
3、缺点
性能开销比较高。因为其需要动态产生、销毁瞬时节点来实现锁功能。
4、实现
可以直接采用zookeeper第三方库curator即可方便地实现分布式锁。以下为基于curator实现的zk分布式锁核心代码：
@Override
public boolean tryLock(LockInfo info) {
InterProcessMutex mutex = getMutex(info);
int tryTimes = info.getTryTimes();
long tryInterval = info.getTryInterval();
boolean flag = true;// 代表是否需要重试
while (flag && --tryTimes >= 0) {
try {
if (mutex.acquire(info.getWaitLockTime(), TimeUnit.MILLISECONDS)) {
LOGGER.info(LogConstant.DST_LOCK + "acquire lock successfully!");
flag = false;
break;
}
} catch (Exception e) {
LOGGER.error(LogConstant.DST_LOCK + "acquire lock error!", e);
} finally {
checkAndRetry(flag, tryInterval, tryTimes);
}
}
return !flag;// 最后还需要重试，说明没拿到锁
}
@Override
public boolean releaseLock(LockInfo info) {
InterProcessMutex mutex = getMutex(info);
int tryTimes = info.getTryTimes();
long tryInterval = info.getTryInterval();
boolean flag = true;// 代表是否需要重试
while (flag && --tryTimes >= 0) {
try {
mutex.release();
LOGGER.info(LogConstant.DST_LOCK + "release lock successfully!");
flag = false;
break;
} catch (Exception e) {
LOGGER.error(LogConstant.DST_LOCK + "release lock error!", e);
} finally {
checkAndRetry(flag, tryInterval, tryTimes);
}
}
return !flag;// 最后还需要重试，说明没拿到锁
}
/**
* 获取锁。此处需要加同步，concurrentHashmap无法避免此处的同步问题
* @param info 锁信息
* @return 锁实例
*/
private synchronized InterProcessMutex getMutex(LockInfo info) {
InterProcessReadWriteLock lock = null;
if (locksCache.get(info.getLock()) != null) {
lock = locksCache.get(info.getLock());
} else {
lock = new InterProcessReadWriteLock(client, BASE_DIR + info.getLock());
locksCache.put(info.getLock(), lock);
}
InterProcessMutex mutex = null;
switch (info.getIsolate()) {
case READ:
mutex = lock.readLock();
break;
case WRITE:
mutex = lock.writeLock();
break;
default:
throw new IllegalArgumentException();
}
return mutex;
}
/**
* 判断是否需要重试
* @param flag 是否需要重试标志
* @param tryInterval 重试间隔
* @param tryTimes 重试次数
*/
private void checkAndRetry(boolean flag, long tryInterval, int tryTimes) {
try {
if (flag) {
Thread.sleep(tryInterval);
LOGGER.info(LogConstant.DST_LOCK + "retry getting lock! now retry time left: " + tryTimes);
}
} catch (InterruptedException e) {
LOGGER.error(LogConstant.DST_LOCK + "retry interval thread interruptted!", e);
}
}
二、memcached分布式锁
1、实现原理：
memcached带有add函数，利用add函数的特性即可实现分布式锁。add和set的区别在于：如果多线程并发set，则每个set都会成功，但最后存储的值以最后的set的线程为准。而add的话则相反，add会添加第一个到达的值，并返回true，后续的添加则都会返回false。利用该点即可很轻松地实现分布式锁。
2、优点
并发高效。
3、缺点
- （1）memcached采用列入LRU置换策略，所以如果内存不够，可能导致缓存中的锁信息丢失。
- （2）memcached无法持久化，一旦重启，将导致信息丢失。
三、redis分布式锁
redis分布式锁即可以结合zk分布式锁锁高度安全和memcached并发场景下效率很好的优点，可以利用jedis客户端实现。参考http://blog.csdn.net/java2000_wl/article/details/8740911
/**
* @author http://blog.csdn.net/java2000_wl
* @version <b>1.0.0</b>
*/
public class RedisBillLockHandler implements IBatchBillLockHandler {
private static final Logger LOGGER = LoggerFactory.getLogger(RedisBillLockHandler.class);
private static final int DEFAULT_SINGLE_EXPIRE_TIME = 3;
private static final int DEFAULT_BATCH_EXPIRE_TIME = 6;
private final JedisPool jedisPool;
/**
* 构造
* @author http://blog.csdn.net/java2000_wl
*/
public RedisBillLockHandler(JedisPool jedisPool) {
this.jedisPool = jedisPool;
}
/**
* 获取锁 如果锁可用 立即返回true， 否则返回false
* @author http://blog.csdn.net/java2000_wl
* @param billIdentify
* @return
*/
public boolean tryLock(IBillIdentify billIdentify) {
return tryLock(billIdentify, 0L, null);
}
/**
* 锁在给定的等待时间内空闲，则获取锁成功 返回true， 否则返回false
* @author http://blog.csdn.net/java2000_wl
* @param billIdentify
* @param timeout
* @param unit
* @return
*/
public boolean tryLock(IBillIdentify billIdentify, long timeout, TimeUnit unit) {
String key = (String) billIdentify.uniqueIdentify();
Jedis jedis = null;
try {
jedis = getResource();
long nano = System.nanoTime();
do {
LOGGER.debug("try lock key: " + key);
Long i = jedis.setnx(key, key);
if (i == 1) {
jedis.expire(key, DEFAULT_SINGLE_EXPIRE_TIME);
LOGGER.debug("get lock, key: " + key + " , expire in " + DEFAULT_SINGLE_EXPIRE_TIME + " seconds.");
return Boolean.TRUE;
} else { // 存在锁
if (LOGGER.isDebugEnabled()) {
String desc = jedis.get(key);
LOGGER.debug("key: " + key + " locked by another business：" + desc);
}
}
if (timeout == 0) {
break;
}
Thread.sleep(300);
} while ((System.nanoTime() - nano) < unit.toNanos(timeout));
return Boolean.FALSE;
} catch (JedisConnectionException je) {
LOGGER.error(je.getMessage(), je);
returnBrokenResource(jedis);
} catch (Exception e) {
LOGGER.error(e.getMessage(), e);
} finally {
returnResource(jedis);
}
return Boolean.FALSE;
}
/**
* 如果锁空闲立即返回 获取失败 一直等待
* @author http://blog.csdn.net/java2000_wl
* @param billIdentify
*/
public void lock(IBillIdentify billIdentify) {
String key = (String) billIdentify.uniqueIdentify();
Jedis jedis = null;
try {
jedis = getResource();
do {
LOGGER.debug("lock key: " + key);
Long i = jedis.setnx(key, key);
if (i == 1) {
jedis.expire(key, DEFAULT_SINGLE_EXPIRE_TIME);
LOGGER.debug("get lock, key: " + key + " , expire in " + DEFAULT_SINGLE_EXPIRE_TIME + " seconds.");
return;
} else {
if (LOGGER.isDebugEnabled()) {
String desc = jedis.get(key);
LOGGER.debug("key: " + key + " locked by another business：" + desc);
}
}
Thread.sleep(300);
} while (true);
} catch (JedisConnectionException je) {
LOGGER.error(je.getMessage(), je);
returnBrokenResource(jedis);
} catch (Exception e) {
LOGGER.error(e.getMessage(), e);
} finally {
returnResource(jedis);
}
}
/**
* 释放锁
* @author http://blog.csdn.net/java2000_wl
* @param billIdentify
*/
public void unLock(IBillIdentify billIdentify) {
List<IBillIdentify> list = new ArrayList<IBillIdentify>();
list.add(billIdentify);
unLock(list);
}
/**
* 批量获取锁 如果全部获取 立即返回true, 部分获取失败 返回false
* @author http://blog.csdn.net/java2000_wl
* @date 2013-7-22 下午10:27:44
* @param billIdentifyList
* @return
*/
public boolean tryLock(List<IBillIdentify> billIdentifyList) {
return tryLock(billIdentifyList, 0L, null);
}
/**
* 锁在给定的等待时间内空闲，则获取锁成功 返回true， 否则返回false
* @author http://blog.csdn.net/java2000_wl
* @param billIdentifyList
* @param timeout
* @param unit
* @return
*/
public boolean tryLock(List<IBillIdentify> billIdentifyList, long timeout, TimeUnit unit) {
Jedis jedis = null;
try {
List<String> needLocking = new CopyOnWriteArrayList<String>();
List<String> locked = new CopyOnWriteArrayList<String>();
jedis = getResource();
long nano = System.nanoTime();
do {
// 构建pipeline，批量提交
Pipeline pipeline = jedis.pipelined();
for (IBillIdentify identify : billIdentifyList) {
String key = (String) identify.uniqueIdentify();
needLocking.add(key);
pipeline.setnx(key, key);
}
LOGGER.debug("try lock keys: " + needLocking);
// 提交redis执行计数
List<Object> results = pipeline.syncAndReturnAll();
for (int i = 0; i < results.size(); ++i) {
Long result = (Long) results.get(i);
String key = needLocking.get(i);
if (result == 1) { // setnx成功，获得锁
jedis.expire(key, DEFAULT_BATCH_EXPIRE_TIME);
locked.add(key);
}
}
needLocking.removeAll(locked); // 已锁定资源去除
if (CollectionUtils.isEmpty(needLocking)) {
return true;
} else {
// 部分资源未能锁住
LOGGER.debug("keys: " + needLocking + " locked by another business：");
}
if (timeout == 0) {
break;
}
Thread.sleep(500);
} while ((System.nanoTime() - nano) < unit.toNanos(timeout));
// 得不到锁，释放锁定的部分对象，并返回失败
if (!CollectionUtils.isEmpty(locked)) {
jedis.del(locked.toArray(new String[0]));
}
return false;
} catch (JedisConnectionException je) {
LOGGER.error(je.getMessage(), je);
returnBrokenResource(jedis);
} catch (Exception e) {
LOGGER.error(e.getMessage(), e);
} finally {
returnResource(jedis);
}
return true;
}
/**
* 批量释放锁
* @author http://blog.csdn.net/java2000_wl
* @param billIdentifyList
*/
public void unLock(List<IBillIdentify> billIdentifyList) {
List<String> keys = new CopyOnWriteArrayList<String>();
for (IBillIdentify identify : billIdentifyList) {
String key = (String) identify.uniqueIdentify();
keys.add(key);
}
Jedis jedis = null;
try {
jedis = getResource();
jedis.del(keys.toArray(new String[0]));
LOGGER.debug("release lock, keys :" + keys);
} catch (JedisConnectionException je) {
LOGGER.error(je.getMessage(), je);
returnBrokenResource(jedis);
} catch (Exception e) {
LOGGER.error(e.getMessage(), e);
} finally {
returnResource(jedis);
}
}
/**
* @author http://blog.csdn.net/java2000_wl
* @date 2013-7-22 下午9:33:45
* @return
*/
private Jedis getResource() {
return jedisPool.getResource();
}
/**
* 销毁连接
* @author http://blog.csdn.net/java2000_wl
* @param jedis
*/
private void returnBrokenResource(Jedis jedis) {
if (jedis == null) {
return;
}
try {
//容错
jedisPool.returnBrokenResource(jedis);
} catch (Exception e) {
LOGGER.error(e.getMessage(), e);
}
}
/**
* @author http://blog.csdn.net/java2000_wl
* @param jedis
*/
private void returnResource(Jedis jedis) {
if (jedis == null) {
return;
}
try {
jedisPool.returnResource(jedis);
} catch (Exception e) {
LOGGER.error(e.getMessage(), e);
}
}
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