使用MySQL实现一个分布式锁

CREATE TABLE `globallocktable` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `lockKey` varchar(60) NOT NULL COMMENT '锁名称',
 `createTime` datetime NOT NULL COMMENT '创建时间',
 PRIMARY KEY (`id`),
 UNIQUE KEY `lockKey` (`lockKey`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC COMMENT='全局锁';

@Component
public class GlobalLockComponent {
 @Resource
 GlobalLockTableDAO globalLockDAO;
 /
   尝试获得锁，成功为true，失败为false
  /
 public boolean tryLock(String key) {
  return GlobalLockUtil.tryLock(this.globalLockDAO, key);
 }
 /
   如果已经有其他程序占用该锁，并且超过timeoutMs（毫秒）时间，就强制清除这个锁占用
   即根据key先删除记录，再添加记录
  /
 public boolean tryLockWithClear(String key, Long timeoutMs) {
  return GlobalLockUtil.tryLockWithClear(this.globalLockDAO, key, timeoutMs);
 }
 /
   释放锁，根据key删除记录
  /
 public void releasLock(String key) {
  GlobalLockUtil.releasLock(this.globalLockDAO, key);
 }
}

public class GlobalLockTable {

 private Integer id;
 private String lockKey;
 private Date createTime;
 // 省略get和set方法
}

GlobalLockTableDAO定义如下

public interface GlobalLockTableDAO {
 int deleteByPrimaryKey(Integer id);
 int deleteByLockKey(String lockKey);
 GlobalLockTable selectByLockKey(String key);
 int insertSelectiveWithTest(GlobalLockTable record);
}

public class GlobalLockUtil {
 private static Logger logger = LoggerFactory.getLogger(GlobalLockUtil.class);
 private static GlobalLockTable tryLockInternal(GlobalLockTableDAO lockDAO, String key) {
  GlobalLockTable insert = new GlobalLockTable();
  insert.setCreateTime(new Date());
  insert.setLockKey(key);
  // 注意的地方1
  int count = lockDAO.insertSelectiveWithTest(insert);
  if (count == 0) {
   GlobalLockTable ready = lockDAO.selectByLockKey(key);
   logger.warn("can not lock the key: {}, {}, {}", insert.getLockKey(), ready.getCreateTime(),
     ready.getId());
   return ready;
  }
  logger.info("yes got the lock by key: {}", insert.getId(), insert.getLockKey());
  return null;
 }
 / 超时清除锁占用，并重新加锁 /
 public static boolean tryLockWithClear(GlobalLockTableDAO lockDAO, String key, Long timeoutMs) {
  GlobalLockTable lock = tryLockInternal(lockDAO, key);
  if (lock == null) return true;
  if (System.currentTimeMillis() - lock.getCreateTime().getTime() <= timeoutMs) {
   logger.warn("sorry, can not get the key. : {}, {}, {}", key, lock.getId(), lock.getCreateTime());
   return false;
  }
  logger.warn("the key already timeout wthin : {}, {}, will clear", key, timeoutMs);
  // 注意的地方2
  int count = lockDAO.deleteByPrimaryKey(lock.getId());
  if (count == 0) {
   logger.warn("sorry, the key already preemptived by others: {}, {}", lock.getId(), lock.getLockKey());
   return false;
  }
  lock = tryLockInternal(lockDAO, key);
  return lock != null ? false : true;
 }
 / 加锁 /
 public static boolean tryLock(GlobalLockTableDAO lockDAO, String key) {
  return tryLockInternal(lockDAO, key) == null ? true : false;
 }
 / 解锁 /
 public static void releasLock(GlobalLockTableDAO lockDAO, String key) {
  lockDAO.deleteByLockKey(key);
 }
}

<insert id="insertSelectiveWithTest" useGeneratedKeys="true" keyProperty="id" parameterType=".javashitang.middleware.lock.mysql.pojo.GlobalLockTable">
 insert into `globallocktable` (`id`,
 `lockKey`, `createTime` )
  select #{id,jdbcType=INTEGER}, #{lockKey,jdbcType=VARCHAR}, #{createTime,jdbcType=TIMESTAMP}
  from dual where not exists
  (select 1 from globallocktable where lockKey = #{lockKey,jdbcType=VARCHAR})
</insert>

if (!globalLockComponent.tryLock(name)) {
 // 没有获取到锁返回
 return;
}
try {
 // 这里写业务逻辑
} catch (Exception e) {
} finally {
 globalLockComponent.releasLock(name)