我工作的一个Java应用程序服务器场，需要做一些定期的工作（比如，每2分钟一次）中运行的服务。 该服务必须接入外部实体，它需要将不同的实例，以便只有一个在给定时间工作在工作同步。 由于服务这项工作中采用了DB，我想实现基于一个简单的数据库表同步的：
id, owner, stamp
其中id是锁的ID，所有者是当前所有者和邮票是它被锁定的时间。
该方法是：
tryLock(id, maxAge, owner) - to try to lock a record or break an old record
refresh(id, owner) - to update the stamp to signal we're still around working on the job
release(id, owner) - to release the lock
你将如何实现这一点？
编辑：删除我的实现，我会后它作为一个“答案”
我想出了下面的实现，但我不知道这是否可以处理所有的情况（我不是很确定我使用的是正确BeanManagedTransaction）。 另外，如果你觉得这个同步化的问题可以在一个更简单的方式来处理，我指向正确的方向。
@Service(objectName=Sync.EjbName)
@Management(SyncMgt.class)
@TransactionManagement(value=TransactionManagementType.BEAN)
public class SyncSvc implements SyncMgt {
@PersistenceContext
protected EntityManager entityManager_;
@Resource
protected UserTransaction utx_;
@TransactionAttribute(TransactionAttributeType.REQUIRED)
private boolean update(SyncRecord sr, String owner) {
Date stamp = (owner != null) ? new Date() : null;
Query q;
if (sr.getOwner() != null) {
q = entityManager_.createQuery("UPDATE SyncRecord sr SET sr.owner = :newOwner, sr.stamp = :stamp WHERE sr.id = :id AND sr.owner = :origOwner AND sr.stamp = :origStamp");
q.setParameter("origOwner", sr.getOwner());
q.setParameter("origStamp", sr.getStamp()); // make it fail if someone refreshed in the meantime
}
else {
q = entityManager_.createQuery("UPDATE SyncRecord sr SET sr.owner = :newOwner, sr.stamp = :stamp WHERE sr.id = :id AND sr.owner IS NULL");
}
q.setParameter("id", sr.getId());
q.setParameter("newOwner", owner);
q.setParameter("stamp", stamp);
int res = q.executeUpdate();
if (res != 1) {
return false;
}
return true;
}
@TransactionAttribute(TransactionAttributeType.REQUIRED)
private boolean tryLockImpl(String id, long maxAge, String owner) {
SyncRecord sr = entityManager_.find(SyncRecord.class, id);
if (sr == null) {
// no record yet, create one
sr = new SyncRecord(id, owner);
sr.touch();
entityManager_.persist(sr);
entityManager_.flush();
return true;
}
// found a SyncRecord, let's see who owns it
if (owner.equals(sr.getOwner())) {
// log some warning, re-locking old lock, should use refresh instead
return update(sr, owner);
}
if (sr.getOwner() == null) {
// sr is not held by anyone, safe to grab it
return update(sr, owner);
}
// someone else holds it, let's check the age
if (maxAge >= 0) {
long maxAgeStamp = System.currentTimeMillis() - maxAge;
if (sr.getStamp().getTime() < maxAgeStamp) {
if (update(sr, owner)) {
return true;
}
return false;
}
}
return false;
}
// Sync impl:
/**
* Try to lock "id" for "owner"
* If the lock is held by someone else, but is older than maxAge, break it
*/
@TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
public boolean tryLock(String id, long maxAge, String owner) {
if (id == null)
throw new IllegalArgumentException("id is null");
try {
utx_.begin();
if (tryLockImpl(id, maxAge, owner)) {
utx_.commit();
return true;
}
}
catch (EntityExistsException e) {
// failed to lock, someone beat us to it
}
catch (Throwable e) {
// some fishy error, raise alarm, log, etc
}
try {
utx_.rollback();
}
catch (Throwable e) {
// log the error, not much else we can do at this point
}
return false;
}
/**
* Refresh lock "id" belonging to "owner" (update its stamp)
*/
@TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
public boolean refresh(String id, String owner) {
if (id == null)
throw new IllegalArgumentException("id is null");
try {
utx_.begin();
SyncRecord sr = entityManager_.find(SyncRecord.class, id);
if (sr == null || !owner.equals(sr.getOwner())) {
utx_.rollback();
return false;
}
if (update(sr, owner)) {
utx_.commit();
return true;
}
}
catch (Throwable e) {
// some fishy error, raise alarm, log, etc
}
try {
utx_.rollback();
}
catch (Throwable e) {
// log the error, not much else we can do at this point
}
return false;
}
/**
* release lock "id" held by "owner"
*/
@TransactionAttribute(TransactionAttributeType.REQUIRES_NEW)
public void release(String id, String owner) {
if (id == null)
throw new IllegalArgumentException("id is null");
try {
utx_.begin();
SyncRecord sr = entityManager_.find(SyncRecord.class, id);
if (sr == null || !owner.equals(sr.getOwner())) {
// we don't own it
utx_.rollback();
return;
}
if (update(sr, null)) {
utx_.commit();
return;
}
}
catch (Throwable e) {
// some fishy error, raise alarm, log, etc
}
try {
utx_.rollback();
}
catch (Throwable e) {
// log the error, not much else we can do at this point
}
}
// LifeCycle impl:
public void start() {}
public void stop() {}
}
