虽然这是一个旧线程，但我想分享我的解决方案，并希望得到一些反馈。请注意，我仅在某些 JUnit 测试用例中使用本地数据库测试了此解决方案。所以到目前为止，这还不是一个有效的功能。
我通过引入一个名为 Sequence 且没有属性的自定义注释为我解决了这个问题。它只是应该从递增序列中分配值的字段的标记。
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Sequence
{
}
使用此注释我标记了我的实体。
public class Area extends BaseEntity implements ClientAware, IssuerAware
{
@Column(name = "areaNumber", updatable = false)
@Sequence
private Integer areaNumber;
....
}
为了保持数据库独立，我引入了一个名为 SequenceNumber 的实体，它保存序列当前值和增量大小。我选择了 className 作为唯一键，所以每个实体类都会有自己的序列。
@Entity
@Table(name = "SequenceNumber", uniqueConstraints = { @UniqueConstraint(columnNames = { "className" }) })
public class SequenceNumber
{
@Id
@Column(name = "className", updatable = false)
private String className;
@Column(name = "nextValue")
private Integer nextValue = 1;
@Column(name = "incrementValue")
private Integer incrementValue = 10;
... some getters and setters ....
}
最后一步也是最困难的一步是处理序列号分配的 PreInsertListener。请注意，我使用 spring 作为 bean 容器。
@Component
public class SequenceListener implements PreInsertEventListener
{
private static final long serialVersionUID = 7946581162328559098L;
private final static Logger log = Logger.getLogger(SequenceListener.class);
@Autowired
private SessionFactoryImplementor sessionFactoryImpl;
private final Map<String, CacheEntry> cache = new HashMap<>();
@PostConstruct
public void selfRegister()
{
// As you might expect, an EventListenerRegistry is the place with which event listeners are registered
// It is a service so we look it up using the service registry
final EventListenerRegistry eventListenerRegistry = sessionFactoryImpl.getServiceRegistry().getService(EventListenerRegistry.class);
// add the listener to the end of the listener chain
eventListenerRegistry.appendListeners(EventType.PRE_INSERT, this);
}
@Override
public boolean onPreInsert(PreInsertEvent p_event)
{
updateSequenceValue(p_event.getEntity(), p_event.getState(), p_event.getPersister().getPropertyNames());
return false;
}
private void updateSequenceValue(Object p_entity, Object[] p_state, String[] p_propertyNames)
{
try
{
List<Field> fields = ReflectUtil.getFields(p_entity.getClass(), null, Sequence.class);
if (!fields.isEmpty())
{
if (log.isDebugEnabled())
{
log.debug("Intercepted custom sequence entity.");
}
for (Field field : fields)
{
Integer value = getSequenceNumber(p_entity.getClass().getName());
field.setAccessible(true);
field.set(p_entity, value);
setPropertyState(p_state, p_propertyNames, field.getName(), value);
if (log.isDebugEnabled())
{
LogMF.debug(log, "Set {0} property to {1}.", new Object[] { field, value });
}
}
}
}
catch (Exception e)
{
log.error("Failed to set sequence property.", e);
}
}
private Integer getSequenceNumber(String p_className)
{
synchronized (cache)
{
CacheEntry current = cache.get(p_className);
// not in cache yet => load from database
if ((current == null) || current.isEmpty())
{
boolean insert = false;
StatelessSession session = sessionFactoryImpl.openStatelessSession();
session.beginTransaction();
SequenceNumber sequenceNumber = (SequenceNumber) session.get(SequenceNumber.class, p_className);
// not in database yet => create new sequence
if (sequenceNumber == null)
{
sequenceNumber = new SequenceNumber();
sequenceNumber.setClassName(p_className);
insert = true;
}
current = new CacheEntry(sequenceNumber.getNextValue() + sequenceNumber.getIncrementValue(), sequenceNumber.getNextValue());
cache.put(p_className, current);
sequenceNumber.setNextValue(sequenceNumber.getNextValue() + sequenceNumber.getIncrementValue());
if (insert)
{
session.insert(sequenceNumber);
}
else
{
session.update(sequenceNumber);
}
session.getTransaction().commit();
session.close();
}
return current.next();
}
}
private void setPropertyState(Object[] propertyStates, String[] propertyNames, String propertyName, Object propertyState)
{
for (int i = 0; i < propertyNames.length; i++)
{
if (propertyName.equals(propertyNames[i]))
{
propertyStates[i] = propertyState;
return;
}
}
}
private static class CacheEntry
{
private int current;
private final int limit;
public CacheEntry(final int p_limit, final int p_current)
{
current = p_current;
limit = p_limit;
}
public Integer next()
{
return current++;
}
public boolean isEmpty()
{
return current >= limit;
}
}
}
从上面的代码可以看出，监听器为每个实体类使用了一个 SequenceNumber 实例，并保留了一对由 SequenceNumber 实体的 incrementValue 定义的序列号。如果序列号用完，它会为目标类加载 SequenceNumber 实体，并为下一次调用保留 incrementValue 值。这样我就不需要在每次需要序列值时查询数据库。
请注意为保留下一组序列号而打开的 StatelessSession。您不能使用目标实体当前持久化的同一会话，因为这会导致 EntityPersister 中的 ConcurrentModificationException。
希望这对某人有所帮助。