3.4.5.5. 事务监听器

事务监听器旨在对事务生命周期事件做出响应。与实体监听器不同，它们不与何实体类型绑定，可以被每个事务调用。

监听器是一个托管 Bean，它实现了 BeforeCommitTransactionListener 或 AfterCompleteTransactionListener 接口或者同时实现这两个接口。

BeforeCommitTransactionListener

如果事务不是只读的，则在所有实体监听器之后，事务提交之前调用 beforeCommit() 方法。该方法接受当前持久化上下文中的实体集合和当前的EntityManager作为参数。

监听器可用于执行涉及多个实体的复杂业务规则。在下面的例子中，Order 实体的 amount 属性必须根据订单中的 discount 值计算，OrderLine 实体的 price 和 quantity 构成订单。

@Component("demo_OrdersTransactionListener")
public class OrdersTransactionListener implements BeforeCommitTransactionListener {

    @Inject
    private PersistenceTools persistenceTools;

    @Override
    public void beforeCommit(EntityManager entityManager, Collection<Entity> managedEntities) {
        // gather all orders affected by changes in the current transaction
        Set<Order> affectedOrders = new HashSet<>();

        for (Entity entity : managedEntities) {
            // skip not modified entities
            if (!persistenceTools.isDirty(entity))
                continue;

            if (entity instanceof Order)
                affectedOrders.add((Order) entity);
            else if (entity instanceof OrderLine) {
                Order order = ((OrderLine) entity).getOrder();
                // a reference can be detached, so merge it into current persistence context
                affectedOrders.add(entityManager.merge(order));
            }
        }
        // calculate amount for each affected order by its lines and discount
        for (Order order : affectedOrders) {
            BigDecimal amount = BigDecimal.ZERO;
            for (OrderLine orderLine : order.getOrderLines()) {
                if (!orderLine.isDeleted()) {
                    amount = amount.add(orderLine.getPrice().multiply(orderLine.getQuantity()));
                }
            }
            BigDecimal discount = order.getDiscount().divide(BigDecimal.valueOf(100), 2, BigDecimal.ROUND_DOWN);
            order.setAmount(amount.subtract(amount.multiply(discount)));
        }
    }
}
AfterCompleteTransactionListener

事务完成后调用 afterComplete() 方法。该方法接受一个参数，该参数表明事务是否已成功提交，以及已完成事务的持久化上下文中包含的已分离实体的集合。

用法示例:

@Component("demo_OrdersTransactionListener")
public class OrdersTransactionListener implements AfterCompleteTransactionListener {

    private Logger log = LoggerFactory.getLogger(OrdersTransactionListener.class);

    @Override
    public void afterComplete(boolean committed, Collection<Entity> detachedEntities) {
        if (!committed)
            return;

        for (Entity entity : detachedEntities) {
            if (entity instanceof Order) {
                log.info("Order: " + entity);
            }
        }
    }
}