推荐网站建设的书,网站去除前台验证码,wordpress网站排行榜,咸阳做网站的公司有哪些显式乐观锁定 在上一篇文章中 #xff0c;我介绍了Java持久性锁定的基本概念。 隐式锁定机制可防止丢失更新 #xff0c;它适用于我们可以主动修改的实体。 虽然隐式乐观锁定是一种广泛使用的技术#xff0c;但是很少有人了解显式乐观锁定模式的内部工作原理。 当锁定的实… 显式乐观锁定 在上一篇文章中 我介绍了Java持久性锁定的基本概念。 隐式锁定机制可防止丢失更新 它适用于我们可以主动修改的实体。 虽然隐式乐观锁定是一种广泛使用的技术但是很少有人了解显式乐观锁定模式的内部工作原理。 当锁定的实体始终由某些外部机制修改时显式乐观锁定可以防止数据完整性异常。 产品订购用例 假设我们有以下域模型 我们的用户爱丽丝想订购产品。 购买过程分为以下步骤 爱丽丝加载产品实体 因为价格方便她决定订购产品 价格引擎批处理作业更改了产品价格考虑了货币更改税项更改和市场营销活动 爱丽丝发出订单而没有注意到价格变动 隐式锁定的缺点 首先我们将测试隐式锁定机制是否可以防止此类异常。 我们的测试用例如下所示 doInTransaction(new TransactionCallableVoid() {Overridepublic Void execute(Session session) {final Product product (Product) session.get(Product.class, 1L);try {executeAndWait(new CallableVoid() {Overridepublic Void call() throws Exception {return doInTransaction(new TransactionCallableVoid() {Overridepublic Void execute(Session _session) {Product _product (Product) _session.get(Product.class, 1L);assertNotSame(product, _product);_product.setPrice(BigDecimal.valueOf(14.49));return null;}});}});} catch (Exception e) {fail(e.getMessage());}OrderLine orderLine new OrderLine(product);session.persist(orderLine);return null;}
}); 测试生成以下输出 #Alice selects a Product
Query:{[select abstractlo0_.id as id1_1_0_, abstractlo0_.description as descript2_1_0_, abstractlo0_.price as price3_1_0_, abstractlo0_.version as version4_1_0_ from product abstractlo0_ where abstractlo0_.id?][1]} #The price engine selects the Product as well
Query:{[select abstractlo0_.id as id1_1_0_, abstractlo0_.description as descript2_1_0_, abstractlo0_.price as price3_1_0_, abstractlo0_.version as version4_1_0_ from product abstractlo0_ where abstractlo0_.id?][1]}
#The price engine changes the Product price
Query:{[update product set description?, price?, version? where id? and version?][USB Flash Drive,14.49,1,1,0]}
#The price engine transaction is committed
DEBUG [pool-2-thread-1]: o.h.e.t.i.j.JdbcTransaction - committed JDBC Connection#Alice inserts an OrderLine without realizing the Product price change
Query:{[insert into order_line (id, product_id, unitPrice, version) values (default, ?, ?, ?)][1,12.99,0]}
#Alice transaction is committed unaware of the Product state change
DEBUG [main]: o.h.e.t.i.j.JdbcTransaction - committed JDBC Connection 隐式乐观锁定机制无法检测到外部更改除非实体也被当前的持久性上下文更改。 为了防止发出过时的Product状态订单我们需要在Product实体上应用显式锁定。 明确锁定救援 Java Persistence LockModeType.OPTIMISTIC是此类情况的合适候选者因此我们将对其进行测试。 Hibernate带有LockModeConverter实用程序该实用程序能够将任何Java Persistence LockModeType映射到与其关联的Hibernate LockMode 。 为了简单起见我们将使用特定于Hibernate的LockMode.OPTIMISTIC 该方法实际上与其Java持久性对应项相同。 根据Hibernate文档显式的OPTIMISTIC锁定模式将 假设交易不会对实体产生竞争。 实体版本将在交易结束时进行验证。 我将调整测试用例改为使用显式OPTIMISTIC锁定 try {doInTransaction(new TransactionCallableVoid() {Overridepublic Void execute(Session session) {final Product product (Product) session.get(Product.class, 1L, new LockOptions(LockMode.OPTIMISTIC));executeAndWait(new CallableVoid() {Overridepublic Void call() throws Exception {return doInTransaction(new TransactionCallableVoid() {Overridepublic Void execute(Session _session) {Product _product (Product) _session.get(Product.class, 1L);assertNotSame(product, _product);_product.setPrice(BigDecimal.valueOf(14.49));return null;}});}});OrderLine orderLine new OrderLine(product);session.persist(orderLine);return null;}});fail(It should have thrown OptimisticEntityLockException!);
} catch (OptimisticEntityLockException expected) {LOGGER.info(Failure: , expected);
} 新的测试版本将生成以下输出 #Alice selects a Product
Query:{[select abstractlo0_.id as id1_1_0_, abstractlo0_.description as descript2_1_0_, abstractlo0_.price as price3_1_0_, abstractlo0_.version as version4_1_0_ from product abstractlo0_ where abstractlo0_.id?][1]} #The price engine selects the Product as well
Query:{[select abstractlo0_.id as id1_1_0_, abstractlo0_.description as descript2_1_0_, abstractlo0_.price as price3_1_0_, abstractlo0_.version as version4_1_0_ from product abstractlo0_ where abstractlo0_.id?][1]}
#The price engine changes the Product price
Query:{[update product set description?, price?, version? where id? and version?][USB Flash Drive,14.49,1,1,0]}
#The price engine transaction is committed
DEBUG [pool-1-thread-1]: o.h.e.t.i.j.JdbcTransaction - committed JDBC Connection#Alice inserts an OrderLine
Query:{[insert into order_line (id, product_id, unitPrice, version) values (default, ?, ?, ?)][1,12.99,0]}
#Alice transaction verifies the Product version
Query:{[select version from product where id ?][1]}
#Alice transaction is rolled back due to Product version mismatch
INFO [main]: c.v.h.m.l.c.LockModeOptimisticTest - Failure:
org.hibernate.OptimisticLockException: Newer version [1] of entity [[com.vladmihalcea.hibernate.masterclass.laboratory.concurrency.
AbstractLockModeOptimisticTest$Product#1]] found in database 操作流程如下 在交易结束时检查产品版本。 任何版本不匹配都会触发异常和事务回滚。 比赛条件风险 不幸的是应用程序级别的版本检查和事务提交不是原子操作。 该检查发生在EntityVerifyVersionProcess中 在交易之前提交阶段 public class EntityVerifyVersionProcess implements BeforeTransactionCompletionProcess {private final Object object;private final EntityEntry entry;/*** Constructs an EntityVerifyVersionProcess** param object The entity instance* param entry The entitys referenced EntityEntry*/public EntityVerifyVersionProcess(Object object, EntityEntry entry) {this.object object;this.entry entry;}Overridepublic void doBeforeTransactionCompletion(SessionImplementor session) {final EntityPersister persister entry.getPersister();final Object latestVersion persister.getCurrentVersion( entry.getId(), session );if ( !entry.getVersion().equals( latestVersion ) ) {throw new OptimisticLockException(object,Newer version [ latestVersion ] of entity [ MessageHelper.infoString( entry.getEntityName(), entry.getId() ) ] found in database);}}
} 调用AbstractTransactionImpl.commit方法将执行before-transaction-commit阶段然后提交实际的事务 Override
public void commit() throws HibernateException {if ( localStatus ! LocalStatus.ACTIVE ) {throw new TransactionException( Transaction not successfully started );}LOG.debug( committing );beforeTransactionCommit();try {doCommit();localStatus LocalStatus.COMMITTED;afterTransactionCompletion( Status.STATUS_COMMITTED );}catch (Exception e) {localStatus LocalStatus.FAILED_COMMIT;afterTransactionCompletion( Status.STATUS_UNKNOWN );throw new TransactionException( commit failed, e );}finally {invalidate();afterAfterCompletion();}
} 在支票和实际交易提交之间其他交易在很短的时间内默默地提交产品价格变化。 结论 显式的OPTIMISTIC锁定策略为过时的状态异常提供了有限的保护。 此竞争条件是“检查时间”到“使用时间数据完整性异常”的典型情况。 在下一篇文章中我将解释如何使用explicit lock upgrade技术保存该示例。 代码可在GitHub上获得 。 翻译自: https://www.javacodegeeks.com/2015/01/hibernate-locking-patterns-how-does-optimistic-lock-mode-work.html
