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hibernate工厂模式_Hibernate锁定模式–乐观锁定模式如何工作

發(fā)布時間：2023/12/3 编程问答 49 豆豆

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hibernate工廠模式

顯式樂觀鎖定

在上一篇文章中，我介紹了Java持久性鎖定的基本概念。

隱式鎖定機制可防止丟失更新，它適用于我們可以主動修改的實體。盡管隱式樂觀鎖定是一種廣泛使用的技術，但很少有人了解顯式樂觀鎖定模式的內(nèi)部工作原理。

當鎖定的實體始終由某些外部機制修改時，顯式樂觀鎖定可以防止數(shù)據(jù)完整性異常。

產(chǎn)品訂購用例

假設我們有以下域模型：

我們的用戶愛麗絲想訂購產(chǎn)品。購買過程如下：

愛麗絲加載產(chǎn)品實體
因為價格方便，她決定訂購產(chǎn)品
價格引擎批處理作業(yè)更改了產(chǎn)品價格（考慮了貨幣更改，稅項更改和市場營銷活動）
愛麗絲發(fā)出訂單而沒有注意到價格變動

隱式鎖定的缺點

首先，我們將測試隱式鎖定機制是否可以防止此類異常。我們的測試用例如下所示：

doInTransaction(new TransactionCallable<Void>() {@Overridepublic Void execute(Session session) {final Product product = (Product) session.get(Product.class, 1L);try {executeAndWait(new Callable<Void>() {@Overridepublic Void call() throws Exception {return doInTransaction(new TransactionCallable<Void>() {@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;} });

測試生成以下輸出：

#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

隱式的樂觀鎖定機制無法檢測到外部更改，除非實體也被當前的持久性上下文更改。為了防止發(fā)出過時的Product狀態(tài)訂單，我們需要在Product實體上應用顯式鎖。

明確鎖定救援

Java Persistence LockModeType.OPTIMISTIC是此類情況的合適候選者，因此我們將對其進行測試。

Hibernate帶有LockModeConverter實用程序，該實用程序能夠將任何Java Persistence LockModeType映射到與其關聯(lián)的Hibernate LockMode 。

為簡單起見，我們將使用特定于Hibernate的LockMode.OPTIMISTIC ，該方法實際上與其Java持久性對應項相同。

根據(jù)Hibernate文檔，顯式的OPTIMISTIC鎖定模式將：

假設交易不會對實體產(chǎn)生競爭。實體版本將在交易結束時進行驗證。

我將調整測試用例，改為使用顯式OPTIMISTIC鎖定：

try {doInTransaction(new TransactionCallable<Void>() {@Overridepublic Void execute(Session session) {final Product product = (Product) session.get(Product.class, 1L, new LockOptions(LockMode.OPTIMISTIC));executeAndWait(new Callable<Void>() {@Overridepublic Void call() throws Exception {return doInTransaction(new TransactionCallable<Void>() {@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); }

新的測試版本生成以下輸出：

#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：

在交易結束時檢查產(chǎn)品版本。任何版本不匹配都會觸發(fā)異常和事務回滾。

比賽條件風險

不幸的是，應用程序級別的版本檢查和事務提交不是原子操作。該檢查發(fā)生在EntityVerifyVersionProcess中，在交易之前提交階段：

public class EntityVerifyVersionProcess implements BeforeTransactionCompletionProcess {private final Object object;private final EntityEntry entry;/*** Constructs an EntityVerifyVersionProcess** @param object The entity instance* @param entry The entity's 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");}} }

調用AbstractTransactionImpl.commit（）方法，將執(zhí)行before-transaction-commit階段，然后提交實際事務：

@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();} }

在支票和實際交易提交之間，其他交易在很短的時間內(nèi)默默地提交產(chǎn)品價格變化。

結論

顯式的OPTIMISTIC鎖定策略為過時的狀態(tài)異常提供了有限的保護。此競爭條件是“檢查時間”到“使用時間數(shù)據(jù)完整性異?！钡牡湫颓闆r。

在下一篇文章中，我將解釋如何使用explicit lock upgrade技術保存該示例。

代碼可在GitHub上獲得。

翻譯自: https://www.javacodegeeks.com/2015/01/hibernate-locking-patterns-how-does-optimistic-lock-mode-work.html