休眠锁定模式– PESSIMISTIC_FORCE_INCREMENT锁定模式如何工作

介绍

在我以前的文章中，我介绍了OPTIMISTIC_FORCE_INCREMENT锁定模式，并将其应用于将子实体版本更改传播到锁定的父实体。在本文中，我将介绍PESSIMISTIC_FORCE_INCREMENT锁定模式，并将其与乐观的锁定模式进行比较。

相像多于不同

正如我们已经发现的那样，即使当前事务没有修改锁定的实体状态， OPTIMISTIC_FORCE_INCREMENT锁定模式也可以增加实体版本。对于每种锁定模式，Hibernate定义一个关联的LockingStrategy ，并且OPTIMISTIC_FORCE_INCREMENT锁定模式事件由OptimisticForceIncrementLockingStrategy处理：

public class OptimisticForceIncrementLockingStrategy implements LockingStrategy {//code omitted for brevity@Overridepublic void lock(Serializable id, Object version, Object object, int timeout, SessionImplementor session) {if ( !lockable.isVersioned() ) {throw new HibernateException( "[" + lockMode + "] not supported for non-versioned entities [" + lockable.getEntityName() + "]" );}final EntityEntry entry = session.getPersistenceContext().getEntry( object );// Register the EntityIncrementVersionProcess action to run just prior to transaction commit.( (EventSource) session ).getActionQueue().registerProcess( new EntityIncrementVersionProcess( object, entry ) );}
}

此策略在当前持久性上下文操作队列中注册EntityIncrementVersionProcess 。在完成当前正在运行的事务之前，锁定的实体版本会增加。

public class EntityIncrementVersionProcess implements BeforeTransactionCompletionProcess {//code omitted for brevity@Overridepublic void doBeforeTransactionCompletion(SessionImplementor session) {final EntityPersister persister = entry.getPersister();final Object nextVersion = persister.forceVersionIncrement( entry.getId(), entry.getVersion(), session );entry.forceLocked( object, nextVersion );}
}

与OPTIMISTIC_FORCE_INCREMENT相似， PESSIMISTIC_FORCE_INCREMENT锁定模式由PessimisticForceIncrementLockingStrategy处理：

public class PessimisticForceIncrementLockingStrategy implements LockingStrategy {//code omitted for brevity@Overridepublic void lock(Serializable id, Object version, Object object, int timeout, SessionImplementor session) {if ( !lockable.isVersioned() ) {throw new HibernateException( "[" + lockMode + "] not supported for non-versioned entities [" + lockable.getEntityName() + "]" );}final EntityEntry entry = session.getPersistenceContext().getEntry( object );final EntityPersister persister = entry.getPersister();final Object nextVersion = persister.forceVersionIncrement( entry.getId(), entry.getVersion(), session );entry.forceLocked( object, nextVersion );}
}

锁定的实体立即增加，因此这两种锁定模式执行相同的逻辑，但时间不同。 PESSIMISTIC_FORCE_INCREMENT的命名可能使您想到您正在使用悲观的锁定策略，而实际上，此锁定模式只是一个乐观的锁定变体。

悲观锁需要显式物理锁（共享或独占），而乐观锁则依赖于当前事务隔离级别的隐式锁。

存储库用例

我将重用之前的练习，然后切换到使用PESSIMISTIC_FORCE_INCREMENT锁定模式。回顾一下，我们的域模型包含：

一个存储库实体，其版本随每次新的提交而增加
一个Commit实体，封装了一个原子存储库状态转换
一个CommitChange组件，封装了一个存储库资源更改

防止并行修改

爱丽丝和鲍勃同时访问我们的系统。从数据库中获取存储库实体后，它总是被锁定：

private final CountDownLatch startLatch = new CountDownLatch(1);
private final CountDownLatch endLatch = new CountDownLatch(1);@Test
public void testConcurrentPessimisticForceIncrementLockingWithLockWaiting() throws InterruptedException {LOGGER.info("Test Concurrent PESSIMISTIC_FORCE_INCREMENT Lock Mode With Lock Waiting");doInTransaction(new TransactionCallable<Void>() {@Overridepublic Void execute(Session session) {try {Repository repository = (Repository) session.get(Repository.class, 1L);session.buildLockRequest(new LockOptions(LockMode.PESSIMISTIC_FORCE_INCREMENT)).lock(repository);executeNoWait(new Callable<Void>() {@Overridepublic Void call() throws Exception {return doInTransaction(new TransactionCallable<Void>() {@Overridepublic Void execute(Session _session) {LOGGER.info("Try to get the Repository row");startLatch.countDown();Repository _repository = (Repository) _session.get(Repository.class, 1L);_session.buildLockRequest(new LockOptions(LockMode.PESSIMISTIC_FORCE_INCREMENT)).lock(_repository);Commit _commit = new Commit(_repository);_commit.getChanges().add(new Change("index.html", "0a1,2..."));_session.persist(_commit);_session.flush();endLatch.countDown();return null;}});}});startLatch.await();LOGGER.info("Sleep for 500ms to delay the other transaction PESSIMISTIC_FORCE_INCREMENT Lock Mode acquisition");Thread.sleep(500);Commit commit = new Commit(repository);commit.getChanges().add(new Change("README.txt", "0a1,5..."));commit.getChanges().add(new Change("web.xml", "17c17..."));session.persist(commit);return null;} catch (InterruptedException e) {fail("Unexpected failure");}return null;}});endLatch.await();
}

该测试用例生成以下输出：

#Alice selects the Repository
Query:{[select lockmodeop0_.id as id1_2_0_, lockmodeop0_.name as name2_2_0_, lockmodeop0_.version as version3_2_0_ from repository lockmodeop0_ where lockmodeop0_.id=?][1]} #Alice locks the Repository using a PESSIMISTIC_FORCE_INCREMENT Lock Mode
Query:{[update repository set version=? where id=? and version=?][1,1,0]} #Bob tries to get the Repository but the SELECT is blocked by Alice lock 
INFO  [pool-1-thread-1]: c.v.h.m.l.c.LockModePessimisticForceIncrementTest - Try to get the Repository row#Alice sleeps for 500ms to prove that Bob is waiting for her to release the acquired lock
Sleep for 500ms to delay the other transaction PESSIMISTIC_FORCE_INCREMENT Lock Mode acquisition#Alice makes two changes and inserts a new Commit<a href="https://vladmihalcea.files.wordpress.com/2015/02/explicitlockingpessimisticforceincrementfailfast.png"><img src="https://vladmihalcea.files.wordpress.com/2015/02/explicitlockingpessimisticforceincrementfailfast.png?w=585" alt="ExplicitLockingPessimisticForceIncrementFailFast" width="585" height="224" class="alignnone size-large wp-image-3955" /></a>
Query:{[insert into commit (id, repository_id) values (default, ?)][1]} 
Query:{[insert into commit_change (commit_id, diff, path) values (?, ?, ?)][1,0a1,5...,README.txt]#The Repository version is bumped up to version 1 and a conflict is raised
Query:{[insert into commit_change (commit_id, diff, path) values (?, ?, ?)][1,17c17...,web.xml]} Query:{[update repository set version=? where id=? and version=?][1,1,0]}#Alice commits the transaction, therefore releasing all locks
DEBUG [main]: o.h.e.t.i.j.JdbcTransaction - committed JDBC Connection#Bob Repository SELECT can proceed 
Query:{[select lockmodepe0_.id as id1_2_0_, lockmodepe0_.name as name2_2_0_, lockmodepe0_.version as version3_2_0_ from repository lockmodepe0_ where lockmodepe0_.id=?][1]} #Bob can insert his changes
Query:{[update repository set version=? where id=? and version=?][2,1,1]} 
Query:{[insert into commit (id, repository_id) values (default, ?)][1]} 
Query:{[insert into commit_change (commit_id, diff, path) values (?, ?, ?)][2,0a1,2...,index.html]}

下图可以很容易地看到这个锁定过程：

显式锁定悲观力量增加

每当修改数据库行时， HSQLDB测试数据库“ 两阶段锁定”实现都会使用过程粒度表锁定。

这就是Bob不能获得Alice刚刚更新的Repository数据库行上的读取锁的原因。其他数据库（例如Oracle，PostgreSQL）使用MVCC ，因此允许SELECT继续执行（使用当前的修改事务撤消日志来重新创建前一个行状态），同时阻止冲突的数据修改语句（例如，当其他并发事务已经停止时更新存储库行）尚未提交锁定的实体状态更改）。

快速失败

即时版本增加具有一些有趣的好处：

如果版本UPDATE成功（获取了排他级锁），则其他任何并发事务都无法修改锁定的数据库行。这是将逻辑锁（版本递增）升级为物理锁（数据库互斥锁）的时刻。
如果版本UPDATE失败（因为其他一些并发事务已经提交了版本更改），那么我们当前正在运行的事务可以立即回滚（而不是在提交期间等待事务失败）

后一个用例可以如下所示：

显式锁定悲观力量递增失败

对于这种情况，我们将使用以下测试用例：

@Test
public void testConcurrentPessimisticForceIncrementLockingFailFast() throws InterruptedException {LOGGER.info("Test Concurrent PESSIMISTIC_FORCE_INCREMENT Lock Mode fail fast");doInTransaction(new TransactionCallable<Void>() {@Overridepublic Void execute(Session session) {try {Repository repository = (Repository) session.get(Repository.class, 1L);executeAndWait(new Callable<Void>() {@Overridepublic Void call() throws Exception {return doInTransaction(new TransactionCallable<Void>() {@Overridepublic Void execute(Session _session) {Repository _repository = (Repository) _session.get(Repository.class, 1L);_session.buildLockRequest(new LockOptions(LockMode.PESSIMISTIC_FORCE_INCREMENT)).lock(_repository);Commit _commit = new Commit(_repository);_commit.getChanges().add(new Change("index.html", "0a1,2..."));_session.persist(_commit);_session.flush();return null;}});}});session.buildLockRequest(new LockOptions(LockMode.PESSIMISTIC_FORCE_INCREMENT)).lock(repository);fail("Should have thrown StaleObjectStateException!");} catch (StaleObjectStateException expected) {LOGGER.info("Failure: ", expected);}return null;}});
}

生成以下输出：

#Alice selects the Repository
Query:{[select lockmodeop0_.id as id1_2_0_, lockmodeop0_.name as name2_2_0_, lockmodeop0_.version as version3_2_0_ from repository lockmodeop0_ where lockmodeop0_.id=?][1]} #Bob selects the Repository too
Query:{[select lockmodepe0_.id as id1_2_0_, lockmodepe0_.name as name2_2_0_, lockmodepe0_.version as version3_2_0_ from repository lockmodepe0_ where lockmodepe0_.id=?][1]} #Bob locks the Repository using a PESSIMISTIC_FORCE_INCREMENT Lock Mode
Query:{[update repository set version=? where id=? and version=?][1,1,0]} #Bob makes a change and inserts a new Commit
Query:{[insert into commit (id, repository_id) values (default, ?)][1]} 
Query:{[insert into commit_change (commit_id, diff, path) values (?, ?, ?)][1,0a1,2...,index.html]} #Bob commits the transaction
DEBUG [pool-3-thread-1]: o.h.e.t.i.j.JdbcTransaction - committed JDBC Connection#Alice tries to lock the Repository
Query:{[update repository set version=? where id=? and version=?][1,1,0]} #Alice cannot lock the Repository, because the version has changed
INFO  [main]: c.v.h.m.l.c.LockModePessimisticForceIncrementTest - Failure: 
org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) : [com.vladmihalcea.hibernate.masterclass.laboratory.concurrency.LockModePessimisticForceIncrementTest$Repository#1]