java - 并行事务如何产生OptimistickLockException？

Question

我知道多种原因可以触发OptimistickLockException，但当前的测试用例要求两个事务同时写入相同的数据才能触发异常。更像是通过并行事务创建读/写违规问题。这就是我所做的:

1. 在测试类中，创建一个私有(private)字段来保存事务和一些事务方法:

   private List<Transaction> transactions = new ArrayList<Transaction>();
   
   // This method only generate transactions and assign
   // them to the member field
   @Transactional 
   public void retrieveTransactions(){
       transactions = generateTransactions(); // generateTransactions() is transactional
   }
   
   // Following two methods just write those
   // transactions into same summery, only the first one sleeps
   // during the process to make sure the second transaction
   // can occur in a parallel way
   @Transactional
   public void processTransactionsOne(){
       for(Transaction transaction:transactions){
           logger.info("====Transaction One=====");
           transaction.writeToSameSummary(); // writeToSameSummary() is transactional
           try {Thread.sleep(10000); } // sleep for 10 seconds before commit
           catch(InterruptedException ex) {}
       }
   }
   @Transactional
   public void processTransactionsTwo(){
       for(Transaction transaction:transactions){
           logger.info("====Transaction Two=====");
           transaction.writeToSameSummary(); // writeToSameSummary() is transactional
       }
   }
   

2. 在applicationContext.xml中，我将上述三个方法设置为调度程序，retrieveTransactions每60秒运行一次，processTransactionsOne和processTransactionsTwo 每 10 秒运行一次

   <task:scheduled ref="testBeanName" method="retrieveTransactions" cron="*/60 * * * * *"/>
   <task:scheduled ref="testBeanName" method="processTransactionsOne" cron="*/10 * * * * *"/>
   <task:scheduled ref="testBeanName" method="processTransactionsTwo" cron="*/10 * * * * *"/>
   

但是，我永远无法重现 OptimistickLockException 和显示的日志

====Transaction One=====
====Transaction One=====
====Transaction One=====
......
====Transaction Two=====
====Transaction Two=====
====Transaction Two=====

因此事务是同步运行的，而不是并行运行的。那么我如何(1)进行并行事务和(2)重现OptimistickLockException

Answer 1

要重现乐观锁定异常，请尝试从本身不是事务性的方法运行代码。例如，调用读取和修改实体的方法，并返回分离的副本。

然后调用相同的方法来修改相同的实体，并返回一个新的分离副本。与数据库相比，第一个副本现在已经过时了。尝试将第一个过时的副本合并到 session 中，它将抛出乐观锁定异常。

这是版本化实体的一些示例代码:

@Entity
public class MyEntity {
   ...
   @Version
   private Long version;
}

这是读取并返回分离实体的服务示例:

@Repository
public class SomeRepository {
    @PersistenceContext
    EntityManager em;

    @Transactional
    public MyEntity readAndModify(Long key) {
        MyEntity myEntity = em.find( key, MyEntity.class);
        myEntity.setSomeField(...); // this will trigger update via dirty checking
        return myEntity;
    }
}

测试中的这段代码可以触发乐观锁定异常:

public someNonTransactionalTest() {
    //read and modify entity, get detached copy
    MyEntity detachedCopy1 = someRepository.readAndModify(1L); 

    //read and modify entity, get another detached copy - copy 1 is stale now
    MyEntity detachedCopy2 = someRepository.readAndModify(1L); 

    // try to merge the stale copy to a session, the optimistic locking exception is thrown
    detachedCopy1 = entityManager.merge(detachedCopy1);

    ... assertions ...
}