执行时：
public void beginTransaction() {
em.getTransaction().begin();
}
在以相同方式启动活动事务后，我得到以下异常：
Exception Description: Transaction is currently active
java.lang.IllegalStateException:
Exception Description: Transaction is currently active
at org.eclipse.persistence.internal.jpa.transaction.EntityTransactionImpl.begin(EntityTransactionImpl.java:45)
at com.mysimpatico.memoplatform.persistence.Database.beginTransaction(Database.java:44)
at com.mysimpatico.memoplatform.persistence.tests.PersistenceTest.setUp(PersistenceTest.java:39)
底层DBMS是嵌入式apache derby，据我所知，它不支持来自不同JVMS的多个连接，但是支持并发事务。Documentation
此测试代码失败：
public <Entity> int getNo(final Class<Entity> entityClass) {
final CriteriaBuilder qb = em.getCriteriaBuilder();
final CriteriaQuery<Long> cq = qb.createQuery(Long.class);
cq.select(qb.count(cq.from(entityClass)));
final int ret = em.createQuery(cq).getSingleResult().intValue();
return ret;
}
protected void insertWords(final List<String[]> expressionDefTriples, MUser usr, final MeaningsDatabase mengsDB) throws Exception {
for (String[] expressionDefTriple : expressionDefTriples) {
mengsDB.persistSynonyms(MeaningsDatabase.toEnumMap(expressionDefTriple, usr));
testSynonymsPersistance(expressionDefTriple,mengsDB);
}
}
protected void testSynonymsPersistance(final String[] expDefTriple, final MeaningsDatabase mengsDB) {
final Expression exp = mengsDB.get(Expression.class, expDefTriple[0]);
testGender(exp, expDefTriple[2]);
final Expression def = mengsDB.get(Expression.class, expDefTriple[1]);
assertNotNull(def);
assertTrue(exp.getMengs().size() > 0);
assertTrue(def.getMengs().size() > 0);
assertTrue(mengsDB.getNo(Meaning.class) > 0); //fails here
}
public void persistSynonyms(final EnumMap<Input, MemoEntity> input) throws MultipleMengsException {
final Expression def = extractExp(input, Input.definition);
final Expression exp = extractExp(input, Input.expression);
final MUser usr = extractUsr(input);
beginTransaction();
final Meaning meng = persistMeng(exp, usr, def);
commitTransaction();
signalContentAvailability();
logInsertion(meng, exp, def);
}
基本上我在同一个em中启动并提交一个事务，但是当我使用相同的em来执行getNo（）方法时，它找不到前一个提交的内容。我没有看到问题出在哪里。显然存在问题，因为像em.find（..）这样的“本地”代码会找到已提交的更改，但不会查找条件查询。
答案 0 :(得分：7)
EntityManager#getTransaction()实际上会返回 当前resource local transaction ，并且当事务处于活动状态时，您无法调用EntityTransaction#begin()：
/**
* Start a resource transaction.
* @throws IllegalStateException if isActive() is true
*/
public void begin();
您可以做的是创建EntityManager#getTransaction()的javadoc暗示的串行本地事务：
/**
* Return the resource-level <code>EntityTransaction</code> object.
* The <code>EntityTransaction</code> instance may be used serially to
* begin and commit multiple transactions.
* @return EntityTransaction instance
* @throws IllegalStateException if invoked on a JTA
* entity manager
*/
public EntityTransaction getTransaction();
总而言之，JPA（和JTA）不支持nested transactions。