Hibernate如何处理LockMode.PESSIMISTIC_WRITE？它是否以与本机查询中的SELECT FOR UPDATE相同的方式处理它？
我运行2个实验性事务（T1和T2），它们执行相同的本机SQL，该本机SQL使用SELECT FOR UPDATE选择一个实体，然后对其进行更新。我在2个线程中运行它们，以确保时间安排如下：
1) T1 SELECT ... FOR UPDATE (based on a condition that becomes false after update)
2) T2 SELECT ... FOR UPDATE (based on a condition that becomes false after update)
3) T2 updates the entity
4) T1 updates the entity
这总是导致我理解和期望的相同流程和结果：
1) T1 reads the single entity based on a condition
2) T2 doesn't select anything
3) T1 updates the entity so that it cannot be selected based on the condition anymore
4) T2 doesn't update anything
该实体的类型为FIELD，具有4个属性：ID, NAME, TYPE, DESCRIPTION。仅DESCRIPTION被更新。
这是运行此类事务的代码：
private void selectForUpdateNativeQueryTransaction(String name, int delayBeforeRead, int delayBeforeCommit) {
Transaction tx = null;
Session session = null;
try {
session = factory.openSession();
tx = session.beginTransaction();
System.out.println(name + " BEGIN");
try {
TimeUnit.SECONDS.sleep(delayBeforeRead);
} catch (InterruptedException e) {
e.printStackTrace();
}
SQLQuery sqlQuery = session.createSQLQuery("SELECT * FROM FIELD WHERE DESCRIPTION=?1 FOR UPDATE");
sqlQuery.setParameter("1", DESC);
List results = sqlQuery.list();
System.out.println(name + " SELECT EXECUTED");
results.forEach(r -> {
Object[] row = (Object[]) r;
for (int i = 0; i < row.length; i++) {
System.out.println(name + " : FIELD READ [" + row[i] + "]");
}
});
try {
TimeUnit.SECONDS.sleep(delayBeforeCommit);
} catch (InterruptedException e) {
e.printStackTrace();
}
Query query = session.createSQLQuery("UPDATE FIELD SET DESCRIPTION=?1 WHERE DESCRIPTION=?2");
query.setParameter("1", name);
query.setParameter("2", DESC);
int result = query.executeUpdate();
System.out.println(name + " UPDATED ROWS: " + result);
tx.commit();
} catch (Exception e) {
fail();
if (tx != null) {
tx.rollback();
}
} finally {
session.close();
}
System.out.println(name + " COMMITTED");
}
下面是在不同线程中运行两个事务的代码：
@Test
public void firstReadsTheOtherRejected() {
ExecutorService es = Executors.newFixedThreadPool(3);
Runnable selectForUpdateNativeQueryTransaction1 = () -> {
selectForUpdateNativeQueryTransaction("T1", 1, 8);
};
Runnable selectForUpdateNativeQueryTransaction2 = () -> {
selectForUpdateNativeQueryTransaction("T2", 3, 2);
};
Runnable selectForUpdateHqlQueryTransaction1 = () -> {
selectForUpdateHqlQueryTransaction("T1", 1, 8);
};
Runnable selectForUpdateHqlQueryTransaction2 = () -> {
selectForUpdateHqlQueryTransaction("T2", 3, 2);
};
// es.execute(selectForUpdateHqlQueryTransaction1);
// es.execute(selectForUpdateHqlQueryTransaction2);
es.execute(selectForUpdateNativeQueryTransaction1);
es.execute(selectForUpdateNativeQueryTransaction2);
es.shutdown();
try {
es.awaitTermination(1, TimeUnit.MINUTES);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
这是输出：
T2 BEGIN
T1 BEGIN
Hibernate: SELECT * FROM FIELD WHERE DESCRIPTION=? FOR UPDATE
T1 SELECT EXECUTED
T1 : FIELD READ [16]
T1 : FIELD READ [Test field]
T1 : FIELD READ [Test type]
T1 : FIELD READ [This is a field for testing]
Hibernate: SELECT * FROM FIELD WHERE DESCRIPTION=? FOR UPDATE
Hibernate: UPDATE FIELD SET DESCRIPTION=? WHERE DESCRIPTION=?
T1 UPDATED ROWS: 1
T1 COMMITTED
T2 SELECT EXECUTED
Hibernate: UPDATE FIELD SET DESCRIPTION=? WHERE DESCRIPTION=?
T2 UPDATED ROWS: 0
T2 COMMITTED
注意：T2 UPDATED ROWS: 0
但是，如果我尝试使用Hql达到相同的效果，则会给我一些奇怪的输出。 这是交易的代码：
private void selectForUpdateHqlQueryTransaction(String name, int delayBeforeRead, int delayBeforeCommit) {
Transaction tx = null;
Session session = null;
try {
session = factory.withOptions().interceptor(new HibernateInterceptor()).openSession();
tx = session.beginTransaction();
System.out.println(name + " BEGIN");
try {
TimeUnit.SECONDS.sleep(delayBeforeRead);
} catch (InterruptedException e) {
e.printStackTrace();
}
Query query = session.createQuery("SELECT f FROM Field f WHERE f.description=?1").setLockMode("this", lockMode);
query.setString("1", DESC);
List fields = query.list();
System.out.println(name + " SELECT EXECUTED");
fields.forEach(obj -> {
Field field = (Field) obj;
System.out.println(name + " : FIELD READ [" + field.getId() + "]");
System.out.println(name + " : FIELD READ [" + field.getName() + "]");
System.out.println(name + " : FIELD READ [" + field.getType() + "]");
System.out.println(name + " : FIELD READ [" + field.getDescription() + "]");
});
try {
TimeUnit.SECONDS.sleep(delayBeforeCommit);
} catch (InterruptedException e) {
e.printStackTrace();
}
Session finalSession = session;
fields.forEach(obj -> {
Field field = (Field) obj;
field.setDescription(name);
finalSession.update(field);
});
System.out.println(name + " UPDATED ROWS: " + fields.size());
tx.commit();
} catch (Exception e) {
fail();
if (tx != null) {
tx.rollback();
}
} finally {
session.close();
}
System.out.println(name + " COMMITTED");
}
输出：
T2 BEGIN
T1 BEGIN
апр 22, 2019 4:29:37 PM org.hibernate.loader.Loader determineFollowOnLockMode
WARN: HHH000445: Alias-specific lock modes requested, which is not currently supported with follow-on locking; all acquired locks will be [PESSIMISTIC_WRITE]
апр 22, 2019 4:29:37 PM org.hibernate.loader.Loader shouldUseFollowOnLocking
WARN: HHH000444: Encountered request for locking however dialect reports that database prefers locking be done in a separate select (follow-on locking); results will be locked after initial query executes
Hibernate: select field0_.ID as ID1_9_, field0_.DESCRIPTION as DESCRIPTION2_9_, field0_.NAME as NAME3_9_, field0_.TYPE as TYPE4_9_ from FIELD field0_ where field0_.DESCRIPTION=?
Hibernate: select ID from FIELD where ID =? for update
T1 SELECT EXECUTED
T1 : FIELD READ [16]
T1 : FIELD READ [Test field]
T1 : FIELD READ [Test type]
T1 : FIELD READ [This is a field for testing]
апр 22, 2019 4:29:39 PM org.hibernate.loader.Loader determineFollowOnLockMode
WARN: HHH000445: Alias-specific lock modes requested, which is not currently supported with follow-on locking; all acquired locks will be [PESSIMISTIC_WRITE]
апр 22, 2019 4:29:39 PM org.hibernate.loader.Loader shouldUseFollowOnLocking
WARN: HHH000444: Encountered request for locking however dialect reports that database prefers locking be done in a separate select (follow-on locking); results will be locked after initial query executes
Hibernate: select field0_.ID as ID1_9_, field0_.DESCRIPTION as DESCRIPTION2_9_, field0_.NAME as NAME3_9_, field0_.TYPE as TYPE4_9_ from FIELD field0_ where field0_.DESCRIPTION=?
Hibernate: select ID from FIELD where ID =? for update
T1 UPDATED ROWS: 1
previousState in onFlushDirty(): []
previousState in onFlushDirty(): This is a field for testing
previousState in onFlushDirty(): Test field
previousState in onFlushDirty(): []
previousState in onFlushDirty(): Test type
currentState in onFlushDirty(): []
currentState in onFlushDirty(): T1
currentState in onFlushDirty(): Test field
currentState in onFlushDirty(): []
currentState in onFlushDirty(): Test type
Hibernate: update FIELD set DESCRIPTION=?, NAME=?, TYPE=? where ID=?
T2 SELECT EXECUTED
T1 COMMITTED
T2 : FIELD READ [16]
T2 : FIELD READ [Test field]
T2 : FIELD READ [Test type]
T2 : FIELD READ [This is a field for testing]
T2 UPDATED ROWS: 1
previousState in onFlushDirty(): []
previousState in onFlushDirty(): This is a field for testing
previousState in onFlushDirty(): Test field
previousState in onFlushDirty(): []
previousState in onFlushDirty(): Test type
currentState in onFlushDirty(): []
currentState in onFlushDirty(): T2
currentState in onFlushDirty(): Test field
currentState in onFlushDirty(): []
currentState in onFlushDirty(): Test type
Hibernate: update FIELD set DESCRIPTION=?, NAME=?, TYPE=? where ID=?
T2 COMMITTED
您可能已经注意到，我使用Hibernate Interceptor来拦截SQL操作。 Interceptor.onFlushDirty()在SQL UPDATE之前被调用。因此，我不清楚为什么我会在输出中看到两个事务都更新了值：
...
currentState in onFlushDirty(): T1
...
currentState in onFlushDirty(): T2
...
第一个问题是： 真的只有1次更新，还是两个交易都更新了价值？当我更改了负责UPDATE的代码时，必须只有1个更新，以便它不会覆盖旧值，而是将事务名称附加到该值上。在两个事务完成之后，该列的末尾不包含“ ... T1：T2”之类的内容。很好。
但是下一个问题是： 为什么HQL版本未选择本机SQL版本却选择了旧实体？是因为缓存吗？我想确切地知道哪个交易“成功”了，哪个没有。