How to stop session to save unsaved objects during query execution?_问答_开发者

I have problem with session and query execution,please see code below.

class A implements Lifecycle{

 public boolean onUpdate(Session session){
     Query test=session.createQuery("select * from Unknown");
     List list=test.list();
     B b=new B();
     session.save(b);
 }

}
class B{
  C c;
  public B(){
    c=new 开发者_开发问答C();
   c.a=new A();
  }

}
class C implements Lifecycle{
  A a;
  public boolean onSave(Session session){
     a.onUpdate(session);
  }

}

I modified A object and called onUpdate method.but exception whenever I call method a.onUpdate();

org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.test.C

I know that above exception due to query execution in onUpdate method. please suggest me is there is way to stop save unsaved objects when queries execute. or any other suggest this problem is great helpful.

please suggest me is there is way to stop to save unsaved objects during queries execution

To strictly answer this question, by default Hibernate will indeed flush the session when performing a query so that you don't get stale results. You can change this behavior using a custom FlushMode (COMMIT or NEVER). From the documentation:

10.10. Flushing the Session

Sometimes the Session will execute the SQL statements needed to synchronize the JDBC connection's state with the state of objects held in memory. This process, called flush, occurs by default at the following points:

before some query executions

from org.hibernate.Transaction.commit()

from Session.flush()

The SQL statements are issued in the following order:

all entity insertions in the same order the corresponding objects were saved using Session.save()

all entity updates

all collection deletions

all collection element deletions, updates and insertions

all collection insertions

all entity deletions in the same order the corresponding objects were deleted using Session.delete()

An exception is that objects using native ID generation are inserted when they are saved.

Except when you explicitly flush(), there are absolutely no guarantees about when the Session executes the JDBC calls, only the order in which they are executed. However, Hibernate does guarantee that the Query.list(..) will never return stale or incorrect data.

It is possible to change the default behavior so that flush occurs less frequently. The FlushMode class defines three different modes: only flush at commit time when the Hibernate Transaction API is used, flush automatically using the explained routine, or never flush unless flush() is called explicitly. The last mode is useful for long running units of work, where a Session is kept open and disconnected for a long time (see Section 11.3.2, "Extended session and automatic versioning").
sess = sf.openSession();
Transaction tx = sess.beginTransaction();
sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state

Cat izi = (Cat) sess.load(Cat.class, id);
izi.setName(iznizi);

// might return stale data
sess.find("from Cat as cat left outer join cat.kittens kitten");

// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();
During flush, an exception might occur (e.g. if a DML operation violates a constraint). Since handling exceptions involves some understanding of Hibernate's transactional behavior, we discuss it in Chapter 11, Transactions and Concurrency.