JPA：如何确保被管理实体的属性值以事务方式写入数据库？

Question

It seems like a pretty basic question to me, so probably I'm either lacking the right search terms or I'm completely missing something about the way how managed entites work, but nevertheless I was unable to find out how to do this: Writing new attribute values of a managed entity to the database in a transactional way, meaning I want to set a bunch of values to an entity bean and have them persisted all at once and without other threads seeing a „dirty“ intermediate state of the bean or interrupting the writing process.

This is the entity class:

@Entity
public class MyEntityClass
{
    ...
    private String status;
    private String value;
    ...
    public void setStatus(String status)
    {
    ...
    public void setValue(String vlaue)
    {
    ...
}

I'm using it here:

import javax.ejb.Stateless;
import javax.ejb.TransactionManagement;
import javax.ejb.TransactionManagementType;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless
@TransactionManagement(TransactionManagementType.CONTAINER)
public class MyService
{
    @PersistenceContext
    protected EntityManager entityManager;
    ...
    MyEntityClass entity = entityManager.find(entityClass, id);

    //Now I'd like to set some attributes in a transactional way
    entity.setStatus(newStatus);

    //what if the new status got persisted and another thread reads from the database now
    entity.setValue(newValue);

    //flushing, just to make sure that at least from here on the database is in a consistent state
    entityManager.flush();
}

I need to ensure that no other thread can see the entity in a „half-written“ state, ie with the new status already persisted but the old value still present in the database. I tried using a lock:

...
MyEntityClass entity = entityManager.find(entityClass, id);
entityManager.lock(entity, LockModeType.WRITE);
entity.setStatus(newStatus);
entity.SetValue(newValue);
entityManager.flush();
entityManager.lock(entity, LockTypeMode.NONE);
...

But this throws:

java.lang.IllegalArgumentException: entity not in the persistence context because the transaction used for reading the entity ends after the entityManager.find() is completed and therefore also the persistence context is gone, as I learned from this answer .

Also, I read here that I cannot manually create transactions with an EntityManager that uses container-managed transactions. But isn't there any way now to manually ensure that the entity's attributes are persisted together (or not at all)? Or is this somehow already done automatically?

Answer 1

Since you run in EJB container and your transaction is managed by the container, every business method of your EJB bean is invoked inside a transaction, so as far as your isolation level of the transaction is no set to Read uncommitted , changes that where made in the method will be visible when the transaction commits, that is when the method finishes.

UPDATE:

As you don't use business methods it looks like your code is running without any transaction. In this case, every invocation of EnitytMangers method will create a new PersistenceContext. This is why you are getting exception. Entity that you get form find method is detached when next method of EntityManager is called (as new PersistenceContext is created).