Managed entity not synchronised with DB?

[John Stone]

I came across this paragraph in "EJB 3 in action"

Since the persist method only issues the INSERT statement and does not load
the data that was changed by the database as a result of the INSERT, the entity
returned by the method would not include the generated postingDate field. This
problem could be fixed by using the refresh method as follows:
public Item addItem(String title, String description,
byte[] picture, double initialPrice, long sellerId) {
Item item = new Item();
item.setTitle(title);
...
entityManager.persist(item);
entityManager.flush();
Entity lifecycle listeners 333
entityManager.refresh(item);
return item;
}

Isn't entity managed after line 'entityManager.persist(item);'?
If it is managed, it should automatically synchronised with db, right?
Then why is there need to call 'refresh'?

[Satya Maheshwari]

Being a managed entity does not automatically ensure that the entity is refreshed from DB. Quoting from the ejb spec:

3.2.3 Synchronization to the Database
The state of persistent entities is synchronized to the database at transaction commit. This synchronization involving writing to the database any updates to persistent entities and their relationships as specified
above.An update to the state of an entity includes both the assignment of a new value to a persistent property or field of the entity as well as the modification of a mutable value of a persistent property or field.
Synchronization to the database does not involve a refresh of any managed entities unless the refresh operation is explicitly invoked on those entities

[John Stone]

Thanks, I thought it works automatically in both ways.

[Satya Maheshwari]

I was trying this out practically. Strangely it turns out that the managed is indeed refreshed from the DB after the persist. Here is the code:

    public Cust updateNameAndCity(Integer custId, String name,java.lang.String city) {
        Cust cust = em.find(Cust.class, custId);
        cust.setName(name);
        persist(cust);
        System.out.println("Name:"+cust.getName());
        System.out.println("City:"+cust.getCity());
        System.out.println("----------------------");
        updateCity(custId,city);
        System.out.println("Name:"+cust.getName());
        System.out.println("City:"+cust.getCity());
        System.out.println("----------------------");        
        return cust;
    }
 
    public void updateCity(Integer custId, String city) {
        Cust cust = em.find(Cust.class, custId);
        cust.setCity(city);
        persist(cust);
    }

And here is the output:

Name:bbb
City:null
----------------------
Name:bbb
City:San Bruno
----------------------

The output suggests that Cust is refreshed with the updated city after it is persisted. I am confused here :confused