休眠事务计数器行为

Question

I have the following JPA entity:

@Entity(name="metrics")
public class Metrics {

    @Id
    private String metricId;

    @Column
    private long count;

    public Metrics() {
        count = 0;
    }

And I try to update it atomically like this:

//Begin transaction
Metrics result = em.find(Metrics.class, id);

if (result == null) {
    result = new Metrics();
    result.metricId = id;
    result.count++;
    em.persist(result);
} else {
    result.count++;
    em.merge(result);
}
//Commit transaction

However for some reason, this doesn't seem to make updates atomic and I end up with lost updates in concurrent environments. I can solve this by implementing optimistic locking with Hibernate using @Version , but I'm a little surprised that is required.

Why does the above code suffer from lost updates, even with a transaction?

Answer 1

Your are right, only optimistic or pessimistic locking can helps you, if you want to hold valid values exactly in database. In your case pessimistic locking would be better, because application works in competitive environment, but optimistic locking would be throw exception again and again. All advantages of Atomic was lost after application server restart or hangups.

Answer 2

Atomic means only one operation at a time. The java language specification guarantees that reading or writing a variable is atomic unless the variable is of type long or double . Because the variable (long/double) is written using two separate operations: one that writes the first 32 bits, and a second one which writes the last 32 bits. That means that another thread might read the value of count , and see the intermediate state.

A simple way to make this operation atomic is to make the variable volatile:

private volatile long count;

Now, the count is never cached by any Thread and it is assured that this variable will always be read by main memory and there will never be a loss of your count in concurrent environment.

AtomicLong might interest you too given that you aint using it in an entity class.