使方法同步将确保它是线程安全的吗？

Question

I have a method in which some database insert operations are happening using hibernate and i want them to be thread safe. The method is getting some data in parametres and its a possiblity that sometimes two calls are made with same data at same point of time.

I can't lock those tables because of performance degradation. Can anyone suggest making the method as synchronized will solve issue?

Answer 1

Synchronizing a method will ensure that it can only be accessed by one thread at a time. If this method is your only means of writing to the database, then yes, this will stop two threads from writing at the same time. However, you still have to deal with the fact that you have multiple insert operations with the same data.

Answer 2

You should let Hibernate handle the concurrency, that's what it is meant to do. Don't assume Hibernate will lock anything: it supports optimistic transactions for exactly this purpose. Quote from the above link:

The only approach that is consistent with high concurrency and high scalability, is optimistic concurrency control with versioning. Version checking uses version numbers, or timestamps, to detect conflicting updates and to prevent lost updates. Hibernate provides three possible approaches to writing application code that uses optimistic concurrency.

Answer 3

Database Concurrency is handled by transactions. Transactions have the Atomic Consistent Isolated Durable (ACID) properties. They provide isolation between programs accessing a database concurrently. In the Hibernate DAO template of spring framework there are single line methods for CRUD operations on the database. When used individually these don't need to be synchronized by method. Spring provides declarative (XML), programmatic and annotation meta-data driven transaction management if you need to declare "your method" as transactional with specific propagation settings, rollbackFor settings, isolation settings. So in "your method" you can do multiple save,update,deletes etc and the ORM will ensure that it is executed with the transaction settings you have given in the meta-data.

Another issue is that the thread has to have the lock on all the objects that are taking part in the transaction.Otherwise the transaction might fail or the ORM will persist stale data. In another situation it can result in a deadlock because of lock-ordering. I think this is what really answers your question.

Both objects a and b have an instance variable of the type Lock. A boolean flag can be used to indicate the success of the transaction. The client code can retry the same transaction if it fails.

             if (a.lock.tryLock()) {
                try {
                    if (b.lock.tryLock()) {
                        try {
                            // persist or update object a and b
                        } finally {
                            b.lock.unlock();
                        }
                    }
                } finally {
                    a.lock.unlock();
                }
            }

The problem with using synchronized methods is that it locks up the entire Service or DAO class making other service methods unavailable to other threads. By using individual locks on objects we can gain the advantage of fine grained concurrency.

Answer 4

No. This method probably uses another methods and objects, which may be not thread safe. synchronized makes threads to use that's method's object monitor only once at a time, so it makes thread-safe a method with respect to the object.

If you are sure that all other threads use shared functionality only with this method, then making it synchronized may be sufficient.

Answer 5

Choosing the best strategy depends on the architecture, sometimes to increase performance seems to be easier to use the trick like method synchronization, but this is bad approach.

There's no doubts, you should use transactions, and if with that strategy you're facing performance issues you should optimize your db queries or db structure.

Please remember that "Synchronization" should be as much as possible atomic.