Spring synchronized method NOT SYNCHRONIZED

Question

Environment:

• apache tomcat 7

• java 7

• oracle 11g

• eclipse

• apache jmeter 2.1

• spring

• hibernate

  I am working on a web application that receives requests from clients and generate sequence number for them according to the request type to be used in further processing. For generating a unique sequence number I have a method to get current sequence number from DB and increment it by 1 then update that record by new sequence number.

The function:

    @Transactional
public synchronized Long generateSequenseNumber(String requestType) {
     //get current sequence number for this requestType
            //increment it by one
            //update it in DB

}

The function works fine, but the problem is when I call the application from stress testing tool (JMeter) to send 50 requests per second I get below exception:

org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect): [

although the function is synchronized.

Any suggestion will be helpful. Thanks.

Answer 1

The problem is that @Transactional begins the session before entering the synchronized method and commits the changes after the method is finished, so changes to the database will not be applied inside the synchronized method.

Please check Spring @Transactional section 10.5.1.

You can try adding a synchronized block when calling this method instead of making it synchronized:

synchronized(this){
   generateSequenseNumber();
}

Answer 2

Extending Azizi's answer...

@Transactional makes Spring wrap the class in AOP proxy. The target method execution is then wrapped in transaction interceptor. So the overall call looks like this:

-> FooProxy#generateSequenseNumber
   -> TransactionInterceptor#invoke
      -> BEGIN TRANSACTION
         -> Foo#generateSequenceNumber (synchronized)
      -> COMMIT|ROLLBACK TRANSACTION

You can (and should) try to put breakpoint inside your method to see what is on the stack.

If you want to solve the synchronization inside your generateSequenseNumber method, then you can use TransactionTemplate and REQUIRES_NEW propagation. Of course then the @Transactional annotation would make no sense.