Question

So I have a Singleton class and in it a non-static public method that is called by multiple threads. In the non-static method I have local references to objects created by a Stored Procedure which I surmise means the ‘new’ keyword gets called somewhere.

 1 public void someMethod(SomeObjectParameter parameter) {
 2      
 3     Thing thingOne = synchornizedStoredProcedureCall(): 
 4     doSomethingWith(thingOne);
 5     doSomethingElseWith(thingOne);
 6
 7 }

Currently lines 3 through 5 are in a synchronized code block which I would like to reduce down to just having the stored procedure call as synchronized.

So… Say we have two threads, Thread1 and Thread2. Thread1 is about to execute line 3 and Thread2 is about to execute line 4. Since this is a local reference, will each thread maintain a different reference or will synchornizedStoredProcedureCall overwrite the reference to thingOne as doSomethingWith is about to use it?

What if thingOne is declared final or if I made it immutable?

Answer 1

will each thread maintain a different reference or will synchornizedStoredProcedureCall overwrite the reference to thingOne as doSomethingWith is about to use it?

The different threads have different stacks. The thingOne is stored in the per-thread stack so cannot be overwritten by the other thread. You also need to make sure that the synchornizedStoredProcedureCall() is returning a different instance of Thing each time and not a static or instance variable. Both threads need to be working on different Thing instances.

As long as the doSomethingWith(...) and doSomethingElseWith(...) calls are thread safe and only work with thingOne argument and constants (etc), protecting just the synchornizedStoredProcedureCall() should be fine.

As @Marko points out, without seeing the Thing class, we cannot be guaranteed that it is thread-safe and not storing internal state.