There are two situations:
- The local variable is of a primitive type like
int
or double
.
- The local variable is of a reference type like
ArrayList
.
In the first situation, you can't synchronize, as you can only synchronize on Objects (which are pointed to by reference-type variables).
In the second situation, it all depends on what the local variable points to. If it points to an object that other threads (can) also point to, then you need to make sure that your code is properly synchronized.
Examples: you assigned the local variable from a static
or instance field, or you got the object from a shared collection.
If, however, the object was created in your thread and only assigned to that local variable, and you never give out a reference to it from your thread to another thread, and the objects implementation itself also doesn't give out references, then you don't need to worry about synchronization.