The documentation of ObjectOutputStream says:
The default serialization mechanism for an object writes the class of the object, the class signature, and the values of all non-transient and non-static fields. References to other objects (except in transient or static fields) cause those objects to be written also.
So when you declare a variable as transient, it should be ignored by ObjectOutputStream. Make sure that you use the transient keyword and not a @Transient annotation. Such annotations are used by some ORM frameworks to mark fields which are not supposed to be saved in databases. They are meaningless for the buildin serialization framework.
private transient String foo; // Field gets ignored by ObjectOutputStream
@Transient private String bar; // Treated normally by ObjectOutputStream (might mean something for some other framework)