Wednesday, September 9, 2009

writeObject() invoked although never declared in any supertype

Usually, a common behavior must be "declared" in a supertype. If base type B.java declares method m1(), then anyone having a B pointer can invoke m1() on our object, which could be a B subtype.

However, writeObject(ObjectOutputStream) [and readObject] is different. You can create a direct subtype of Object.java and put a private writeObject() in it. Say you have an object myOb and you serialize it. In the classic Hollywood tradition, Hollywood calls myOb.writeObject(), even though this method is private and never declared in any supertype. Trick is reflection -- Hollywood looks up the method named writeObject --

writeObjectMethod = getPrivateMethod(cl, "writeObject", ...

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