Serializable and transient

Question

To make class serializable we do the following:

class A implements Serializable {
    transient Object a;
}

And why not:

serializable class A {
   transient Object a;
}

Why if we want to make class serializable we implement special interface. And if we want to exclude some fields we use keyword "transient"? Why special keywords aren't used in both cases? I mean were there any reasons to make the same thing in different ways? I know, there is no such keyword as "serializabe" but why it wasn't introduced instead of special interface Serializable?

Answer 1

Serializable is a marker interface. Interfaces are a standard way (in Java and in some other languages) of indicating features of a class; an "is a" relaionship. Making Serializable an interface means we can declare methods that accept or return Serializables just like we can methods that work with other interfaces. Anything else would have required syntax changes to the language (at the time; now we have annotations, but I think an interface would still be used).

Answer 2

So you're asking why you can't mark a class as not serializable (like a transient member)? Why wouldn't you just not mark class members of the not-to-serialize type as transient? Or use a serialization delegate for that class type when you do the serialization? It seems a little weird that you would want to tell Java to not do something at this level instead of telling it to do something.

Answer 3

Why isn't used some special keyword to mark classes as serializable too? Serializable interface looks like a magic numbers in code and not like the language feature.

I think you have to look at it the other way: language keywords exist mainly to support compile-time language constructs. Serialization is a runtime mechanism. Additionally, you don't want to have an extra keyword for everything, because you then can't use it as an identifier. A marker interface on the other hand is much less intrusive.

The question is thus: why do we need a language keyword to mark transient fields? And the answer is that there simply was no other way to mark specific fields at that time.

Nowadays, one would use annotations for this purpose in both cases (and for other things like the obscure strictfp keyword as well).

Answer 4

Serializable is a marker interface (like Cloneable) that is used to set a flag for standard Java runtime library code that an object can be serialised according to the designer of that class.

The transient keyword can be used to specify that an attribute does not need to be serialised, for instance because it is a derived attribute.

See also this reply to a similar question on SO and this one about designing marker interfaces.

Update

Why marker interfaces and no keywords for things like serializable, cloneable, etc? My guess would be the possibility to consistently extend the Java runtime lib with new marker interfaces combined with too many keywords if behavioural aspects made it into the language.

The fact that class attributes cannot implement Interfaces and transient can be seen as a generic property of an attribute makes sense of introducing transient as a language keyword.