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To make a class serializable we do the following:

class A implements Serializable {
    transient Object a;

And not this:

serializable class A {
   transient Object a;

Why, if we want to make a class serializable, do we implement a special interface. And if we want to exclude some fields we use the keyword transient? Why aren't special keywords 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 serializable but why wasn't it introduced instead of the special interface Serializable?

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Why do the birds go on singing? Why does the sea rush to shore? Why do the stars glow above? Why does my heart go on beating? Why do these eyes of mine cry? –  Jonathan Feinberg Dec 17 '09 at 16:25
Why is the sad cebu sad? Is the canoe wood or aluminum? –  Michael Myers Dec 17 '09 at 16:26
But I think the question is asking why Serializable feels grafted on to the language. And I think the answer is because it is. –  Michael Myers Dec 17 '09 at 16:27
@Jonathan, why don't you stay on topic? –  rsp Dec 17 '09 at 16:28
What's a topic? –  Jonathan Feinberg Dec 17 '09 at 16:31
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4 Answers

up vote 20 down vote accepted

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).

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Thanks. I agree that transient was introduced only due to annotations absense. But I still can not understand why was not introduced "serializable" keyword. –  Andrew Dashin Dec 17 '09 at 16:39
As I said: adding keywords to a language is actually something you want to avoid. With classes, it could be avoided by using a marker interface instead. With fields, there was no alternative. –  Michael Borgwardt Dec 17 '09 at 16:41
I've clarified the question a bit. –  Andrew Dashin Dec 17 '09 at 16:43
This is the answer to the question. Given some feature X that you want to add, you try to avoid adding new keywords at all costs if there are already language constructs to handle it. Just because you have to compromise on one thing ("transient" in this case) doesn't mean you automatically turn everything else into a keyword too. –  PSpeed Dec 17 '09 at 18:44
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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).

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I hope you don't write methods that have Serializable as parameter or return types. –  Tom Hawtin - tackline Dec 17 '09 at 16:31
Why not? For a general storage or communication interface, it may make perfect sense - your need stuff to be serializable, but that's all. –  Michael Borgwardt Dec 17 '09 at 16:37
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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.

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Well, in boo you tell the compiler not to do that. Everything is serializable by default and there's the transient keyword for non-serializable stuff. No need for markers. –  R. Martinho Fernandes Dec 17 '09 at 16:35
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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.


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.

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