We work heavily with serialization and having to specify Serializable tag on every object we use is kind of a burden. Especially when it's a 3rd-party class that we can't really change.
The question is: since Serializable is an empty interface and Java provides robust serialization once you add implements Serializable - why didn't they make everything serializable and that's it?
What am I missing?
Not everything is genuinely serializable. Take a network socket connection, for example. You could serialize the data/state of your socket object, but the essence of an active connection would be lost.
I think the though was to make sure you, as the programmer, know that your object my be serialized.
For some classes, especially those that represent something more physical like a File, a Socket, a Thread, or a DB connection, it makes absolutely no sense to serialize instances. For many others, Serialization may be problematic because it destroys uniqueness constraints or simply forces you to deal with instances of different versions of a class, which you may not want to.
Arguably, it might have been better to make everything Serializable by default and make classes non-serializable through a keyword or marker interface - but then, those who should use that option probably would not think about it. The way it is, if you need to implement Serializable, you'll be told so by an Exception.
Serialization is fraught with pitfalls. Automatic serialization support of this form makes the class internals part of the public API (which is why javadoc gives you the persisted forms of classes).
For long-term persistence, the class must be able to decode this form, which restricts the changes you can make to class design. This breaks encapsulation.
Serialization can also lead to security problems. By being able to serialize any object it has a reference to, a class can access data it would not normally be able to (by parsing the resultant byte data).
There are other issues, such as the serialized form of inner classes not being well defined.
Making all classes serializable would exacerbate these problems. Check out Effective Java Second Edition, in particular Item 74: Implement Serializable judiciously.
I think both Java and .Net people got it wrong this time around, would have been better to make everything serializable by default and only need to mark those classes that can't be safely serialized instead.
For example in Smalltalk (a language created in 70s) every object is serializable by default. I have no idea why this is not the case in Java, considering the fact that the vast majority of objects are safe to serialize and just a few of them aren't.
Marking an object as serializable (with an interface) doesn't magically make that object serializable, it was serializable all along, it's just that now you expressed something that the system could have found on his own, so I see no real good reason for serialization being the way it is now.
I think it was either a poor decision made by designers or serialization was an afterthought, or the platform was never ready to do serialization by default on all objects safely and consistently.
Well, my answer is that this is for no good reason. And from your comments I can see that you've already learned that. Other languages happily try serializing everything that doesn't jump on a tree after you've counted to 10. An Object should default to be serializable.
So, what you basically need to do is read all the properties of your 3rd-party class yourself. Or, if that's an option for you: decompile, put the damn keyword there, and recompile.
The main role of Serializable in Java is to actually make, by default, all other objects nonserializable. Serialization is a very dangerous mechanism, especially in its default implementation. Hence, like friendship in C++, it is off by default, even if it costs a little to make things serializable.
Serialization adds constraints and potential problems since structure compatibility is not insured. It is good that it is off by default.
I have to admit that I have seen very few nontrivial classes where standard serialization does what I want it to. Especially in the case of complex data structures. So the effort you'd spend making the class serializble properly dwarves the cost of adding the interface.
Having to state explicitely that instances of a certain class are Serializable the language forces you to think about if you you should allow that. For simple value objects serialization is trivial, but in more complex cases you need to really think things through.
By just relying on the standard serialization support of the JVM you expose yourself to all kinds of nasty versioning issues.
Uniqueness, references to 'real' resources, timers and lots of other types of artifacts are NOT candidates for serialization.
There are some things in Java that simply cannot be serialized because they are runtime specific. Things like streams, threads, runtime, etc. and even some GUI classes (which are connected to the underlying OS) cannot be serialized.
I just recompiled groovy to make all objects serializable and I inject all classes with a serialVersionUID that don;t have one. Did I do a bad thing ?