How does Java's serialization work and when it should be used instead of some other persistence technique?

Question

I've been lately trying to learn more and generally test Java's serialization for both work and personal projects and I must say that the more I know about it, the less I like it. This may be caused by misinformation though so that's why I'm asking these two things from you all:

1: On byte level, how does serialization know how to match serialized values with some class?

One of my problems right here is that I made a small test with ArrayList containing values "one", "two", "three". After serialization the byte array took 78 bytes which seems awfully lot for such low amount of information(19+3+3+4 bytes). Granted there's bound to be some overhead but this leads to my second question:

2: Can serialization be considered a good method for persisting objects at all? Now obviously if I'd use some homemade XML format the persistence data would be something like this

<object>
    <class="java.util.ArrayList">
    <!-- Object array inside Arraylist is called elementData -->
    <field name="elementData">
        <value>One</value>
        <value>Two</value>
        <value>Three</value>
    </field>
</object>

which, like XML in general, is a bit bloated and takes 138 bytes(without whitespaces, that is). The same in JSON could be

{
    "java.util.ArrayList": {
        "elementData": [
            "one",
            "two",
            "three"
        ]
    }
}

which is 75 bytes so already slightly smaller than Java's serialization. With these text-based formats it's of course obvious that there has to be a way to represent your basic data as text, numbers or any combination of both.

So to recap, how does serialization work on byte/bit level, when it should be used and when it shouldn't be used and what are real benefits of serialization besides that it comes standard in Java?

Answer 1

I would personally try to avoid Java's "built-in" serialization:

• It's not portable to other platforms
• It's not hugely efficient
• It's fragile - getting it to cope with multiple versions of a class is somewhat tricky. Even changing compilers can break serialization unless you're careful.

For details of what the actual bytes mean, see the Java Object Serialization Specification.

There are various alternatives, such as:

• XML and JSON, as you've shown (various XML flavours, of course)
• YAML
• Facebook's Thrift (RPC as well as serialization)
• Google Protocol Buffers
• Hessian (web services as well as serialization)
• Apache Avro
• Your own custom format

(Disclaimer: I work for Google, and I'm doing a port of Protocol Buffers to C# as my 20% project, so clearly I think that's a good bit of technology :)

Cross-platform formats are almost always more restrictive than platform-specific formats for obvious reasons - Protocol Buffers has a pretty limited set of native types, for example - but the interoperability can be incredibly useful. You also need to consider the impact of versioning, with backward and forward compatibility, etc. The text formats are generally hand-editable, but tend to be less efficient in both space and time.

Basically, you need to look at your requirements carefully.