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Spring MVC - should my domain classes implement Serializable for over-the-wire transfer?

I'm trying to learn Spring Boot by implementing a simple REST API.
My understanding was that if I need to transfer an object over the wire, that object should implement Serializable.
In many examples on the net though, including official ones, domain classes that need to be transferred from server to client (or vice-versa) do not to implement Serializable.
For instance: https://spring.io/guides/gs/rest-service/
But in some cases, they do:
For instance: https://github.com/szerhusenBC/jwt-spring-security-demo/blob/master/src/main/java/org/zerhusen/security/JwtAuthenticationRequest.java
Is there a general rule of thumb on when to implement Serializable?

To update this, advice about Serializable has changed, the recommendation currently seems to be Don’t use Serializable for anything.
Using the Java serialization API means you need something in Java on the other side of the wire to deserialize the objects, so you have to control the code that deserializes as well as the code that serializes.
This typically isn't relevant for REST applications, consuming the application response is the business of someone else's code, usually outside your organization. When building a REST application it's normal to try to avoid imposing limitations on what is consuming it, picking a format that is more technology-agnostic and broadly available.
Some reasons for having an object implement java.io.Serializable would be:
so you can put it in an HttpSession
so you can pass it across a network between parts of a distributed application
so you can save it to the file system and restore it later (for instance, you could make the contents of a queue serializable and have the queue contents saved when the application shuts down, reading from the save location when the application starts to restore the queue to its state on shutdown).
In all these cases, you serialize so you can save something to a filesystem or send it across a network.

There are many ways to serialize an object. Java's object serialization is just one of them. From the official documentation:
To serialize an object means to convert its state to a byte stream
REST APIs usually send and receive JSON or XML. In that case serializing an object means converting its state to a String.
There is no direct connection between "sending an object over the wire" and implementing Serializable. The technologies you use dictate whether or not Serializable has to be implemented.

The specific examples you have mentioned do not transfer objects over the wire. From the example links I see that the controller methods return a domain object with ResponseBody annotation. Just because the return type of the method is the domain object it is not necessary that the whole object is being sent to the client. One of the handler method in Spring mvc framework internally intercepts the invocation and determines that the method return type does not translate to direct ModelAndView object. RequestResponseBoodyMethodProcessor which handles the return value of such annotated methods and uses one of the message converters to write the return object to the http response body. In the case the message converter used would be MappingJackson2HttpMessageConverter. So if are to follow the same coding style you are not required to implement Serializable for your domain objects.
Have a look at this link for the Http message converters provided by default from spring. The list is quiet extensive however not exhaustive and if requirements arise you can implement your own custom message converter to user as-well.

that's a good question when to implement Serializable interface.
these links can provides some useful contents:
Serializing java.io.Serializable instance into JSON with Spring and Jackson JSON
When and why JPA entities should implement the Serializable interface?
I sometimes wonder about this, and I think
Because Java is a open source language, and more libraries providered by third party. for tells who will serialize and deserialize the object, the java offical declare a constract interface, makes transfer easy and safety throught different library.
It's just a constract, most third-party libraries can serialize/deserialize when checking implement this constract. and jackson's jar library is not use it.
So you can deem if you use serialize/deserialize object data in your own system, and simple process, likes just serialize and response it(jackson in spring MVC), you needn't to implements it.
but if you used in other jar library, likes saving in HttpSession, or other third-party componens/library, you should(or have to) implement Serializable, otherwise the libraries will throw a exception to tell you the constract interfaced which it knows is not provide.
But they said it's a good habit and best properties that to implement the Serializable when serialize a custom class. :)

you should serialize if you are using caching for database operations.Usually the third party cache providers like (hazle cast, Jboss cache etc..) internally serialize/ de serialise objects.In that case model classes should implement Serializable to facilitate caching.

How to extend Jackson property discovery in a "catch-all-types" way?

Following my previous question about serialization only, I'd like to go further and support JsonFormatVisitor.
I have the same requirements, that is:
I have objects of several types (interfaces).
I don't know the type of theses objets in advance.
I can't add annotations on theses types.
I can introspect all theses objets to get their state data.
Now that serialization works, I need to generate JsonSchema and hence do something like that:
SchemaFactoryWrapper visitor = WHAT?
mapper.acceptJsonFormatVisitor( mapper.constructType( Foo.class ), visitor );
JsonSchema jsonSchema = visitor.finalSchema();
String schemaString = mapper.writeValueAsString( jsonSchema );
I've implemented a SchemaFactoryWrapper that gets its expectAnyFormat called but I don't know what to do inside it. Looks like there's no schema for "any" objects.
Maybe I can hook elsewhere in jackson? Maybe it is possible to extends the whole Bean/Property introspection mechanism to support a completely different model (ie. not beans)?
I'm a bit lost, please help me find the treasure room :)

I can try to suggest some approaches that may be helpful.
First, even if you can not annotate classes directly, "mix-in annotations" can help -- this does assume static knowledge, however
Second, since schema-generation uses type detection used for serialization, you may want to register custom serializers; but this does not necessarily mean having to hand-write all. The most flexible way to register custom serializers is via Module interface (mapper.registerModule(new MyModule()); Modules can register Serializers instance which gets called when trying to locate a JsonSerializer for a type for the first time (after this, instance is cached to be re-used for other properties of same type).
This is where you could configure and return your custom JsonSerializer; but it might only need to handle schema-related callback(s) (one(s) called by schema generator).
It is also possible to extend/modify property discovery mechanism; whether this is easier depends. But the thing to look for is registering BeanSerializerModifier via Module.
It gets called during construction of BeanSerializer (general POJO serializer used unless something more specific is registered), and with it you can add/modify properties; or just replace resulting serializer altogether (and also then allows chaining of custom serializer with default one, if needed).

Better way to store data in java?

I am currently working on a videogame, and i want to have the user be able to save their character to a new file. I know how to use the file io (for the most part), but i have been using the 'serialize' to serialize a whole object (that contains all the variables for the character) and save it to a file. The problem is that i am constantly updating the object and making changes to it, so when i try to load the old character with the new object, it errors and crashes. Same with levels as-well (an object holding a few 2d-array of variables).
There must be a better way to do this so it is compatible with future versions. If there is a way, would anybody please offer some source code and/or a link to a nice tutorial? All help is appreciated, thanks!!!

Use XML or an embedded database (fast and lightweight) such as Derby or H2. You could even use a plain old properties file.
In fact, see if the properties file will work for you. And only if that won't work, try XML or the embedded database approach.

if you are looking for java serializers here is the benchmark for you https://github.com/eishay/jvm-serializers/wiki/
Apache Avro seems to perform well.
Another way is to store the values in the persistent store like HSQLDB or H2 db and load it to memory at startup and persist when needed.You can also use SQLite (for driver check this)

You can implement Externalizable instead of Serializable, and in the readExternal() and writeExternal() methods you can put the logic to read/write the object. This way you have full control of serialization/deserialization and can make changes fairly easily. Alternatively you can use JSON serialization by using Gson. I would not recommend XML, but if you want to you can check out xstream for the same thing.

If you are extending your objects in backwards compatible ways, i.e. add fields, and not removing fields. Make sure that you have declared a serialVersionUID as per the serializable javadoc.
http://download.oracle.com/javase/1.5.0/docs/api/java/io/Serializable.html

One additional option to consider since you're already using serialization, you could implement Externalizable instead of Serializable. The code you use to serialize objects would remain the same. However in your class you would specify exactly how you want it serialized by overriding readExternal() and writeExternal(). E.g.:
public class MyClass implements Externalizable {
private int foo;
private String bar;
public readExternal(ObjectInput in) {
foo = in.readInt();
bar = in.readUTF();
}
public writeExternal(ObjectOutput out) {
out.writeInt(foo);
out.writeUTF(bar);
}
}
Just be sure to keep the order the same when reading and writing. Try to only add fields, however if you need to remove a field leave a gap to account for old versions.
Ultimately though if you're making a lot of changes it might best to switch to a properties or XML file as LES2 suggested. It'll be more portable and readable that way.

This game uses java.util.prefs.Preferences for cross-platform convenience. Because keys are stored individually, new additions rarely interfere with existing entries.

How can I avoid the MarshalException for Java RMI?

I just finished a small program with java rmi and somehow it doesn't work. Everytime I want to start the server I'm getting the MarshalException. Are there any important points that I should be aware of them on how to implement the interface for the remote method invocation? I thought it would be possible to create an implementation but also include some additional methods like a constructor or private variables inside of the implementing class.
Shouldn't this just work?
Greetings

In order to be able to transfer objects you need to make them implement Serializable. And perhaps have a default (no-arg) constructor (this is not a requirement for serialization though)
As helios noted, not only the class, but all your field hierarchy (classes of fields, and the classes of their fields) must be Serializable)

Caused by:
java.lang.ClassNotFoundException:
vsys.ue04.server.RemoteChargeImplementation
There's your problem. That class is required at the client.

Serialize KeyguardLock

I am writing an android application and I need to have two classes use the same KeyguardLock object but I am experiencing extreme difficulty in sharing (via serialization) that object. I have tried using the serialization stackoverflow example link but that didn't work at all. I get a "not serializable" IO exception trying to save the object. I have also tried using JSONObject.
Any ideas? Has anyone run into a similar problem?

Why are you trying to serialize it? A object can only be serialize if it implements Serializable which KeyguardLock doesn't.
If you're trying to pass it around Activities, either create a custom Application object and store it there. Or use a public static variable in a class and access it via that. The static variable is probably the better option for this.