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

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.