Serializable is a marker interface but Externalizable is not a marker

interface.

When we implement Serializable interface, the class is serialized

automatically by default. We can override writeObject() and

readObject()methods to control more complex object Serialization

process.

In case of Externalizable, we use readExternal() and

writeExternal() methods to give control to class for class's

serialization process.

Serializable interface is based on recursive algorithm.

Serializable gives you two options. One option is to provide custom

way of serialization, the other default way. In Externalizable, you

have to always implement readExternal() and writeExternal()

methods.

A public no-arg constructor is needed while using Externalizable

interface.

In Serialization, we need to define serialVersionUID. If it is not

explicitly defined it will be generated automatically based on all the

fields, methods of the class

https://www.baeldung.com/java-externalizable#:~:text=Performance-,The%20java.,control%20over%20the%20serialization%20process.