What is the difference between Serializable and Externalizable in Java?

Question

What is the difference between Serializable and Externalizable in Java?

Answer 1

https://docs.oracle.com/javase/8/docs/platform/serialization/spec/serialTOC.html

Default serialization is somewhat verbose, and assumes the widest possible usage scenario of the serialized object, and accordingly the default format (Serializable) annotates the resultant stream with information about the class of the serialized object.

Externalization give the producer of the object stream complete control over the precise class meta-data (if any) beyond the minimal required identification of the class (e.g. its name). This is clearly desirable in certain situations, such as closed environments, where producer of the object stream and its consumer (which reifies the object from the stream) are matched, and additional metadata about the class serves no purpose and degrades performance.

Additionally (as Uri point out) externalization also provides for complete control over the encoding of the data in the stream corresponding to Java types. For (a contrived) example, you may wish to record boolean true as 'Y' and false as 'N'. Externalization allows you to do that.