Externalization efficiency comes at a price. The default serialization mechanism adapts to application changes due to the fact that metadata is automatically extracted from the class definitions (observe the format above and you will see that when the object is serialized by implementing Serializable interface, the class metadata(definitions) are written to the persistent store while when you serialize by implementing Externalizable interface, the class metadata is not written to the persistent store).
Externalization on the other hand isn't very flexible and requires you to rewrite your marshalling and demarshalling code whenever you change your class definitions.
As you know a default public no-arg constructor will be called when serializing the objects that implements Externalizable interface. Hence, Externalizable interface can't be implemented by Inner Classes in Java as all the constructors of an inner class in Java will always accept the instance of the enclosing class as a prepended parameter and therefore you can't have a no-arg constructor for an inner class. Inner classes can achieve object serialization by only implementing Serializable interface.
If you are subclassing your externalizable class, you have to invoke your superclass’s implementation. So this causes overhead while you subclass your externalizable class. Observe the examples above where the superclass writeExternal method is explicitly called in the subclass writeExternal method.
Methods in externalizable interface are public. So any malicious program can invoke which results into loosing the prior serialized state.
Once your class is tagged with either Serializable or Externalizable, you can't change any evolved version of your class to the other format. You alone are responsible for maintaining compatibility across versions. That means that if you want the flexibility to add fields in the future, you'd better have your own mechanism so that you can skip over additional information possibly added by those future versions.
Externalization on the other hand isn't very flexible and requires you to rewrite your marshalling and demarshalling code whenever you change your class definitions.
As you know a default public no-arg constructor will be called when serializing the objects that implements Externalizable interface. Hence, Externalizable interface can't be implemented by Inner Classes in Java as all the constructors of an inner class in Java will always accept the instance of the enclosing class as a prepended parameter and therefore you can't have a no-arg constructor for an inner class. Inner classes can achieve object serialization by only implementing Serializable interface.
If you are subclassing your externalizable class, you have to invoke your superclass’s implementation. So this causes overhead while you subclass your externalizable class. Observe the examples above where the superclass writeExternal method is explicitly called in the subclass writeExternal method.
Methods in externalizable interface are public. So any malicious program can invoke which results into loosing the prior serialized state.
Once your class is tagged with either Serializable or Externalizable, you can't change any evolved version of your class to the other format. You alone are responsible for maintaining compatibility across versions. That means that if you want the flexibility to add fields in the future, you'd better have your own mechanism so that you can skip over additional information possibly added by those future versions.
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