Marshalling/Unmarshalling nulls in JAXB

Question

I have several hand-written Java classes in a REST Server that uses JAXB to marshall/unmarshall from XML (JAX-RS).

I have implemented a small class hierarchy borrowed from Scala: Option<T> , Some<T> , and None<T> and would like to use these in my model to distinguish missing values from null values in the XML.

For example, in the following XML, the value phone_number is missing and the corresponding Java model field should be set to None :

<person>
  <last_name>Jones</last_name>
  <first_name>Abe</first_name>
</person>

class Person {
  @XmlElement("last_name")
  Option<String> lastName;

  @XmlElement("first_name")
  Option<String> firstName;

  @XmlElement("phone_number")
  Option<String> phoneNumber;
}

whereas in the this XML message, the phone_number should be set to new Some(null) :

<person>
  <last_name>Jones</last_name>
  <first_name>Abe</first_name>
  <phone_number></phone_number>
</person>

I realize that with this scheme, I cannot distinguish a zero-length String from a null String.

I thought of using @XmlJavaTypeAdapter , but I have many model classes and would rather use a "centralized" solution that works for all of the classes.

I believe that the correct solution involves using a MessageBodyWriter and MessageBodyReader , but I still want the built-in machinery to handle each of the classes; I'll handle the individual fields (including String and Date).

Answer 1

You can always set the Person class attributes to default values. If you default PhoneNumber to "None", saving it to XML will give you the following tag

 <phone_number>None</phone_number>

In any case, the PhoneNumber attribute will stay at "None" until you call a setter to set the phone number real value.

I hope this helps,