Question

What happened if there is no value for some class fields in xml file during JAXB unmarshalling? JAXB just “omit” this value and left them uninitialized?

Answer 1

they will get initialized.

Initial values for fields will be set and the no-arg constructor will also run.

for example:

class AClass {

    private int x = 5;
    private int y = 16;
    private Object object;

    public AClass() {
        this.x = 100;
    }

    // getters and setters
    // ...

}

if the values in the xml for x, y, z and object are omitted there valus will be:

x: 100 (the constructor runs after the field value initialization)
y: 16 (from the field value initialization)
object: null (objects get initialized to null if no value is given)