Designates a boolean field/property as a flag to indicate whether another property is present or not.

Sometimes you'd want to map a Java primitive type to an optional element/attribute. Doing this makes it impossible to represent the absence of the property, thus you always end up producing the value when you marshal to XML. For example,

XmlElement
class Foo {
     XmlElement
     int x;
}

marshaller.marshal(new Foo());

and you get:

<foo><x>0</x></foo>

By creating a side boolean field/property that has this annotation, you can indicate the absence of the property by setting this boolean to false.

XmlElement
class Foo {
     XmlElement
     int x;
     XmlIsSet("x")
     boolean xIsPresent;
}

Foo f = new Foo();
f.x = 5;
f.xIsPresent = false;

marshaller.marshal(f);

<foo/>


f.xIsPresent = true;
<foo><x>5</x></foo>

A property/field annotated with XmlIsSet itself will not show up in XML. It is an error to use this annotation on the same property/field as XmlElement, XmlAttribute, XmlValue, or XmlElementRef, ...TBD.