public class ImplicitDefaultXmlValueBinding extends StandardXmlValueBindingImpl {

	@Override
	public String read() {
		String value = super.read();
		if (value != null && !value.equals(XmlUtil.EMPTY_STRING))
			return value;
		else
			return defaultValue();
	}

	@Override
	public void write(String value) {
		final String defaultValue = defaultValue();
		if (defaultValue.equals(value)) {
			super.write(null);
		} else {
			super.write(value);
		}
	}

	private String defaultValue() {
		...
	}

}

@XmlValueBinding(path = "@MyBooleanAttribute", mapExistanceToValue = "true;")

<Element MyBooleanAttribute="">

<Element>