Martin Desruisseaux created SIS-586:
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Summary: Abandon NilObject support of primitive wrappers
Key: SIS-586
URL: https://issues.apache.org/jira/browse/SIS-586
Project: Spatial Information Systems
Issue Type: Task
Components: Metadata
Affects Versions: 1.3
Reporter: Martin Desruisseaux
Assignee: Martin Desruisseaux
Fix For: 1.4
The ISO 19115-1 standard (which provides the metadata model) defines each
attribute as optional, mandatory or conditional. For example in the
{{CI_Citation}} class, the {{title}} attribute is mandatory but the {{edition}}
attribute is optional. A mandatory attribute shall never be null.
The ISO 19115-3 standard (which provides the XML representation of ISO 19115-1)
maps mandatory attributes to _required_ XML elements. However in practice,
sometime a mandatory attribute is nevertheless missing. ISO 19115-3 recognizes
the following reasons why a mandatory attribute may be missing:
* {{INAPPLICABLE}} — There is no value.
* {{MISSING}} — The correct value is not readily available to the sender of
this data.
* {{TEMPLATE}} — The value will be available later.
* {{UNKNOWN}} — The correct value is not known to, and not computable by, the
sender of this data.
* {{WITHHELD}} — The value is not divulged.
Apache SIS represents those special cases by sentinel values with all
properties set to NaN, 0 or empty strings, depending on what the value permits.
Those special values implement the {{org.apache.sis.xml.NilObject}} interface,
which provides a method giving the reason why the value is nil. However this
approach is not possible when the class of the value is final. In particular,
it is not possible to return a {{String}}, {{Integer}} or {{Double}} value
implementing the {{NilObject}} interface. As a workaround, Apache SIS creates a
new instance of those classes with, for example, {{new Double(NaN)}} —
intentionally *not* with {{Double.valueOf(NaN)}} because we really want a new
instance — then uses identity check (i.e. using {{==}} instead of
{{Double.equals(Object)}} for identifying the nil sentinel value.
However above trick will soon be impossible. The constructors of all primitive
wrappers have been deprecated for removal, so this part of Apache SIS code will
not compile or even execute on future Java versions. Even if the constructors
were not removed, the wrappers are planed to become _value objects_ when this
feature will be added to the Java language ([Project
Valhalla|https://openjdk.org/projects/valhalla/]), which will make the identity
checks ineffective.
We have no workaround at this time. The only thing we can do is to drop the
support of {{NilObject}} on {{Integer}} and {{Double}} values (or any other
primitive wrapper classes). We are better to do that soon because all Apache
SIS releases having explicit calls to {{new Double(…)}} may break in future
Java environments.
As a replacement, a future SIS version could use {{BigDecimal}} or
{{BigInteger}} instead of {{Double}} and {{Long}}. This is actually desirable
even if it was not for this {{NilObject}} issue. Performance and memory
consumption of {{BigDecimal}} or {{BigInteger}} are not a big issue in metadata
(they would be much more important in referencing, features or coverage
classes), and there is a desire to express as closely as possible the data
distributor intend, which includes the use of base 10 and the number of
significant decimal digits. See [issue #87 on
GeoAPI|https://github.com/opengeospatial/geoapi/issues/87].
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