A number of issues and/or concerns have been come up recently relating
to the reflective support for records. These arose when finalizing and
completing the runtime CSR. Taken together they seem to lead back to a
few small, but significant, omissions in the spec that would be good to
tighten up. It is important that it be possible to reflectively reason
about record classes in a way that is unambiguous and provides
certainty that the record class is well-formed.
For a class to be a record class, then:
1) It's direct superclass must be java.lang.Record, and
2) It must have a Record attribute.
The entry point to the reflective API for records is the two methods:
Class::isRecord and Class::getRecordComponents.
The isRecord method should, in it's specification, guaranteed both of
point 1 and point 2 above. That is to say, for a class to be considered
a record class, then its direct superclass must be java.lang.Record, and
it must have a Record attribute. The implementation already behaves
this way, but the specification should require it.
The getRecordComponents method currently returns an empty array for both
a record class with no components and a non-record class. We thought it
kinda nice to be able to avoid returning null, but with hindsight I
think that it would be better to remove this potential ambiguity. The
getRecordComponents method should only return a non-null value if both
point 1 ( the class is a record class ) and point 2 above are true.
There are many other null returning methods in Class, so this is not
unusual or out of place. The implementation only requires a minor change
to support this ( return null for non-record classes ).
The most significant part of the changes proposed are to the
specification, so I've included that here inline. The proposed changes
tightly couple the pair of methods as part of their specification,
something that will hopefully be cleaner to do (or even unnecessary)
when we have full pattern matching.
/src/java.base/share/classes/java/lang/Class.java
/**
* {@preview Associated with records, a preview feature of the Java
language.
*
* This method is associated with <i>records</i>, a preview
* feature of the Java language. Preview features
* may be removed in a future release, or upgraded to permanent
* features of the Java language.}
*
* Returns {@code true} if and only if this class is a record class.
- * It returns {@code false} otherwise. Note that class {@link Record} is
not a
- * record type and thus invoking this method on class {@link
java.lang.Record}
- * returns {@code false}.
- *
- * @return true if and only if this class is a record class
+ *
+ * <p> The {@linkplain #getSuperclass() direct superclass} of a record
+ * class is {@code java.lang.Record}. A record class has (possibly empty)
+ * record components, that is, {@link #getRecordComponents()} returns a
+ * non-null value.
+ *
+ * <p> Note that class {@link Record} is not a record type and thus
invoking
+ * this method on class {@code Record} returns {@code false}.
+ *
+ * @return true if and only if this class is a record class, otherwise
false
* @jls 8.10 Record Types
* @since 14
*/
public boolean isRecord() { ... }
/**
* {@preview Associated with records, a preview feature of the Java
language.
*
* This method is associated with <i>records</i>, a preview
* feature of the Java language. Preview features
* may be removed in a future release, or upgraded to permanent
* features of the Java language.}
*
- * Returns an array containing {@code RecordComponent} objects reflecting
all the
- * declared record components of the record represented by this {@code
Class} object.
- * The components are returned in the same order that they are declared in
the
- * record header.
- *
- * @return The array of {@code RecordComponent} objects representing all
the
- * record components of this record. The array is empty if this
class
- * is not a record, or if this class is a record with no
components.
+ * Returns an array of {@code RecordComponent} objects representing all the
+ * record components of this record class, or {@code null} if this class is
+ * not a record class.
+ *
+ * <p> The components are returned in the same order that they are declared
+ * in the record header. The array is empty if this record class has no
+ * components. If the class is not a record class, that is {@link
+ * #isRecord()} returns false, then this method returns null. Conversely,
if
+ * {@link #isRecord()} returns true, then this method returns a non-null
+ * value.
+ *
+ * @return An array of {@code RecordComponent} objects representing all
the
+ * record components of this record class, or {@code null} if this
+ * class is not a record class
* @throws SecurityException
* ...
*
* @jls 8.10 Record Types
* @since 14
*/
public RecordComponent[] getRecordComponents() { … }
-Chris.
