Clarifying record reflective support
Maurizio Cimadamore
maurizio.cimadamore at oracle.com
Tue Dec 3 21:40:11 UTC 2019
I feel torn here - on the one hand, flash-forward in time and we'll have
a way to transform isRecord/getRecordComponents and
isArray/getComponentType pairs into deconstruction patterns. If we go
down that route, it is actually beneficial if the set of return values
for getRecordComponents and getComponentType is disjoint in the two
cases (record/non-record, array/non-array). Because then you can turn
"returns null" into "does not match" and everything else follows pretty
nicely from that.
That said, it is true that all methods in j.l.Class which return arrays
return 0-length arrays; for instance, getClasses doc says:
> Returns an array containing |Class| objects representing all the
> public classes and interfaces that are members of the class
> represented by this |Class| object. This includes public class and
> interface members inherited from superclasses and public class and
> interface members declared by the class. This method returns an array
> of length 0 if this |Class| object has no public member classes or
> interfaces. This method also returns an array of length 0 if this
> |Class| object represents a primitive type, an array class, or void.
On the other hand, there something off about "getRecordComponents"
because the very name of the method _screams_ that it should only really
be called on record classes. In fact, if we were writing an API form
scratch, it wouldn't be unreasonable to throw
UnsupportedOperationException if this method was called when isRecord()
== false. So, the very biased nature of the "getRecordComponents" method
is what makes me uncomfortable in just saying "return a zero-length
array". If you are calling a method that's only really supposed to be
called on a record on something that's not a record, wouldn't you want
to know about it?
Maurizio
On 03/12/2019 21:10, Remi Forax wrote:
> Hi Chris,
>
> Please do not add another java.io.File.listFiles() !
>
> People will test a code that use Class.getRecordComponents() only with records forgetting as usual to have negative tests and it will blow in production.
>
> A method that returns an array should never return null.
> I repeat
> A method that returns an array should never return null.
>
> If someone want to disambiguate between i've no record component because i'm a record or i've no record component because i'm not a record, isRecord is exactly the kind of method you want to call.
>
> regards,
> Rémi
>
>
> ----- Mail original -----
>> De: "Chris Hegarty" <chris.hegarty at oracle.com>
>> À: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>, "joe darcy" <joe.darcy at oracle.com>, "Maurizio Cimadamore"
>> <maurizio.cimadamore at oracle.com>
>> Envoyé: Mardi 3 Décembre 2019 12:26:53
>> Objet: Clarifying record reflective support
>> 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.
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