RFR: 8264124: Update MXBean specification and implementation to extend mapping of CompositeType to records

[Chris Hegarty]

On Thu, 25 Mar 2021 17:30:52 GMT, Daniel Fuchs <dfuchs at openjdk.org> wrote:

> This RFE proposes to extend the MXBean framework to define a mapping to records.
> 
> The MXBean framework already defines a mapping of `CompositeType` to plain java objects. Records are a natural representation of CompositeTypes. A record can be easily reconstructed from a `CompositeData` through the record canonical constructor. A clear advantage of records over plain java objects is that the canonical constructor will not need to be annotated in order to map composite data property names to constructor parameter names.
> 
> With this RFE, here is an example comparing coding a composite type `NamedNumber` that consists of an `int` and a `String`, using records and using a plain java class. In both case, the `CompositeType` looks like this:
> 
> CompositeType(
>     "NamedNumber",                      // typeName
>     "NamedNumber",                      // description
>     new String[] {"number", "name"},    // itemNames
>     new String[] {"number", "name"},    // itemDescriptions
>     new OpenType[] {SimpleType.INTEGER,
>                     SimpleType.STRING}  // itemTypes
> );
> 
> The plain Java class needs a public constructor annotated with `@ConstructorParameters` annotation:
> 
> public class NamedNumber {
>     public int getNumber() {return number;}
>     public String getName() {return name;}
>     @ConstructorParameters({"number", "name"})
>     public NamedNumber(int number, String name) {
>         this.number = number;
>         this.name = name;
>     }
>     private final int number;
>     private final String name;
> }
> 
> And the equivalent with a record class: 
> 
> public record NamedNumber(int number, String name) {}

Generally, I think that this is a good change. It nice to see that the semantics of record classes can be leverage to build the CompositeType in this way. And equally how that maps back to the canonical constructor.

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PR: https://git.openjdk.java.net/jdk/pull/3201