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[CLJ-1255] Support Abstract Base Classes with Java-only variant of "reify" Created: 06/Sep/13  Updated: 15/May/17

Status: Open
Project: Clojure
Component/s: None
Affects Version/s: Release 1.6
Fix Version/s: None

Type: Feature Priority: Major
Reporter: Mike Anderson Assignee: Unassigned
Resolution: Unresolved Votes: 10
Labels: interop

Attachments: Text File 0001-CLJ-1225-add-defclass-extend-class.patch    
Approval: Triaged

 Description   

Problem:

  • Various Java APIs depend on extension of abstract base classes rather than interfaces
  • "proxy" has limitations (no access to protected fields or super)
  • "proxy" has performance overhead because of an extra layer of functions / parameter boxing etc.
  • "gen-class" is complex and is complected with compilation / bytecode generation

In summary: Clojure does not currently have a good / convenient way to extend a Java abstract base class dynamically.

The proposal is to create a variant of "reify" that allows the extension of a single abstract base class (optionally also with interfaces/protocols). Code generation would occur as if the abstract base class had been directly extended in Java (i.e. with full access to protected members and with fully type-hinted fields).

Since this is a JVM-only construct, it should not affect the portable extension methods in Clojure (deftype etc.). We propose that it is placed in an separate namespace that could become the home for other JVM-specific interop functionality, e.g. "clojure.java.interop"

Proposed solution: The attached patch proposes an implementation for this feature, providing a new `clojure.interop` namespace, containing the `defclass` and `extend-class` macros.

"(defclass name [fields*] options* super-class super-args specs*)

   Like clojure.core/deftype but can extend a concrete class, override
   and invoke public and protected methods defined in the super class or
   one of its base classes and access/set! public and protected fields of
   those.

   super-args is a (possibly empty) vector of arguments to the superclass
   constructor

   It is possible to invoke a super method while overriding it, by type-hinting
   the parameter `this` as the super class: (. ^SuperClass this method args*)"
"(extend-class options* super-class super-args specs*)

   Like clojure.core/reify but can extend a concrete class, override
   and invoke public and protected methods defined in the super class or
   one of its base classes and access/set! public and protected fields of
   those.

   super-args is a (possibly empty) vector of arguments to the superclass
   constructor

   It is possible to invoke a super method while overriding it, by type-hinting
   the parameter `this` as the super class: (. ^SuperClass this method args*)"

Patch: 0001-CLJ-1225-add-defclass-extend-class.patch



 Comments   
Comment by Alex Miller [ 30/Jun/14 8:18 AM ]

From Rich: we do not want to support abstract classes in a portable construct (reify, deftype). However, this would be considered as a new Java-only construct (extend-class or reify-class). If you could modify the ticket appropriately, will move back to Triaged.

Comment by Nicola Mometto [ 15/May/17 3:19 PM ]

Attached a proposed impl for this feature

Comment by Nicola Mometto [ 15/May/17 3:22 PM ]

More documentation about the proposed impmlementation can be found at https://docs.google.com/document/d/1OcewjSpxmeFRQ3TizcaRRwlV34T8wl4wVED138FHFFE, non squashed commits at https://github.com/clojure/clojure/compare/master...Bronsa:defclass

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