Expose a subset of java.lang.ref to enable hooking into the destruction/GC of objects (#1359)

* reify java.lang.Object with optional toString and finalize

* reify java.lang.Runnable

* reflection support for java.lang.Runnable

* java.lang.ref.* class exposure

* test for detection of garbage collection of Object

* added change to changelog

CHANGELOG.md

@@ -5,6 +5,10 @@ For a list of breaking changes, check [here](#breaking-changes).
 A preview of the next release can be installed from
 [babashka-dev-builds](https://github.com/babashka/babashka-dev-builds).
 
+## Unreleased
+
+- [#1358](https://github.com/babashka/babashka/issues/1358): Expose a subset of java.lang.ref to enable hooking into the destruction/GC of objects
+
 ## 0.9.162 (2022-09-04)
 
 Check out our new project: [bbin](https://github.com/babashka/bbin): install any Babashka script or project with one command.

reify/src/babashka/impl/reify2.clj

@@ -61,6 +61,19 @@
       java.lang.Object
       (toString [this] (toString-fn this)))))
 
+(defn reify-runnable [m]
+  (let [methods (:methods m)
+        run-fn (or (get methods 'run)
+                   (fn [_]))]
+    (reify
+      sci.impl.types.IReified
+      (getMethods [_] (:methods m))
+      (getInterfaces [_] (:interfaces m))
+      (getProtocols [_] (:protocols m))
+      java.lang.Runnable
+      (run [this] (run-fn this))))
+  )
+
 (defmacro gen-reify-fn []
   `(fn [~'m]
      (when (> (count (:interfaces ~'m)) 1)
@@ -78,7 +91,9 @@
                     ["clojure.lang.IFn"
                      `(reify-ifn ~'m)
                      "java.lang.Object"
-                     `(reify-object ~'m)]
+                     `(reify-object ~'m)
+                     "java.lang.Runnable"
+                     `(reify-runnable ~'m)]
                     (for [i interfaces]
                       (let [in (.getName ^Class i)]
                         [in

src/babashka/impl/classes.clj

@@ -84,6 +84,8 @@
     java.lang.reflect.Array
     {:methods [{:name "newInstance"}
                {:name "set"}]}
+    java.lang.Runnable
+    {:methods [{:name "run"}]}
     java.net.Inet4Address
     {:methods [{:name "getHostAddress"}]}
     java.net.Inet6Address
@@ -264,6 +266,9 @@
           java.lang.System
           java.lang.Throwable
           ;; java.lang.UnsupportedOperationException
+          java.lang.ref.WeakReference
+          java.lang.ref.ReferenceQueue
+          java.lang.ref.Cleaner
           java.math.BigDecimal
           java.math.BigInteger
           java.math.MathContext

test/babashka/reify_test.clj

@@ -128,3 +128,26 @@
 (reify
   java.lang.Object (toString [_] \"foo\")
   clojure.lang.Seqable (seq [_] '(1 2 3)))")))))
+
+(deftest reify-runnable-and-garbage-collection-test
+  (is (bb nil "
+(def cleaner (java.lang.ref.Cleaner/create))
+(def deleted? (atom false))
+(defn make-cleanable-ref []
+  (let [obj (Object.)]
+    (.register cleaner obj
+      (reify java.lang.Runnable
+        (run [_]
+          (reset! deleted? true))))
+    nil))
+(defn force-gc []
+  (let [t (atom (Object.))
+        wr (java.lang.ref.WeakReference. @t)]
+    (reset! t nil)
+    (while (.get wr)
+      (System/gc)
+      (System/runFinalization))))
+(make-cleanable-ref)
+(force-gc)
+@deleted?
+")))