Improve performance of foreignKClasses, add comments

core/runtime.jvm/src/kotlin/reflect/jvm/internal/foreignKClasses.kt

@@ -0,0 +1,65 @@
+/*
+ * Copyright 2010-2014 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kotlin.reflect.jvm.internal
+
+import java.lang.ref.WeakReference
+import kotlin.reflect.jvm.internal.pcollections.HashPMap
+
+// TODO: collect nulls periodically
+// Key of the map is Class.getName(), each value is either a WeakReference<KClassImpl<*>> or an Array<WeakReference<KClassImpl<*>>>.
+// Arrays are needed because the same class can be loaded by different class loaders, which results in different Class instances.
+// This variable is not volatile intentionally: we don't care if there's a data race on it and some KClass instances will be lost.
+// We do care however about general performance on read access to it, thus no synchronization is done here whatsoever
+private var FOREIGN_K_CLASSES = HashPMap.empty<String, Any>()!!
+
+// This function is invoked on each reflection access to Java classes, properties, etc. Performance is critical here.
+fun <T> foreignKotlinClass(jClass: Class<T>): KClassImpl<T> {
+    val name = jClass.getName()
+    val cached = FOREIGN_K_CLASSES[name]
+    if (cached is WeakReference<*>) {
+        val kClass = cached.get() as KClassImpl<T>?
+        if (kClass?.jClass == jClass) {
+            return kClass!!
+        }
+    }
+    else if (cached != null) {
+        // If the cached value is not a weak reference, it's an array of weak references
+        cached as Array<WeakReference<KClassImpl<T>>>
+        for (ref in cached) {
+            val kClass = ref.get()
+            if (kClass?.jClass == jClass) {
+                return kClass!!
+            }
+        }
+
+        // This is the most unlikely case: we found a cached array of references of length at least 2 (can't be 1 because
+        // the single element would be cached instead), and none of those classes is the one we're looking for
+        val size = cached.size
+        // Don't use Array constructor because it creates a lambda
+        val newArray = arrayOfNulls<WeakReference<KClassImpl<*>>>(size + 1)
+        // Don't use Arrays.copyOf because it works reflectively
+        System.arraycopy(cached, 0, newArray, 0, size)
+        val newKClass = KClassImpl<T>(jClass)
+        newArray[size] = WeakReference(newKClass)
+        FOREIGN_K_CLASSES = FOREIGN_K_CLASSES.plus(name, newArray)!!
+        return newKClass
+    }
+
+    val newKClass = KClassImpl<T>(jClass)
+    FOREIGN_K_CLASSES = FOREIGN_K_CLASSES.plus(name, WeakReference(newKClass))!!
+    return newKClass
+}

core/runtime.jvm/src/kotlin/reflect/jvm/internal/util.kt

@@ -16,10 +16,7 @@
 
 package kotlin.reflect.jvm.internal
 
-import java.lang.ref.WeakReference
 import java.lang.reflect.Method
-import java.util.Arrays
-import kotlin.reflect.jvm.internal.pcollections.HashPMap
 
 // TODO: use stdlib?
 suppress("PLATFORM_CLASS_MAPPED_TO_KOTLIN")
@@ -48,36 +45,6 @@ private fun Class<*>.getMaybeDeclaredMethod(name: String, vararg parameterTypes:
 }
 
 
-// TODO: collect nulls periodically
-private var FOREIGN_K_CLASSES = HashPMap.empty<String, Array<WeakReference<KClassImpl<*>>>>()!!
-
-fun <T> foreignKotlinClass(jClass: Class<T>): KClassImpl<T> {
-    val name = jClass.getName()
-    val cached = FOREIGN_K_CLASSES[name]
-    if (cached != null) {
-        var i = cached.size - 1
-        while (i >= 0) {
-            val kClass = cached[i].get()
-            if (kClass?.jClass == jClass) {
-                return kClass as KClassImpl<T>
-            }
-            i--
-        }
-
-        val newArray = Arrays.copyOf(cached, cached.size + 1) as Array<WeakReference<KClassImpl<*>>>
-        val newKClass = KClassImpl<T>(jClass)
-        newArray[cached.size] = WeakReference(newKClass)
-        FOREIGN_K_CLASSES = FOREIGN_K_CLASSES.plus(name, newArray)!!
-        return newKClass
-    }
-
-    val newKClass = KClassImpl<T>(jClass)
-    val newArray = arrayOfNulls<WeakReference<KClassImpl<*>>>(1)
-    newArray[0] = WeakReference(newKClass)
-    FOREIGN_K_CLASSES = FOREIGN_K_CLASSES.plus(name, newArray as Array<WeakReference<KClassImpl<*>>>)!!
-    return newKClass
-}
-
 private val K_OBJECT_CLASS = Class.forName("kotlin.jvm.internal.KObject")
 
 fun <T> kotlinClass(jClass: Class<T>): KClassImpl<T> {