Backport cebaad1c94c301304fd146526cac95bfeaac66bf

src/java.base/share/classes/java/io/ClassCache.java

@@ -0,0 +1,116 @@
+/*
+ * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package java.io;
+
+import java.lang.ref.Reference;
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.SoftReference;
+import java.util.Objects;
+
+// Maps Class instances to values of type T. Under memory pressure, the
+// mapping is released (under soft references GC policy) and would be
+// recomputed the next time it is queried. The mapping is bound to the
+// lifetime of the class: when the class is unloaded, the mapping is
+// removed too.
+abstract class ClassCache<T> {
+
+    private static class CacheRef<T> extends SoftReference<T> {
+        private final Class<?> type;
+        private T strongReferent;
+
+        CacheRef(T referent, ReferenceQueue<T> queue, Class<?> type) {
+            super(referent, queue);
+            this.type = type;
+            this.strongReferent = referent;
+        }
+
+        Class<?> getType() {
+            return type;
+        }
+
+        T getStrong() {
+            return strongReferent;
+        }
+
+        void clearStrong() {
+            strongReferent = null;
+        }
+    }
+
+    private final ReferenceQueue<T> queue;
+    private final ClassValue<CacheRef<T>> map;
+
+    protected abstract T computeValue(Class<?> cl);
+
+    protected ClassCache() {
+        queue = new ReferenceQueue<>();
+        map = new ClassValue<>() {
+            @Override
+            protected CacheRef<T> computeValue(Class<?> type) {
+                T v = ClassCache.this.computeValue(type);
+                Objects.requireNonNull(v);
+                return new CacheRef<>(v, queue, type);
+            }
+        };
+    }
+
+    T get(Class<?> cl) {
+        while (true) {
+            processQueue();
+
+            CacheRef<T> ref = map.get(cl);
+
+            // Case 1: A recently created CacheRef.
+            // We might still have strong referent, and can return it.
+            // This guarantees progress for at least one thread on every CacheRef.
+            // Clear the strong referent before returning to make the cache soft.
+            T strongVal = ref.getStrong();
+            if (strongVal != null) {
+                ref.clearStrong();
+                return strongVal;
+            }
+
+            // Case 2: Older or recently cleared CacheRef.
+            // Check if its soft referent is still available, and return it.
+            T val = ref.get();
+            if (val != null) {
+                return val;
+            }
+
+            // Case 3: The reference was cleared.
+            // Clear the mapping and retry.
+            map.remove(cl);
+        }
+    }
+
+    private void processQueue() {
+        Reference<? extends T> ref;
+        while((ref = queue.poll()) != null) {
+            CacheRef<? extends T> cacheRef = (CacheRef<? extends T>)ref;
+            map.remove(cacheRef.getType());
+        }
+    }
+}