8280041: Retry loop issues in java.io.ClassCache

Co-authored-by: Peter Levart <plevart@openjdk.org>
Reviewed-by: rkennke, rriggs, plevart

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

@@ -28,6 +28,7 @@ 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
@@ -38,19 +39,29 @@ 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<SoftReference<T>> map;
+    private final ClassValue<CacheRef<T>> map;
 
     protected abstract T computeValue(Class<?> cl);
 
@@ -58,23 +69,41 @@ abstract class ClassCache<T> {
         queue = new ReferenceQueue<>();
         map = new ClassValue<>() {
             @Override
-            protected SoftReference<T> computeValue(Class<?> type) {
-                return new CacheRef<>(ClassCache.this.computeValue(type), queue, type);
+            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) {
-        processQueue();
-        T val;
-        do {
-            SoftReference<T> ref = map.get(cl);
-            val = ref.get();
-            if (val == null) {
-                map.remove(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;
             }
-        } while (val == null);
-        return val;
+
+            // 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() {

test/jdk/java/io/ClassCache/ContinuousGCTest.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2022, Red Hat, Inc. 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.
+ *
+ * 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.
+ */
+
+import java.io.NameClassCache;
+
+/**
+ * @test
+ * @bug 8280041
+ * @summary Sanity test for ClassCache under continuous GC
+ * @compile/module=java.base java/io/NameClassCache.java
+ * @run main ContinuousGCTest
+ */
+public class ContinuousGCTest {
+    static final NameClassCache CACHE = new NameClassCache();
+    static final String VALUE = "ClassCache-ContinuousGCTest";
+
+    public static void main(String... args) throws Throwable {
+        for (int c = 0; c < 1000; c++) {
+            test();
+            System.gc();
+        }
+    }
+
+    public static void test() {
+        String cached = CACHE.get(ContinuousGCTest.class);
+        if (!cached.equals(VALUE)) {
+            throw new IllegalStateException("Cache failure, got: " + cached);
+        }
+    }
+}

test/jdk/java/io/ClassCache/NullValueTest.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2022, Red Hat, Inc. 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.
+ *
+ * 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.
+ */
+
+import java.io.NullClassCache;
+
+/**
+ * @test
+ * @bug 8280041
+ * @summary Test that ClassCache throws on trying to pass null value
+ * @compile/module=java.base java/io/NullClassCache.java
+ * @run main NullValueTest
+ */
+public class NullValueTest {
+    public static void main(String... args) throws Throwable {
+        try {
+            new NullClassCache().get(Object.class);
+            throw new IllegalStateException("Should have failed");
+        } catch (NullPointerException npe) {
+            // Expected
+        }
+    }
+}

test/jdk/java/io/ClassCache/java.base/java/io/NameClassCache.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2022, Red Hat, Inc. 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.
+ *
+ * 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;
+
+public class NameClassCache extends ClassCache<String> {
+    protected String computeValue(Class<?> cl) {
+        // Return string that is not interned and specific to class
+        return "ClassCache-" + cl.getName();
+    }
+
+    public String get(Class<?> cl) {
+        return super.get(cl);
+    }
+}

test/jdk/java/io/ClassCache/java.base/java/io/NullClassCache.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2022, Red Hat, Inc. 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.
+ *
+ * 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;
+
+public class NullClassCache extends ClassCache<Object> {
+    protected Object computeValue(Class<?> cl) {
+        return null;
+    }
+
+    public Object get(Class<?> cl) {
+        return super.get(cl);
+    }
+}