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src/java.base/share/classes/java/lang/ClassValue.java

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@@ -1,7 +1,7 @@
 /*
- * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 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

@@ -23,16 +23,21 @@
  * questions.
  */
 
 package java.lang;
 
-import java.util.WeakHashMap;
+import jdk.internal.misc.Unsafe;
+import jdk.internal.vm.annotation.ForceInline;
+import jdk.internal.vm.annotation.Stable;
+
+import java.lang.ref.PhantomReference;
+import java.lang.ref.ReferenceQueue;
 import java.lang.ref.WeakReference;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
-
-import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
-import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;
+import java.util.concurrent.atomic.AtomicReference;
 
 /**
  * Lazily associate a computed value with (potentially) every type.
  * For example, if a dynamic language needs to construct a message dispatch
  * table for each class encountered at a message send call site,

@@ -93,27 +98,62 @@
      * @return the current value associated with this {@code ClassValue}, for the given class or interface
      * @throws NullPointerException if the argument is null
      * @see #remove
      * @see #computeValue
      */
+    @SuppressWarnings("unchecked")
+    @ForceInline
     public T get(Class<?> type) {
-        // non-racing this.hashCodeForCache : final int
-        Entry<?>[] cache;
-        Entry<T> e = probeHomeLocation(cache = getCacheCarefully(type), this);
-        // racing e : current value <=> stale value from current cache or from stale cache
-        // invariant:  e is null or an Entry with readable Entry.version and Entry.value
-        if (match(e))
-            // invariant:  No false positive matches.  False negatives are OK if rare.
-            // The key fact that makes this work: if this.version == e.version,
-            // then this thread has a right to observe (final) e.value.
-            return e.value();
-        // The fast path can fail for any of these reasons:
-        // 1. no entry has been computed yet
-        // 2. hash code collision (before or after reduction mod cache.length)
-        // 3. an entry has been removed (either on this type or another)
-        // 4. the GC has somehow managed to delete e.version and clear the reference
-        return getFromBackup(cache, type);
+        ClassValueMap map = type.classValueMap;
+        Object value;
+        if (map != null) {
+            value = map.get(key);
+            if (value != null && !(value instanceof Removed)) {
+                return (value == NULL_MASK) ? null : (T) value;
+            }
+        } else {
+            value = null;
+        }
+        return getSlow(type, value);
+    }
+    // ...the rest in separate method
+    @SuppressWarnings("unchecked")
+    private T getSlow(Class<?> type, Object oldValue) {
+        ClassValueMap.expungeStaleEntries();
+        ClassValueMap map = getMap(type);
+        while (true) {
+            // assert oldValue == null || oldValue instanceof Removed;
+            T newValue = computeValue(type);
+            if (oldValue == null) {
+                oldValue = map.putIfAbsent(key, (newValue == null) ? NULL_MASK : newValue);
+                if (oldValue == null) {
+                    return newValue;
+                } else {
+                    // another thread has beaten us
+                    if (!(oldValue instanceof Removed)) {
+                        return (oldValue == NULL_MASK) ? null : (T) oldValue;
+                    }
+                }
+            } else {
+                // assert oldValue instanceof Removed;
+                if (map.replace(key, oldValue, (newValue == null) ? NULL_MASK : newValue)) {
+                    // this only succeeds if oldValue is still the same instance of
+                    // Removed as observed before computeValue invocation
+                    return newValue;
+                } else {
+                    // another thread has beaten us
+                    oldValue = map.get(key);
+                    if (oldValue != null && !(oldValue instanceof Removed)) {
+                        return (oldValue == NULL_MASK) ? null : (T) oldValue;
+                    }
+                }
+            }
+            // retry compute since we observed another version of Removed
+            // as was current before compute which means that there was
+            // at least one value present in between time which means
+            // that the result of our compute is stale.
+        }
     }
 
     /**
      * Removes the associated value for the given class.
      * If this value is subsequently {@linkplain #get read} for the same class,

@@ -166,592 +206,213 @@
      *
      * @param type the type whose class value must be removed
      * @throws NullPointerException if the argument is null
      */
     public void remove(Class<?> type) {
-        ClassValueMap map = getMap(type);
-        map.removeEntry(this);
+        ClassValueMap.expungeStaleEntries();
+        ClassValueMap map = type.classValueMap;
+        if (map != null) {
+            Object value, removed = null;
+            do {
+                value = map.get(key);
+                if (value == null || value instanceof Removed) {
+                    // non-existence is treated as Removed(V0)
+                    // invoking remove() when state is Removed(VN) does not
+                    // change it to Removed(VN+1)
+                    break;
+                }
+                if (removed == null) {
+                    removed = new Removed();
+                }
+            } while (!map.replace(key, value, removed));
+        }
     }
 
     // Possible functionality for JSR 292 MR 1
     /*public*/ void put(Class<?> type, T value) {
+        ClassValueMap.expungeStaleEntries();
         ClassValueMap map = getMap(type);
-        map.changeEntry(this, value);
+        map.put(key, (value == null) ? NULL_MASK : value);
     }
 
     /// --------
     /// Implementation...
     /// --------
 
-    /** Return the cache, if it exists, else a dummy empty cache. */
-    private static Entry<?>[] getCacheCarefully(Class<?> type) {
-        // racing type.classValueMap{.cacheArray} : null => new Entry[X] <=> new Entry[Y]
-        ClassValueMap map = type.classValueMap;
-        if (map == null)  return EMPTY_CACHE;
-        Entry<?>[] cache = map.getCache();
-        return cache;
-        // invariant:  returned value is safe to dereference and check for an Entry
-    }
-
-    /** Initial, one-element, empty cache used by all Class instances.  Must never be filled. */
-    private static final Entry<?>[] EMPTY_CACHE = { null };
-
     /**
-     * Slow tail of ClassValue.get to retry at nearby locations in the cache,
-     * or take a slow lock and check the hash table.
-     * Called only if the first probe was empty or a collision.
-     * This is a separate method, so compilers can process it independently.
+     * The key used in ClassValueMap for this ClassValue.
+     * Since ClassValue(s) are usually assigned to static final fields, it makes
+     * sense to promote the key to constant too.
      */
-    private T getFromBackup(Entry<?>[] cache, Class<?> type) {
-        Entry<T> e = probeBackupLocations(cache, this);
-        if (e != null)
-            return e.value();
-        return getFromHashMap(type);
-    }
+    @Stable
+    private final Key key = new Key(this);
 
-    // Hack to suppress warnings on the (T) cast, which is a no-op.
-    @SuppressWarnings("unchecked")
-    Entry<T> castEntry(Entry<?> e) { return (Entry<T>) e; }
-
-    /** Called when the fast path of get fails, and cache reprobe also fails.
+    /**
+     * Key of a ClassValue and a tracker of the ClassValue at the same time.
      */
-    private T getFromHashMap(Class<?> type) {
-        // The fail-safe recovery is to fall back to the underlying classValueMap.
-        ClassValueMap map = getMap(type);
-        for (;;) {
-            Entry<T> e = map.startEntry(this);
-            if (!e.isPromise())
-                return e.value();
-            try {
-                // Try to make a real entry for the promised version.
-                e = makeEntry(e.version(), computeValue(type));
-            } finally {
-                // Whether computeValue throws or returns normally,
-                // be sure to remove the empty entry.
-                e = map.finishEntry(this, e);
-            }
-            if (e != null)
-                return e.value();
-            // else try again, in case a racing thread called remove (so e == null)
-        }
-    }
-
-    /** Check that e is non-null, matches this ClassValue, and is live. */
-    boolean match(Entry<?> e) {
-        // racing e.version : null (blank) => unique Version token => null (GC-ed version)
-        // non-racing this.version : v1 => v2 => ... (updates are read faithfully from volatile)
-        return (e != null && e.get() == this.version);
-        // invariant:  No false positives on version match.  Null is OK for false negative.
-        // invariant:  If version matches, then e.value is readable (final set in Entry.<init>)
-    }
-
-    /** Internal hash code for accessing Class.classValueMap.cacheArray. */
-    final int hashCodeForCache = nextHashCode.getAndAdd(HASH_INCREMENT) & HASH_MASK;
+    private static final class Key extends PhantomReference<ClassValue<?>> {
 
-    /** Value stream for hashCodeForCache.  See similar structure in ThreadLocal. */
-    private static final AtomicInteger nextHashCode = new AtomicInteger();
+        /** Value stream for hash. See similar structure in ThreadLocal. */
+        private static final AtomicInteger nextHash = new AtomicInteger();
 
     /** Good for power-of-two tables.  See similar structure in ThreadLocal. */
     private static final int HASH_INCREMENT = 0x61c88647;
 
-    /** Mask a hash code to be positive but not too large, to prevent wraparound. */
-    static final int HASH_MASK = (-1 >>> 2);
+        /** Single global queue for all stale keys to be enqueued */
+        static final ReferenceQueue<ClassValue<?>> queue = new ReferenceQueue<>();
 
-    /**
-     * Private key for retrieval of this object from ClassValueMap.
-     */
-    static class Identity {
-    }
-    /**
-     * This ClassValue's identity, expressed as an opaque object.
-     * The main object {@code ClassValue.this} is incorrect since
-     * subclasses may override {@code ClassValue.equals}, which
-     * could confuse keys in the ClassValueMap.
-     */
-    final Identity identity = new Identity();
+        @Stable
+        private final int hash;
 
-    /**
-     * Current version for retrieving this class value from the cache.
-     * Any number of computeValue calls can be cached in association with one version.
-     * But the version changes when a remove (on any type) is executed.
-     * A version change invalidates all cache entries for the affected ClassValue,
-     * by marking them as stale.  Stale cache entries do not force another call
-     * to computeValue, but they do require a synchronized visit to a backing map.
-     * <p>
-     * All user-visible state changes on the ClassValue take place under
-     * a lock inside the synchronized methods of ClassValueMap.
-     * Readers (of ClassValue.get) are notified of such state changes
-     * when this.version is bumped to a new token.
-     * This variable must be volatile so that an unsynchronized reader
-     * will receive the notification without delay.
-     * <p>
-     * If version were not volatile, one thread T1 could persistently hold onto
-     * a stale value this.value == V1, while another thread T2 advances
-     * (under a lock) to this.value == V2.  This will typically be harmless,
-     * but if T1 and T2 interact causally via some other channel, such that
-     * T1's further actions are constrained (in the JMM) to happen after
-     * the V2 event, then T1's observation of V1 will be an error.
-     * <p>
-     * The practical effect of making this.version be volatile is that it cannot
-     * be hoisted out of a loop (by an optimizing JIT) or otherwise cached.
-     * Some machines may also require a barrier instruction to execute
-     * before this.version.
-     */
-    private volatile Version<T> version = new Version<>(this);
-    Version<T> version() { return version; }
-    void bumpVersion() { version = new Version<>(this); }
-    static class Version<T> {
-        private final ClassValue<T> classValue;
-        private final Entry<T> promise = new Entry<>(this);
-        Version(ClassValue<T> classValue) { this.classValue = classValue; }
-        ClassValue<T> classValue() { return classValue; }
-        Entry<T> promise() { return promise; }
-        boolean isLive() { return classValue.version() == this; }
-    }
-
-    /** One binding of a value to a class via a ClassValue.
-     *  States are:<ul>
-     *  <li> promise if value == Entry.this
-     *  <li> else dead if version == null
-     *  <li> else stale if version != classValue.version
-     *  <li> else live </ul>
-     *  Promises are never put into the cache; they only live in the
-     *  backing map while a computeValue call is in flight.
-     *  Once an entry goes stale, it can be reset at any time
-     *  into the dead state.
-     */
-    static class Entry<T> extends WeakReference<Version<T>> {
-        final Object value;  // usually of type T, but sometimes (Entry)this
-        Entry(Version<T> version, T value) {
-            super(version);
-            this.value = value;  // for a regular entry, value is of type T
-        }
-        private void assertNotPromise() { assert(!isPromise()); }
-        /** For creating a promise. */
-        Entry(Version<T> version) {
-            super(version);
-            this.value = this;  // for a promise, value is not of type T, but Entry!
-        }
-        /** Fetch the value.  This entry must not be a promise. */
-        @SuppressWarnings("unchecked")  // if !isPromise, type is T
-        T value() { assertNotPromise(); return (T) value; }
-        boolean isPromise() { return value == this; }
-        Version<T> version() { return get(); }
-        ClassValue<T> classValueOrNull() {
-            Version<T> v = version();
-            return (v == null) ? null : v.classValue();
-        }
-        boolean isLive() {
-            Version<T> v = version();
-            if (v == null)  return false;
-            if (v.isLive())  return true;
-            clear();
-            return false;
-        }
-        Entry<T> refreshVersion(Version<T> v2) {
-            assertNotPromise();
-            @SuppressWarnings("unchecked")  // if !isPromise, type is T
-            Entry<T> e2 = new Entry<>(v2, (T) value);
-            clear();
-            // value = null -- caller must drop
-            return e2;
-        }
-        static final Entry<?> DEAD_ENTRY = new Entry<>(null, null);
+        Key(ClassValue<?> cv) {
+            super(cv, queue);
+            int h = nextHash.getAndAdd(HASH_INCREMENT);
+            // undo the effect of CHM.spread()
+            this.hash = h ^ (h >>> 16);
     }
 
-    /** Return the backing map associated with this type. */
-    private static ClassValueMap getMap(Class<?> type) {
-        // racing type.classValueMap : null (blank) => unique ClassValueMap
-        // if a null is observed, a map is created (lazily, synchronously, uniquely)
-        // all further access to that map is synchronized
-        ClassValueMap map = type.classValueMap;
-        if (map != null)  return map;
-        return initializeMap(type);
+        @Override
+        public int hashCode() {
+            return hash;
     }
 
-    private static final Object CRITICAL_SECTION = new Object();
-    private static ClassValueMap initializeMap(Class<?> type) {
-        ClassValueMap map;
-        synchronized (CRITICAL_SECTION) {  // private object to avoid deadlocks
-            // happens about once per type
-            if ((map = type.classValueMap) == null)
-                type.classValueMap = map = new ClassValueMap();
-        }
-        return map;
+        // equals is identity
     }
 
-    static <T> Entry<T> makeEntry(Version<T> explicitVersion, T value) {
-        // Note that explicitVersion might be different from this.version.
-        return new Entry<>(explicitVersion, value);
-
-        // As soon as the Entry is put into the cache, the value will be
-        // reachable via a data race (as defined by the Java Memory Model).
-        // This race is benign, assuming the value object itself can be
-        // read safely by multiple threads.  This is up to the user.
-        //
-        // The entry and version fields themselves can be safely read via
-        // a race because they are either final or have controlled states.
-        // If the pointer from the entry to the version is still null,
-        // or if the version goes immediately dead and is nulled out,
-        // the reader will take the slow path and retry under a lock.
-    }
-
-    // The following class could also be top level and non-public:
-
-    /** A backing map for all ClassValues.
-     *  Gives a fully serialized "true state" for each pair (ClassValue cv, Class type).
-     *  Also manages an unserialized fast-path cache.
-     */
-    static class ClassValueMap extends WeakHashMap<ClassValue.Identity, Entry<?>> {
-        private Entry<?>[] cacheArray;
-        private int cacheLoad, cacheLoadLimit;
-
-        /** Number of entries initially allocated to each type when first used with any ClassValue.
-         *  It would be pointless to make this much smaller than the Class and ClassValueMap objects themselves.
-         *  Must be a power of 2.
-         */
-        private static final int INITIAL_ENTRIES = 32;
+    /** A masked null value. */
+    private static final Object NULL_MASK = new Object();
 
-        /** Build a backing map for ClassValues.
-         *  Also, create an empty cache array and install it on the class.
-         */
-        ClassValueMap() {
-            sizeCache(INITIAL_ENTRIES);
-        }
+    /** Removed value(s) */
+    private static final class Removed {}
 
-        Entry<?>[] getCache() { return cacheArray; }
-
-        /** Initiate a query.  Store a promise (placeholder) if there is no value yet. */
-        synchronized
-        <T> Entry<T> startEntry(ClassValue<T> classValue) {
-            @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
-            Entry<T> e = (Entry<T>) get(classValue.identity);
-            Version<T> v = classValue.version();
-            if (e == null) {
-                e = v.promise();
-                // The presence of a promise means that a value is pending for v.
-                // Eventually, finishEntry will overwrite the promise.
-                put(classValue.identity, e);
-                // Note that the promise is never entered into the cache!
-                return e;
-            } else if (e.isPromise()) {
-                // Somebody else has asked the same question.
-                // Let the races begin!
-                if (e.version() != v) {
-                    e = v.promise();
-                    put(classValue.identity, e);
-                }
-                return e;
-            } else {
-                // there is already a completed entry here; report it
-                if (e.version() != v) {
-                    // There is a stale but valid entry here; make it fresh again.
-                    // Once an entry is in the hash table, we don't care what its version is.
-                    e = e.refreshVersion(v);
-                    put(classValue.identity, e);
-                }
-                // Add to the cache, to enable the fast path, next time.
-                checkCacheLoad();
-                addToCache(classValue, e);
-                return e;
-            }
+    /** Return the backing map associated with this type, possibly constructing it. */
+    private static ClassValueMap getMap(Class<?> type) {
+        ClassValueMap map = type.classValueMap;
+        return (map != null) ? map : initializeMap(type);
         }
 
-        /** Finish a query.  Overwrite a matching placeholder.  Drop stale incoming values. */
-        synchronized
-        <T> Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) {
-            @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
-            Entry<T> e0 = (Entry<T>) get(classValue.identity);
-            if (e == e0) {
-                // We can get here during exception processing, unwinding from computeValue.
-                assert(e.isPromise());
-                remove(classValue.identity);
-                return null;
-            } else if (e0 != null && e0.isPromise() && e0.version() == e.version()) {
-                // If e0 matches the intended entry, there has not been a remove call
-                // between the previous startEntry and now.  So now overwrite e0.
-                Version<T> v = classValue.version();
-                if (e.version() != v)
-                    e = e.refreshVersion(v);
-                put(classValue.identity, e);
-                // Add to the cache, to enable the fast path, next time.
-                checkCacheLoad();
-                addToCache(classValue, e);
-                return e;
-            } else {
-                // Some sort of mismatch; caller must try again.
-                return null;
-            }
+    private static ClassValueMap initializeMap(Class<?> type) {
+        ClassValueMap map = ClassValueMap.create();
+        return U.compareAndSwapObject(type, CLASS_VALUE_MAP_OFFSET, null, map)
+               ? map
+               : (ClassValueMap) U.getObjectVolatile(type, CLASS_VALUE_MAP_OFFSET);
         }
 
-        /** Remove an entry. */
-        synchronized
-        void removeEntry(ClassValue<?> classValue) {
-            Entry<?> e = remove(classValue.identity);
-            if (e == null) {
-                // Uninitialized, and no pending calls to computeValue.  No change.
-            } else if (e.isPromise()) {
-                // State is uninitialized, with a pending call to finishEntry.
-                // Since remove is a no-op in such a state, keep the promise
-                // by putting it back into the map.
-                put(classValue.identity, e);
-            } else {
-                // In an initialized state.  Bump forward, and de-initialize.
-                classValue.bumpVersion();
-                // Make all cache elements for this guy go stale.
-                removeStaleEntries(classValue);
-            }
-        }
+    /** A backing map for all ClassValues in a particular Class.
+     */
+    static final class ClassValueMap extends ConcurrentHashMap<Key, Object> {
+        // eh, needed to silent the gods
+        private static final long serialVersionUID = 1L;
 
-        /** Change the value for an entry. */
-        synchronized
-        <T> void changeEntry(ClassValue<T> classValue, T value) {
-            @SuppressWarnings("unchecked")  // one map has entries for all value types <T>
-            Entry<T> e0 = (Entry<T>) get(classValue.identity);
-            Version<T> version = classValue.version();
-            if (e0 != null) {
-                if (e0.version() == version && e0.value() == value)
-                    // no value change => no version change needed
-                    return;
-                classValue.bumpVersion();
-                removeStaleEntries(classValue);
-            }
-            Entry<T> e = makeEntry(version, value);
-            put(classValue.identity, e);
-            // Add to the cache, to enable the fast path, next time.
-            checkCacheLoad();
-            addToCache(classValue, e);
+        // a node in a linked list of WeakReference(s) to ClassValueMap(s)
+        private static final class WeakNode extends WeakReference<ClassValueMap> {
+            WeakNode next;
+            WeakNode(ClassValueMap map) { super(map); }
         }
 
-        /// --------
-        /// Cache management.
-        /// --------
-
-        // Statics do not need synchronization.
+        // the head of the linked list of WeakReference(s) to ClassValueMap(s)
+        private static final AtomicReference<WeakNode> mapsList = new AtomicReference<>();
 
-        /** Load the cache entry at the given (hashed) location. */
-        static Entry<?> loadFromCache(Entry<?>[] cache, int i) {
-            // non-racing cache.length : constant
-            // racing cache[i & (mask)] : null <=> Entry
-            return cache[i & (cache.length-1)];
-            // invariant:  returned value is null or well-constructed (ready to match)
-        }
-
-        /** Look in the cache, at the home location for the given ClassValue. */
-        static <T> Entry<T> probeHomeLocation(Entry<?>[] cache, ClassValue<T> classValue) {
-            return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache));
-        }
-
-        /** Given that first probe was a collision, retry at nearby locations. */
-        static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classValue) {
-            if (PROBE_LIMIT <= 0)  return null;
-            // Probe the cache carefully, in a range of slots.
-            int mask = (cache.length-1);
-            int home = (classValue.hashCodeForCache & mask);
-            Entry<?> e2 = cache[home];  // victim, if we find the real guy
-            if (e2 == null) {
-                return null;   // if nobody is at home, no need to search nearby
-            }
-            // assume !classValue.match(e2), but do not assert, because of races
-            int pos2 = -1;
-            for (int i = home + 1; i < home + PROBE_LIMIT; i++) {
-                Entry<?> e = cache[i & mask];
-                if (e == null) {
-                    break;   // only search within non-null runs
-                }
-                if (classValue.match(e)) {
-                    // relocate colliding entry e2 (from cache[home]) to first empty slot
-                    cache[home] = e;
-                    if (pos2 >= 0) {
-                        cache[i & mask] = Entry.DEAD_ENTRY;
-                    } else {
-                        pos2 = i;
-                    }
-                    cache[pos2 & mask] = ((entryDislocation(cache, pos2, e2) < PROBE_LIMIT)
-                                          ? e2                  // put e2 here if it fits
-                                          : Entry.DEAD_ENTRY);
-                    return classValue.castEntry(e);
-                }
-                // Remember first empty slot, if any:
-                if (!e.isLive() && pos2 < 0)  pos2 = i;
-            }
-            return null;
-        }
+        // the lock used to guard expunging logic
+        private static final AtomicBoolean expungeInProgress = new AtomicBoolean();
 
-        /** How far out of place is e? */
-        private static int entryDislocation(Entry<?>[] cache, int pos, Entry<?> e) {
-            ClassValue<?> cv = e.classValueOrNull();
-            if (cv == null)  return 0;  // entry is not live!
-            int mask = (cache.length-1);
-            return (pos - cv.hashCodeForCache) & mask;
+        private ClassValueMap() {
+            super(1);
         }
 
-        /// --------
-        /// Below this line all functions are private, and assume synchronized access.
-        /// --------
-
-        private void sizeCache(int length) {
-            assert((length & (length-1)) == 0);  // must be power of 2
-            cacheLoad = 0;
-            cacheLoadLimit = (int) ((double) length * CACHE_LOAD_LIMIT / 100);
-            cacheArray = new Entry<?>[length];
+        static ClassValueMap create() {
+            ClassValueMap map = new ClassValueMap();
+            WeakNode node = new WeakNode(map);
+            // register the weakly referenced map into the list
+            do {
+                node.next = mapsList.get();
+            } while (!mapsList.compareAndSet(node.next, node));
+            return map;
         }
 
-        /** Make sure the cache load stays below its limit, if possible. */
-        private void checkCacheLoad() {
-            if (cacheLoad >= cacheLoadLimit) {
-                reduceCacheLoad();
-            }
-        }
-        private void reduceCacheLoad() {
-            removeStaleEntries();
-            if (cacheLoad < cacheLoadLimit)
-                return;  // win
-            Entry<?>[] oldCache = getCache();
-            if (oldCache.length > HASH_MASK)
-                return;  // lose
-            sizeCache(oldCache.length * 2);
-            for (Entry<?> e : oldCache) {
-                if (e != null && e.isLive()) {
-                    addToCache(e);
-                }
+        static void expungeStaleEntries() {
+            // make sure only one thread at a time is expunging stale entries;
+            // this is important for correctness so that no stale enqueued key
+            // is missed from being removed from any map that might possibly
+            // hold it.
+            if (!expungeInProgress.get() &&
+                expungeInProgress.compareAndSet(false, true)) try {
+                expungeStaleEntriesImpl();
+            } finally {
+                expungeInProgress.set(false);
             }
         }
 
-        /** Remove stale entries in the given range.
-         *  Should be executed under a Map lock.
-         */
-        private void removeStaleEntries(Entry<?>[] cache, int begin, int count) {
-            if (PROBE_LIMIT <= 0)  return;
-            int mask = (cache.length-1);
-            int removed = 0;
-            for (int i = begin; i < begin + count; i++) {
-                Entry<?> e = cache[i & mask];
-                if (e == null || e.isLive())
-                    continue;  // skip null and live entries
-                Entry<?> replacement = null;
-                if (PROBE_LIMIT > 1) {
-                    // avoid breaking up a non-null run
-                    replacement = findReplacement(cache, i);
-                }
-                cache[i & mask] = replacement;
-                if (replacement == null)  removed += 1;
-            }
-            cacheLoad = Math.max(0, cacheLoad - removed);
-        }
-
-        /** Clearing a cache slot risks disconnecting following entries
-         *  from the head of a non-null run, which would allow them
-         *  to be found via reprobes.  Find an entry after cache[begin]
-         *  to plug into the hole, or return null if none is needed.
-         */
-        private Entry<?> findReplacement(Entry<?>[] cache, int home1) {
-            Entry<?> replacement = null;
-            int haveReplacement = -1, replacementPos = 0;
-            int mask = (cache.length-1);
-            for (int i2 = home1 + 1; i2 < home1 + PROBE_LIMIT; i2++) {
-                Entry<?> e2 = cache[i2 & mask];
-                if (e2 == null)  break;  // End of non-null run.
-                if (!e2.isLive())  continue;  // Doomed anyway.
-                int dis2 = entryDislocation(cache, i2, e2);
-                if (dis2 == 0)  continue;  // e2 already optimally placed
-                int home2 = i2 - dis2;
-                if (home2 <= home1) {
-                    // e2 can replace entry at cache[home1]
-                    if (home2 == home1) {
-                        // Put e2 exactly where he belongs.
-                        haveReplacement = 1;
-                        replacementPos = i2;
-                        replacement = e2;
-                    } else if (haveReplacement <= 0) {
-                        haveReplacement = 0;
-                        replacementPos = i2;
-                        replacement = e2;
-                    }
-                    // And keep going, so we can favor larger dislocations.
-                }
-            }
-            if (haveReplacement >= 0) {
-                if (cache[(replacementPos+1) & mask] != null) {
-                    // Be conservative, to avoid breaking up a non-null run.
-                    cache[replacementPos & mask] = (Entry<?>) Entry.DEAD_ENTRY;
+        private static void expungeStaleEntriesImpl() {
+            // start with an empty private list
+            WeakNode first = null, last = null;
+            // iterate Key(s) of GCed ClassValue(s)
+            Key key;
+            while ((key = (Key) Key.queue.poll()) != null) {
+                // steal any (newly) registered map(s) from the shared 'mapsList'
+                // as they might already be holding the stale 'key' that has just
+                // been removed from the queue. The following guarantees that
+                // no (stale-key, live-map) combination is missed:
+                // - a map is 1st registered into the mapsList and then non-stale
+                //   keys are inserted into it
+                // - a non-stale key becomes stale when a ClassValue that it
+                //   phantomly references is GCed; the key is then enqueued
+                // - a stale key is 1st removed from the queue and then mapsList
+                //   is checked for any (newly) registered map(s)
+                if (mapsList.get() != null) {
+                    WeakNode stolen = mapsList.getAndSet(null);
+                    // add them to the end of current private list
+                    if (last != null) {
+                        last.next = stolen;
                 } else {
-                    cache[replacementPos & mask] = null;
-                    cacheLoad -= 1;
-                }
+                        // assert first == null;
+                        first = stolen;
             }
-            return replacement;
         }
-
-        /** Remove stale entries in the range near classValue. */
-        private void removeStaleEntries(ClassValue<?> classValue) {
-            removeStaleEntries(getCache(), classValue.hashCodeForCache, PROBE_LIMIT);
+                // iterate the accumulated private list of maps
+                last = null;
+                for (WeakNode node = first; node != null; ) {
+                    ClassValueMap map = node.get();
+                    if (map == null) {
+                        // Class and its ClassValueMap have already been GCed
+                        // -> remove the node from the list
+                        if (last == null) {
+                            first = node = node.next;
+                        } else {
+                            last.next = node = node.next;
         }
-
-        /** Remove all stale entries, everywhere. */
-        private void removeStaleEntries() {
-            Entry<?>[] cache = getCache();
-            removeStaleEntries(cache, 0, cache.length + PROBE_LIMIT - 1);
+                    } else {
+                        // ClassValueMap is still alive
+                        // -> attempt to remove the key from it
+                        map.remove(key);
+                        // navigate to next in list
+                        last = node;
+                        node = node.next;
         }
-
-        /** Add the given entry to the cache, in its home location, unless it is out of date. */
-        private <T> void addToCache(Entry<T> e) {
-            ClassValue<T> classValue = e.classValueOrNull();
-            if (classValue != null)
-                addToCache(classValue, e);
         }
-
-        /** Add the given entry to the cache, in its home location. */
-        private <T> void addToCache(ClassValue<T> classValue, Entry<T> e) {
-            if (PROBE_LIMIT <= 0)  return;  // do not fill cache
-            // Add e to the cache.
-            Entry<?>[] cache = getCache();
-            int mask = (cache.length-1);
-            int home = classValue.hashCodeForCache & mask;
-            Entry<?> e2 = placeInCache(cache, home, e, false);
-            if (e2 == null)  return;  // done
-            if (PROBE_LIMIT > 1) {
-                // try to move e2 somewhere else in his probe range
-                int dis2 = entryDislocation(cache, home, e2);
-                int home2 = home - dis2;
-                for (int i2 = home2; i2 < home2 + PROBE_LIMIT; i2++) {
-                    if (placeInCache(cache, i2 & mask, e2, true) == null) {
-                        return;
+                // at this time, 'last' points to the last WeakNode in the private
+                // list (if any) and the following always holds:
+                // assert (first == null) == (last == null);
                     }
+            // re-insert the stolen and expunged list of map(s) back into shared
+            // mapsList
+            if (last != null) {
+                do {
+                    last.next = mapsList.get();
+                } while (!mapsList.compareAndSet(last.next, first));
                 }
             }
-            // Note:  At this point, e2 is just dropped from the cache.
         }
 
-        /** Store the given entry.  Update cacheLoad, and return any live victim.
-         *  'Gently' means return self rather than dislocating a live victim.
-         */
-        private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) {
-            Entry<?> e2 = overwrittenEntry(cache[pos]);
-            if (gently && e2 != null) {
-                // do not overwrite a live entry
-                return e;
-            } else {
-                cache[pos] = e;
-                return e2;
-            }
-        }
+    /** We need to cas */
+    private static final Unsafe U = Unsafe.getUnsafe();
+    private static final long CLASS_VALUE_MAP_OFFSET;
 
-        /** Note an entry that is about to be overwritten.
-         *  If it is not live, quietly replace it by null.
-         *  If it is an actual null, increment cacheLoad,
-         *  because the caller is going to store something
-         *  in its place.
-         */
-        private <T> Entry<T> overwrittenEntry(Entry<T> e2) {
-            if (e2 == null)  cacheLoad += 1;
-            else if (e2.isLive())  return e2;
-            return null;
+    static {
+        try {
+            CLASS_VALUE_MAP_OFFSET = U.objectFieldOffset(
+                Class.class.getDeclaredField("classValueMap"));
+        } catch (Exception e) {
+            throw new Error(e);
         }
-
-        /** Percent loading of cache before resize. */
-        private static final int CACHE_LOAD_LIMIT = 67;  // 0..100
-        /** Maximum number of probes to attempt. */
-        private static final int PROBE_LIMIT      =  6;       // 1..
-        // N.B.  Set PROBE_LIMIT=0 to disable all fast paths.
     }
 }
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