diff -r 2c33418a6d57 -r 83b611b88ac8 jdk/src/java.base/share/classes/java/lang/WeakPairMap.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/java.base/share/classes/java/lang/WeakPairMap.java	Fri Apr 07 08:05:54 2017 +0000
@@ -0,0 +1,354 @@
+/*
+ * 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
+ * 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.lang;
+
+import java.lang.ref.Reference;
+import java.lang.ref.ReferenceQueue;
+import java.lang.ref.WeakReference;
+import java.util.Collection;
+import java.util.Objects;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.BiFunction;
+
+/**
+ * A WeakHashMap-like data structure that uses a pair of weakly-referenced keys
+ * with identity equality semantics to associate a strongly-referenced value.
+ * Unlike WeakHashMap, this data structure is thread-safe.
+ *
+ * @param <K1> the type of 1st key in key pair
+ * @param <K2> the type of 2nd key in key pair
+ * @param <V>  the type of value
+ * @author Peter Levart
+ */
+final class WeakPairMap<K1, K2, V> {
+
+    private final ConcurrentHashMap<Pair<K1, K2>, V> map = new ConcurrentHashMap<>();
+    private final ReferenceQueue<Object> queue = new ReferenceQueue<>();
+
+    /**
+     * Tests if the specified pair of keys are associated with a value
+     * in the WeakPairMap.
+     *
+     * @param k1 the 1st of the pair of keys
+     * @param k2 the 2nd of the pair of keys
+     * @return true if and only if the specified key pair is in this WeakPairMap,
+     * as determined by the identity comparison; false otherwise
+     * @throws NullPointerException if any of the specified keys is null
+     */
+    public boolean containsKeyPair(K1 k1, K2 k2) {
+        expungeStaleAssociations();
+        return map.containsKey(Pair.lookup(k1, k2));
+    }
+
+    /**
+     * Returns the value to which the specified pair of keys is mapped, or null
+     * if this WeakPairMap contains no mapping for the key pair.
+     * <p>More formally, if this WeakPairMap contains a mapping from a key pair
+     * {@code (_k1, _k2)} to a value {@code v} such that
+     * {@code k1 == _k1 && k2 == _k2}, then this method returns {@code v};
+     * otherwise it returns {@code null}.
+     * (There can be at most one such mapping.)
+     *
+     * @param k1 the 1st of the pair of keys for which the mapped value is to
+     *           be returned
+     * @param k2 the 2nd of the pair of keys for which the mapped value is to
+     *           be returned
+     * @return the value to which the specified key pair is mapped, or null if
+     * this map contains no mapping for the key pair
+     * @throws NullPointerException if any of the specified keys is null
+     */
+    public V get(K1 k1, K2 k2) {
+        expungeStaleAssociations();
+        return map.get(Pair.lookup(k1, k2));
+    }
+
+    /**
+     * Maps the specified key pair to the specified value in this WeakPairMap.
+     * Neither the keys nor the value can be null.
+     * <p>The value can be retrieved by calling the {@link #get} method
+     * with the the same keys (compared by identity).
+     *
+     * @param k1 the 1st of the pair of keys with which the specified value is to
+     *           be associated
+     * @param k2 the 2nd of the pair of keys with which the specified value is to
+     *           be associated
+     * @param v  value to be associated with the specified key pair
+     * @return the previous value associated with key pair, or {@code null} if
+     * there was no mapping for key pair
+     * @throws NullPointerException if any of the specified keys or value is null
+     */
+    public V put(K1 k1, K2 k2, V v) {
+        expungeStaleAssociations();
+        return map.put(Pair.weak(k1, k2, queue), v);
+    }
+
+    /**
+     * If the specified key pair is not already associated with a value,
+     * associates it with the given value and returns {@code null}, else does
+     * nothing and returns the currently associated value.
+     *
+     * @param k1 the 1st of the pair of keys with which the specified value is to
+     *           be associated
+     * @param k2 the 2nd of the pair of keys with which the specified value is to
+     *           be associated
+     * @param v  value to be associated with the specified key pair
+     * @return the previous value associated with key pair, or {@code null} if
+     * there was no mapping for key pair
+     * @throws NullPointerException if any of the specified keys or value is null
+     */
+    public V putIfAbsent(K1 k1, K2 k2, V v) {
+        expungeStaleAssociations();
+        return map.putIfAbsent(Pair.weak(k1, k2, queue), v);
+    }
+
+    /**
+     * If the specified key pair is not already associated with a value,
+     * attempts to compute its value using the given mapping function
+     * and enters it into this WeakPairMap unless {@code null}. The entire
+     * method invocation is performed atomically, so the function is
+     * applied at most once per key pair. Some attempted update operations
+     * on this WeakPairMap by other threads may be blocked while computation
+     * is in progress, so the computation should be short and simple,
+     * and must not attempt to update any other mappings of this WeakPairMap.
+     *
+     * @param k1              the 1st of the pair of keys with which the
+     *                        computed value is to be associated
+     * @param k2              the 2nd of the pair of keys with which the
+     *                        computed value is to be associated
+     * @param mappingFunction the function to compute a value
+     * @return the current (existing or computed) value associated with
+     * the specified key pair, or null if the computed value is null
+     * @throws NullPointerException  if any of the specified keys or
+     *                               mappingFunction is null
+     * @throws IllegalStateException if the computation detectably
+     *                               attempts a recursive update to this map
+     *                               that would otherwise never complete
+     * @throws RuntimeException      or Error if the mappingFunction does so, in
+     *                               which case the mapping is left unestablished
+     */
+    public V computeIfAbsent(K1 k1, K2 k2,
+                             BiFunction<? super K1, ? super K2, ? extends V>
+                                 mappingFunction) {
+        expungeStaleAssociations();
+        try {
+            return map.computeIfAbsent(
+                Pair.weak(k1, k2, queue),
+                pair -> mappingFunction.apply(pair.first(), pair.second()));
+        } finally {
+            Reference.reachabilityFence(k1);
+            Reference.reachabilityFence(k2);
+        }
+    }
+
+    /**
+     * Returns a {@link Collection} view of the values contained in this
+     * WeakPairMap. The collection is backed by the WeakPairMap, so changes to
+     * the map are reflected in the collection, and vice-versa.  The collection
+     * supports element removal, which removes the corresponding
+     * mapping from this map, via the {@code Iterator.remove},
+     * {@code Collection.remove}, {@code removeAll},
+     * {@code retainAll}, and {@code clear} operations.  It does not
+     * support the {@code add} or {@code addAll} operations.
+     *
+     * @return the collection view
+     */
+    public Collection<V> values() {
+        expungeStaleAssociations();
+        return map.values();
+    }
+
+    /**
+     * Removes associations from this WeakPairMap for which at least one of the
+     * keys in key pair has been found weakly-reachable and corresponding
+     * WeakRefPeer(s) enqueued. Called as part of each public operation.
+     */
+    private void expungeStaleAssociations() {
+        WeakRefPeer<?> peer;
+        while ((peer = (WeakRefPeer<?>) queue.poll()) != null) {
+            map.remove(peer.weakPair());
+        }
+    }
+
+    /**
+     * Common interface of both {@link Weak} and {@link Lookup} key pairs.
+     */
+    private interface Pair<K1, K2> {
+
+        static <K1, K2> Pair<K1, K2> weak(K1 k1, K2 k2,
+                                          ReferenceQueue<Object> queue) {
+            return new Weak<>(k1, k2, queue);
+        }
+
+        static <K1, K2> Pair<K1, K2> lookup(K1 k1, K2 k2) {
+            return new Lookup<>(k1, k2);
+        }
+
+        /**
+         * @return The 1st of the pair of keys (may be null for {@link Weak}
+         * when it gets cleared)
+         */
+        K1 first();
+
+        /**
+         * @return The 2nd of the pair of keys (may be null for {@link Weak}
+         * when it gets cleared)
+         */
+        K2 second();
+
+        static int hashCode(Object first, Object second) {
+            // assert first != null && second != null;
+            return System.identityHashCode(first) ^
+                   System.identityHashCode(second);
+        }
+
+        static boolean equals(Object first, Object second, Pair<?, ?> p) {
+            return first != null && second != null &&
+                   first == p.first() && second == p.second();
+        }
+
+        /**
+         * A Pair where both keys are weakly-referenced.
+         * It is composed of two instances of {@link WeakRefPeer}s:
+         * <pre>{@code
+         *
+         *     +-referent-> [K1]                +-referent-> [K2]
+         *     |                                |
+         *   +----------------+               +----------------+
+         *   | Pair.Weak <:   |-----peer----->| (anonymous) <: |
+         *   | WeakRefPeer,   |               | WeakRefPeer    |
+         *   | Pair           |<--weakPair()--|                |
+         *   +----------------+               +----------------+
+         *     |            ^
+         *     |            |
+         *     +-weakPair()-+
+         *
+         * }</pre>
+         * <p>
+         * Pair.Weak is used for CHM keys. Both peers are associated with the
+         * same {@link ReferenceQueue} so when either of their referents
+         * becomes weakly-reachable, the corresponding entries can be
+         * {@link #expungeStaleAssociations() expunged} from the map.
+         */
+        final class Weak<K1, K2> extends WeakRefPeer<K1> implements Pair<K1, K2> {
+
+            // saved hash so it can be retrieved after the reference is cleared
+            private final int hash;
+            // link to <K2> peer
+            private final WeakRefPeer<K2> peer;
+
+            Weak(K1 k1, K2 k2, ReferenceQueue<Object> queue) {
+                super(k1, queue);
+                hash = Pair.hashCode(k1, k2);
+                peer = new WeakRefPeer<>(k2, queue) {
+                    // link back to <K1> peer
+                    @Override
+                    Weak<?, ?> weakPair() { return Weak.this; }
+                };
+            }
+
+            @Override
+            Weak<?, ?> weakPair() {
+                return this;
+            }
+
+            @Override
+            public K1 first() {
+                return get();
+            }
+
+            @Override
+            public K2 second() {
+                return peer.get();
+            }
+
+            @Override
+            public int hashCode() {
+                return hash;
+            }
+
+            @Override
+            public boolean equals(Object obj) {
+                return this == obj ||
+                       (obj instanceof Pair &&
+                        Pair.equals(first(), second(), (Pair<?, ?>) obj));
+            }
+        }
+
+        /**
+         * Optimized lookup Pair, used as lookup key in methods like
+         * {@link java.util.Map#get(Object)} or
+         * {@link java.util.Map#containsKey(Object)}) where
+         * there is a great chance its allocation is eliminated
+         * by escape analysis when such lookups are inlined by JIT.
+         * All its methods are purposely designed so that 'this' is never
+         * passed to any other method or used as identity.
+         */
+        final class Lookup<K1, K2> implements Pair<K1, K2> {
+            private final K1 k1;
+            private final K2 k2;
+
+            Lookup(K1 k1, K2 k2) {
+                this.k1 = Objects.requireNonNull(k1);
+                this.k2 = Objects.requireNonNull(k2);
+            }
+
+            @Override
+            public K1 first() {
+                return k1;
+            }
+
+            @Override
+            public K2 second() {
+                return k2;
+            }
+
+            @Override
+            public int hashCode() {
+                return Pair.hashCode(k1, k2);
+            }
+
+            @Override
+            public boolean equals(Object obj) {
+                return obj instanceof Pair &&
+                       Pair.equals(k1, k2, (Pair<?, ?>) obj);
+            }
+        }
+    }
+
+    /**
+     * Common abstract supertype of a pair of WeakReference peers.
+     */
+    private static abstract class WeakRefPeer<K> extends WeakReference<K> {
+
+        WeakRefPeer(K k, ReferenceQueue<Object> queue) {
+            super(Objects.requireNonNull(k), queue);
+        }
+
+        /**
+         * @return the {@link Pair.Weak} side of the pair of peers.
+         */
+        abstract Pair.Weak<?, ?> weakPair();
+    }
+}