/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
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* 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).
*
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* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
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package java.lang.reflect;
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();
}
}