/* |
|
* 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. |
|
*/ |
|
/* |
|
* This file is available under and governed by the GNU General Public |
|
* License version 2 only, as published by the Free Software Foundation. |
|
* However, the following notice accompanied the original version of this |
|
* file: |
|
* |
|
* Written by Doug Lea with assistance from members of JCP JSR-166 |
|
* Expert Group and released to the public domain, as explained at |
|
* http://creativecommons.org/publicdomain/zero/1.0/ |
|
*/ |
|
package java.util.concurrent.atomic; |
|
import java.lang.invoke.MethodHandles; |
|
import java.lang.invoke.VarHandle; |
|
/** |
|
* An {@code AtomicMarkableReference} maintains an object reference |
|
* along with a mark bit, that can be updated atomically. |
|
* |
|
* <p>Implementation note: This implementation maintains markable |
|
* references by creating internal objects representing "boxed" |
|
* [reference, boolean] pairs. |
|
* |
|
* @since 1.5 |
|
* @author Doug Lea |
|
* @param <V> The type of object referred to by this reference |
|
*/ |
|
public class AtomicMarkableReference<V> { |
|
private static class Pair<T> { |
|
final T reference; |
|
final boolean mark; |
|
private Pair(T reference, boolean mark) { |
|
this.reference = reference; |
|
this.mark = mark; |
|
} |
|
static <T> Pair<T> of(T reference, boolean mark) { |
|
return new Pair<T>(reference, mark); |
|
} |
|
} |
|
private volatile Pair<V> pair; |
|
/** |
|
* Creates a new {@code AtomicMarkableReference} with the given |
|
* initial values. |
|
* |
|
* @param initialRef the initial reference |
|
* @param initialMark the initial mark |
|
*/ |
|
public AtomicMarkableReference(V initialRef, boolean initialMark) { |
|
pair = Pair.of(initialRef, initialMark); |
|
} |
|
/** |
|
* Returns the current value of the reference. |
|
* |
|
* @return the current value of the reference |
|
*/ |
|
public V getReference() { |
|
return pair.reference; |
|
} |
|
/** |
|
* Returns the current value of the mark. |
|
* |
|
* @return the current value of the mark |
|
*/ |
|
public boolean isMarked() { |
|
return pair.mark; |
|
} |
|
/** |
|
* Returns the current values of both the reference and the mark. |
|
* Typical usage is {@code boolean[1] holder; ref = v.get(holder); }. |
|
* |
|
* @param markHolder an array of size of at least one. On return, |
|
* {@code markHolder[0]} will hold the value of the mark. |
|
* @return the current value of the reference |
|
*/ |
|
public V get(boolean[] markHolder) { |
|
Pair<V> pair = this.pair; |
|
markHolder[0] = pair.mark; |
|
return pair.reference; |
|
} |
|
/** |
|
* Atomically sets the value of both the reference and mark to the |
|
* given update values if the current reference is {@code ==} to |
|
* the expected reference and the current mark is equal to the |
|
* expected mark. This operation may fail spuriously and does not |
|
* provide ordering guarantees, so is only rarely an |
|
* appropriate alternative to {@code compareAndSet}. |
|
* |
|
* @param expectedReference the expected value of the reference |
|
* @param newReference the new value for the reference |
|
* @param expectedMark the expected value of the mark |
|
* @param newMark the new value for the mark |
|
* @return {@code true} if successful |
|
*/ |
|
public boolean weakCompareAndSet(V expectedReference, |
|
V newReference, |
|
boolean expectedMark, |
|
boolean newMark) { |
|
return compareAndSet(expectedReference, newReference, |
|
expectedMark, newMark); |
|
} |
|
/** |
|
* Atomically sets the value of both the reference and mark |
|
* to the given update values if the |
|
* current reference is {@code ==} to the expected reference |
|
* and the current mark is equal to the expected mark. |
|
* |
|
* @param expectedReference the expected value of the reference |
|
* @param newReference the new value for the reference |
|
* @param expectedMark the expected value of the mark |
|
* @param newMark the new value for the mark |
|
* @return {@code true} if successful |
|
*/ |
|
public boolean compareAndSet(V expectedReference, |
|
V newReference, |
|
boolean expectedMark, |
|
boolean newMark) { |
|
Pair<V> current = pair; |
|
return |
|
expectedReference == current.reference && |
|
expectedMark == current.mark && |
|
((newReference == current.reference && |
|
newMark == current.mark) || |
|
casPair(current, Pair.of(newReference, newMark))); |
|
} |
|
/** |
|
* Unconditionally sets the value of both the reference and mark. |
|
* |
|
* @param newReference the new value for the reference |
|
* @param newMark the new value for the mark |
|
*/ |
|
public void set(V newReference, boolean newMark) { |
|
Pair<V> current = pair; |
|
if (newReference != current.reference || newMark != current.mark) |
|
this.pair = Pair.of(newReference, newMark); |
|
} |
|
/** |
|
* Atomically sets the value of the mark to the given update value |
|
* if the current reference is {@code ==} to the expected |
|
* reference. Any given invocation of this operation may fail |
|
* (return {@code false}) spuriously, but repeated invocation |
|
* when the current value holds the expected value and no other |
|
* thread is also attempting to set the value will eventually |
|
* succeed. |
|
* |
|
* @param expectedReference the expected value of the reference |
|
* @param newMark the new value for the mark |
|
* @return {@code true} if successful |
|
*/ |
|
public boolean attemptMark(V expectedReference, boolean newMark) { |
|
Pair<V> current = pair; |
|
return |
|
expectedReference == current.reference && |
|
(newMark == current.mark || |
|
casPair(current, Pair.of(expectedReference, newMark))); |
|
} |
|
// VarHandle mechanics |
|
private static final VarHandle PAIR; |
|
static { |
|
try { |
|
MethodHandles.Lookup l = MethodHandles.lookup(); |
|
PAIR = l.findVarHandle(AtomicMarkableReference.class, "pair", |
|
Pair.class); |
|
} catch (ReflectiveOperationException e) { |
|
throw new ExceptionInInitializerError(e); |
|
} |
|
} |
|
private boolean casPair(Pair<V> cmp, Pair<V> val) { |
|
return PAIR.compareAndSet(this, cmp, val); |
|
} |
|
} |