Java tutorial
/* * 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; import java.util.function.BinaryOperator; import java.util.function.UnaryOperator; /** * An object reference that may be updated atomically. See the {@link * VarHandle} specification for descriptions of the properties of * atomic accesses. * @since 1.5 * @author Doug Lea * @param <V> The type of object referred to by this reference */ public class AtomicReference<V> implements java.io.Serializable { private static final long serialVersionUID = -1848883965231344442L; private static final VarHandle VALUE; static { try { MethodHandles.Lookup l = MethodHandles.lookup(); VALUE = l.findVarHandle(AtomicReference.class, "value", Object.class); } catch (ReflectiveOperationException e) { throw new ExceptionInInitializerError(e); } } private volatile V value; /** * Creates a new AtomicReference with the given initial value. * * @param initialValue the initial value */ public AtomicReference(V initialValue) { value = initialValue; } /** * Creates a new AtomicReference with null initial value. */ public AtomicReference() { } /** * Returns the current value, * with memory effects as specified by {@link VarHandle#getVolatile}. * * @return the current value */ public final V get() { return value; } /** * Sets the value to {@code newValue}, * with memory effects as specified by {@link VarHandle#setVolatile}. * * @param newValue the new value */ public final void set(V newValue) { value = newValue; } /** * Sets the value to {@code newValue}, * with memory effects as specified by {@link VarHandle#setRelease}. * * @param newValue the new value * @since 1.6 */ public final void lazySet(V newValue) { VALUE.setRelease(this, newValue); } /** * Atomically sets the value to {@code newValue} * if the current value {@code == expectedValue}, * with memory effects as specified by {@link VarHandle#compareAndSet}. * * @param expectedValue the expected value * @param newValue the new value * @return {@code true} if successful. False return indicates that * the actual value was not equal to the expected value. */ public final boolean compareAndSet(V expectedValue, V newValue) { return VALUE.compareAndSet(this, expectedValue, newValue); } /** * Possibly atomically sets the value to {@code newValue} * if the current value {@code == expectedValue}, * with memory effects as specified by {@link VarHandle#weakCompareAndSetPlain}. * * @deprecated This method has plain memory effects but the method * name implies volatile memory effects (see methods such as * {@link #compareAndExchange} and {@link #compareAndSet}). To avoid * confusion over plain or volatile memory effects it is recommended that * the method {@link #weakCompareAndSetPlain} be used instead. * * @param expectedValue the expected value * @param newValue the new value * @return {@code true} if successful * @see #weakCompareAndSetPlain */ @Deprecated(since = "9") public final boolean weakCompareAndSet(V expectedValue, V newValue) { return VALUE.weakCompareAndSetPlain(this, expectedValue, newValue); } /** * Possibly atomically sets the value to {@code newValue} * if the current value {@code == expectedValue}, * with memory effects as specified by {@link VarHandle#weakCompareAndSetPlain}. * * @param expectedValue the expected value * @param newValue the new value * @return {@code true} if successful * @since 9 */ public final boolean weakCompareAndSetPlain(V expectedValue, V newValue) { return VALUE.weakCompareAndSetPlain(this, expectedValue, newValue); } /** * Atomically sets the value to {@code newValue} and returns the old value, * with memory effects as specified by {@link VarHandle#getAndSet}. * * @param newValue the new value * @return the previous value */ @SuppressWarnings("unchecked") public final V getAndSet(V newValue) { return (V) VALUE.getAndSet(this, newValue); } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the current value with the results of * applying the given function, returning the previous value. The * function should be side-effect-free, since it may be re-applied * when attempted updates fail due to contention among threads. * * @param updateFunction a side-effect-free function * @return the previous value * @since 1.8 */ public final V getAndUpdate(UnaryOperator<V> updateFunction) { V prev = get(), next = null; for (boolean haveNext = false;;) { if (!haveNext) next = updateFunction.apply(prev); if (weakCompareAndSetVolatile(prev, next)) return prev; haveNext = (prev == (prev = get())); } } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the current value with the results of * applying the given function, returning the updated value. The * function should be side-effect-free, since it may be re-applied * when attempted updates fail due to contention among threads. * * @param updateFunction a side-effect-free function * @return the updated value * @since 1.8 */ public final V updateAndGet(UnaryOperator<V> updateFunction) { V prev = get(), next = null; for (boolean haveNext = false;;) { if (!haveNext) next = updateFunction.apply(prev); if (weakCompareAndSetVolatile(prev, next)) return next; haveNext = (prev == (prev = get())); } } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the current value with the results of * applying the given function to the current and given values, * returning the previous value. The function should be * side-effect-free, since it may be re-applied when attempted * updates fail due to contention among threads. The function is * applied with the current value as its first argument, and the * given update as the second argument. * * @param x the update value * @param accumulatorFunction a side-effect-free function of two arguments * @return the previous value * @since 1.8 */ public final V getAndAccumulate(V x, BinaryOperator<V> accumulatorFunction) { V prev = get(), next = null; for (boolean haveNext = false;;) { if (!haveNext) next = accumulatorFunction.apply(prev, x); if (weakCompareAndSetVolatile(prev, next)) return prev; haveNext = (prev == (prev = get())); } } /** * Atomically updates (with memory effects as specified by {@link * VarHandle#compareAndSet}) the current value with the results of * applying the given function to the current and given values, * returning the updated value. The function should be * side-effect-free, since it may be re-applied when attempted * updates fail due to contention among threads. The function is * applied with the current value as its first argument, and the * given update as the second argument. * * @param x the update value * @param accumulatorFunction a side-effect-free function of two arguments * @return the updated value * @since 1.8 */ public final V accumulateAndGet(V x, BinaryOperator<V> accumulatorFunction) { V prev = get(), next = null; for (boolean haveNext = false;;) { if (!haveNext) next = accumulatorFunction.apply(prev, x); if (weakCompareAndSetVolatile(prev, next)) return next; haveNext = (prev == (prev = get())); } } /** * Returns the String representation of the current value. * @return the String representation of the current value */ public String toString() { return String.valueOf(get()); } // jdk9 /** * Returns the current value, with memory semantics of reading as * if the variable was declared non-{@code volatile}. * * @return the value * @since 9 */ public final V getPlain() { return (V) VALUE.get(this); } /** * Sets the value to {@code newValue}, with memory semantics * of setting as if the variable was declared non-{@code volatile} * and non-{@code final}. * * @param newValue the new value * @since 9 */ public final void setPlain(V newValue) { VALUE.set(this, newValue); } /** * Returns the current value, * with memory effects as specified by {@link VarHandle#getOpaque}. * * @return the value * @since 9 */ public final V getOpaque() { return (V) VALUE.getOpaque(this); } /** * Sets the value to {@code newValue}, * with memory effects as specified by {@link VarHandle#setOpaque}. * * @param newValue the new value * @since 9 */ public final void setOpaque(V newValue) { VALUE.setOpaque(this, newValue); } /** * Returns the current value, * with memory effects as specified by {@link VarHandle#getAcquire}. * * @return the value * @since 9 */ public final V getAcquire() { return (V) VALUE.getAcquire(this); } /** * Sets the value to {@code newValue}, * with memory effects as specified by {@link VarHandle#setRelease}. * * @param newValue the new value * @since 9 */ public final void setRelease(V newValue) { VALUE.setRelease(this, newValue); } /** * Atomically sets the value to {@code newValue} if the current value, * referred to as the <em>witness value</em>, {@code == expectedValue}, * with memory effects as specified by * {@link VarHandle#compareAndExchange}. * * @param expectedValue the expected value * @param newValue the new value * @return the witness value, which will be the same as the * expected value if successful * @since 9 */ public final V compareAndExchange(V expectedValue, V newValue) { return (V) VALUE.compareAndExchange(this, expectedValue, newValue); } /** * Atomically sets the value to {@code newValue} if the current value, * referred to as the <em>witness value</em>, {@code == expectedValue}, * with memory effects as specified by * {@link VarHandle#compareAndExchangeAcquire}. * * @param expectedValue the expected value * @param newValue the new value * @return the witness value, which will be the same as the * expected value if successful * @since 9 */ public final V compareAndExchangeAcquire(V expectedValue, V newValue) { return (V) VALUE.compareAndExchangeAcquire(this, expectedValue, newValue); } /** * Atomically sets the value to {@code newValue} if the current value, * referred to as the <em>witness value</em>, {@code == expectedValue}, * with memory effects as specified by * {@link VarHandle#compareAndExchangeRelease}. * * @param expectedValue the expected value * @param newValue the new value * @return the witness value, which will be the same as the * expected value if successful * @since 9 */ public final V compareAndExchangeRelease(V expectedValue, V newValue) { return (V) VALUE.compareAndExchangeRelease(this, expectedValue, newValue); } /** * Possibly atomically sets the value to {@code newValue} * if the current value {@code == expectedValue}, * with memory effects as specified by * {@link VarHandle#weakCompareAndSet}. * * @param expectedValue the expected value * @param newValue the new value * @return {@code true} if successful * @since 9 */ public final boolean weakCompareAndSetVolatile(V expectedValue, V newValue) { return VALUE.weakCompareAndSet(this, expectedValue, newValue); } /** * Possibly atomically sets the value to {@code newValue} * if the current value {@code == expectedValue}, * with memory effects as specified by * {@link VarHandle#weakCompareAndSetAcquire}. * * @param expectedValue the expected value * @param newValue the new value * @return {@code true} if successful * @since 9 */ public final boolean weakCompareAndSetAcquire(V expectedValue, V newValue) { return VALUE.weakCompareAndSetAcquire(this, expectedValue, newValue); } /** * Possibly atomically sets the value to {@code newValue} * if the current value {@code == expectedValue}, * with memory effects as specified by * {@link VarHandle#weakCompareAndSetRelease}. * * @param expectedValue the expected value * @param newValue the new value * @return {@code true} if successful * @since 9 */ public final boolean weakCompareAndSetRelease(V expectedValue, V newValue) { return VALUE.weakCompareAndSetRelease(this, expectedValue, newValue); } }