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/*
 * 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
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/*
 * 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 VarHandle specification for descriptions of the properties of atomic accesses. 
Author:Doug Lea
Type parameters:
  • <V> – The type of object referred to by this reference
Since:1.5
/**
 * 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.

Params:
  • initialValue – the initial 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.

/**
     * Creates a new AtomicReference with null initial value.
     */
    public AtomicReference() {
    }

    
Returns the current value, with memory effects as specified by VarHandle.getVolatile. 
Returns:the current value
/**
     * 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 newValue, with memory effects as specified by VarHandle.setVolatile. 
Params:
  • newValue – the new 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 newValue, with memory effects as specified by VarHandle.setRelease. 
Params:
  • newValue – the new value
Since:1.6
/**
     * 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 newValue if the current value == expectedValue, with memory effects as specified by VarHandle.compareAndSet. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:true if successful. False return indicates that the actual value was not equal to the expected value.
/**
     * 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 newValue if the current value == expectedValue, with memory effects as specified by VarHandle.weakCompareAndSetPlain. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
See Also:
Deprecated:This method has plain memory effects but the method name implies volatile memory effects (see methods such as compareAndExchange and compareAndSet). To avoid confusion over plain or volatile memory effects it is recommended that the method weakCompareAndSetPlain be used instead.
Returns:true if successful
/**
     * 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 newValue if the current value == expectedValue, with memory effects as specified by VarHandle.weakCompareAndSetPlain. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:true if successful
Since:9
/**
     * 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 newValue and returns the old value, with memory effects as specified by VarHandle.getAndSet. 
Params:
  • newValue – the new value
Returns:the previous value
/**
     * 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 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. 
Params:
  • updateFunction – a side-effect-free function
Returns:the previous value
Since:1.8
/**
     * 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 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. 
Params:
  • updateFunction – a side-effect-free function
Returns:the updated value
Since:1.8
/**
     * 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 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. 
Params:
  • x – the update value
  • accumulatorFunction – a side-effect-free function of two arguments
Returns:the previous value
Since:1.8
/**
     * 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 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. 
Params:
  • x – the update value
  • accumulatorFunction – a side-effect-free function of two arguments
Returns:the updated value
Since:1.8
/**
     * 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.

Returns:the String representation of the current value
/**
     * 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-volatile. 
Returns:the value
Since:9
/**
     * 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 newValue, with memory semantics of setting as if the variable was declared non-volatile and non-final. 
Params:
  • newValue – the new value
Since:9
/**
     * 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 VarHandle.getOpaque. 
Returns:the value
Since:9
/**
     * 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 newValue, with memory effects as specified by VarHandle.setOpaque. 
Params:
  • newValue – the new value
Since:9
/**
     * 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 VarHandle.getAcquire. 
Returns:the value
Since:9
/**
     * 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 newValue, with memory effects as specified by VarHandle.setRelease. 
Params:
  • newValue – the new value
Since:9
/**
     * 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 newValue if the current value, referred to as the witness value, == expectedValue, with memory effects as specified by VarHandle.compareAndExchange. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:the witness value, which will be the same as the
expected value if successful
Since:9
/**
     * 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 newValue if the current value, referred to as the witness value, == expectedValue, with memory effects as specified by VarHandle.compareAndExchangeAcquire. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:the witness value, which will be the same as the
expected value if successful
Since:9
/**
     * 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 newValue if the current value, referred to as the witness value, == expectedValue, with memory effects as specified by VarHandle.compareAndExchangeRelease. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:the witness value, which will be the same as the
expected value if successful
Since:9
/**
     * 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 newValue if the current value == expectedValue, with memory effects as specified by VarHandle.weakCompareAndSet. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:true if successful
Since:9
/**
     * 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 newValue if the current value == expectedValue, with memory effects as specified by VarHandle.weakCompareAndSetAcquire. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:true if successful
Since:9
/**
     * 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 newValue if the current value == expectedValue, with memory effects as specified by VarHandle.weakCompareAndSetRelease. 
Params:
  • expectedValue – the expected value
  • newValue – the new value
Returns:true if successful
Since:9
/**
     * 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);
    }

}