https://openjdk.java.net/ 
/*
 * 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.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.Objects;
import java.util.function.BinaryOperator;
import java.util.function.UnaryOperator;
import jdk.internal.misc.Unsafe;
import jdk.internal.reflect.CallerSensitive;
import jdk.internal.reflect.Reflection;
import java.lang.invoke.VarHandle;


A reflection-based utility that enables atomic updates to designated volatile reference fields of designated classes. This class is designed for use in atomic data structures in which several reference fields of the same node are independently subject to atomic updates. For example, a tree node might be declared as 
 
class Node {
  private volatile Node left, right;
  private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
    AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
  private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
    AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
  Node getLeft() { return left; }
  boolean compareAndSetLeft(Node expect, Node update) {
    return leftUpdater.compareAndSet(this, expect, update);
  }
  // ... and so on
 }


Note that the guarantees of the compareAndSet method in this class are weaker than in other atomic classes. Because this class cannot ensure that all uses of the field are appropriate for purposes of atomic access, it can guarantee atomicity only with respect to other invocations of compareAndSet and set on the same updater. 
Object arguments for parameters of type T that are not instances of the class passed to newUpdater will result in a ClassCastException being thrown. 
Author:Doug Lea
Type parameters:
  • <T> – The type of the object holding the updatable field
  • <V> – The type of the field
Since:1.5
/**
 * A reflection-based utility that enables atomic updates to
 * designated {@code volatile} reference fields of designated
 * classes.  This class is designed for use in atomic data structures
 * in which several reference fields of the same node are
 * independently subject to atomic updates. For example, a tree node
 * might be declared as
 *
 * <pre> {@code
 * class Node {
 *   private volatile Node left, right;
 *
 *   private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
 *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
 *   private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
 *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
 *
 *   Node getLeft() { return left; }
 *   boolean compareAndSetLeft(Node expect, Node update) {
 *     return leftUpdater.compareAndSet(this, expect, update);
 *   }
 *   // ... and so on
 * }}</pre>
 *
 * <p>Note that the guarantees of the {@code compareAndSet}
 * method in this class are weaker than in other atomic classes.
 * Because this class cannot ensure that all uses of the field
 * are appropriate for purposes of atomic access, it can
 * guarantee atomicity only with respect to other invocations of
 * {@code compareAndSet} and {@code set} on the same updater.
 *
 * <p>Object arguments for parameters of type {@code T} that are not
 * instances of the class passed to {@link #newUpdater} will result in
 * a {@link ClassCastException} being thrown.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <T> The type of the object holding the updatable field
 * @param <V> The type of the field
 */
public abstract class AtomicReferenceFieldUpdater<T,V> {

    
Creates and returns an updater for objects with the given field.
The Class arguments are needed to check that reflective types and
generic types match.

Params:
  • tclass – the class of the objects holding the field
  • vclass – the class of the field
  • fieldName – the name of the field to be updated
Type parameters:
  • <U> – the type of instances of tclass
  • <W> – the type of instances of vclass
Throws:
  • ClassCastException – if the field is of the wrong type
  • IllegalArgumentException – if the field is not volatile
  • RuntimeException – with a nested reflection-based
exception if the class does not hold field or is the wrong type,
or the field is inaccessible to the caller according to Java language
access control
Returns:the updater
/**
     * Creates and returns an updater for objects with the given field.
     * The Class arguments are needed to check that reflective types and
     * generic types match.
     *
     * @param tclass the class of the objects holding the field
     * @param vclass the class of the field
     * @param fieldName the name of the field to be updated
     * @param <U> the type of instances of tclass
     * @param <W> the type of instances of vclass
     * @return the updater
     * @throws ClassCastException if the field is of the wrong type
     * @throws IllegalArgumentException if the field is not volatile
     * @throws RuntimeException with a nested reflection-based
     * exception if the class does not hold field or is the wrong type,
     * or the field is inaccessible to the caller according to Java language
     * access control
     */
    @CallerSensitive
    public static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass,
                                                                    Class<W> vclass,
                                                                    String fieldName) {
        return new AtomicReferenceFieldUpdaterImpl<U,W>
            (tclass, vclass, fieldName, Reflection.getCallerClass());
    }

    
Protected do-nothing constructor for use by subclasses.

/**
     * Protected do-nothing constructor for use by subclasses.
     */
    protected AtomicReferenceFieldUpdater() {
    }

    
Atomically sets the field of the given object managed by this updater to the given updated value if the current value == the expected value. This method is guaranteed to be atomic with respect to other calls to compareAndSet and set, but not necessarily with respect to other changes in the field. 
Params:
  • obj – An object whose field to conditionally set
  • expect – the expected value
  • update – the new value
Returns:true if successful
/**
     * Atomically sets the field of the given object managed by this updater
     * to the given updated value if the current value {@code ==} the
     * expected value. This method is guaranteed to be atomic with respect to
     * other calls to {@code compareAndSet} and {@code set}, but not
     * necessarily with respect to other changes in the field.
     *
     * @param obj An object whose field to conditionally set
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful
     */
    public abstract boolean compareAndSet(T obj, V expect, V update);

    
Atomically sets the field of the given object managed by this updater to the given updated value if the current value == the expected value. This method is guaranteed to be atomic with respect to other calls to compareAndSet and set, but not necessarily with respect to other changes in the field. 
May fail
spuriously and does not provide ordering guarantees, so is only rarely an appropriate alternative to compareAndSet. 
Params:
  • obj – An object whose field to conditionally set
  • expect – the expected value
  • update – the new value
Returns:true if successful
/**
     * Atomically sets the field of the given object managed by this updater
     * to the given updated value if the current value {@code ==} the
     * expected value. This method is guaranteed to be atomic with respect to
     * other calls to {@code compareAndSet} and {@code set}, but not
     * necessarily with respect to other changes in the field.
     *
     * <p><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, so is
     * only rarely an appropriate alternative to {@code compareAndSet}.
     *
     * @param obj An object whose field to conditionally set
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful
     */
    public abstract boolean weakCompareAndSet(T obj, V expect, V update);

    
Sets the field of the given object managed by this updater to the given updated value. This operation is guaranteed to act as a volatile store with respect to subsequent invocations of compareAndSet. 
Params:
  • obj – An object whose field to set
  • newValue – the new value
/**
     * Sets the field of the given object managed by this updater to the
     * given updated value. This operation is guaranteed to act as a volatile
     * store with respect to subsequent invocations of {@code compareAndSet}.
     *
     * @param obj An object whose field to set
     * @param newValue the new value
     */
    public abstract void set(T obj, V newValue);

    
Eventually sets the field of the given object managed by this
updater to the given updated value.

Params:
  • obj – An object whose field to set
  • newValue – the new value
Since:1.6
/**
     * Eventually sets the field of the given object managed by this
     * updater to the given updated value.
     *
     * @param obj An object whose field to set
     * @param newValue the new value
     * @since 1.6
     */
    public abstract void lazySet(T obj, V newValue);

    
Returns the current value held in the field of the given object
managed by this updater.

Params:
  • obj – An object whose field to get
Returns:the current value
/**
     * Returns the current value held in the field of the given object
     * managed by this updater.
     *
     * @param obj An object whose field to get
     * @return the current value
     */
    public abstract V get(T obj);

    
Atomically sets the field of the given object managed by this updater
to the given value and returns the old value.

Params:
  • obj – An object whose field to get and set
  • newValue – the new value
Returns:the previous value
/**
     * Atomically sets the field of the given object managed by this updater
     * to the given value and returns the old value.
     *
     * @param obj An object whose field to get and set
     * @param newValue the new value
     * @return the previous value
     */
    public V getAndSet(T obj, V newValue) {
        V prev;
        do {
            prev = get(obj);
        } while (!compareAndSet(obj, prev, newValue));
        return prev;
    }

    
Atomically updates (with memory effects as specified by VarHandle.compareAndSet) the field of the given object managed by this updater 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:
  • obj – An object whose field to get and set
  • 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 field of the given object managed
     * by this updater 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 obj An object whose field to get and set
     * @param updateFunction a side-effect-free function
     * @return the previous value
     * @since 1.8
     */
    public final V getAndUpdate(T obj, UnaryOperator<V> updateFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = updateFunction.apply(prev);
        } while (!compareAndSet(obj, prev, next));
        return prev;
    }

    
Atomically updates (with memory effects as specified by VarHandle.compareAndSet) the field of the given object managed by this updater 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:
  • obj – An object whose field to get and set
  • 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 field of the given object managed
     * by this updater 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 obj An object whose field to get and set
     * @param updateFunction a side-effect-free function
     * @return the updated value
     * @since 1.8
     */
    public final V updateAndGet(T obj, UnaryOperator<V> updateFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = updateFunction.apply(prev);
        } while (!compareAndSet(obj, prev, next));
        return next;
    }

    
Atomically updates (with memory effects as specified by VarHandle.compareAndSet) the field of the given object managed by this updater 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:
  • obj – An object whose field to get and set
  • 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 field of the given object managed
     * by this updater 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 obj An object whose field to get and set
     * @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(T obj, V x,
                                    BinaryOperator<V> accumulatorFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSet(obj, prev, next));
        return prev;
    }

    
Atomically updates (with memory effects as specified by VarHandle.compareAndSet) the field of the given object managed by this updater 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:
  • obj – An object whose field to get and set
  • 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 field of the given object managed
     * by this updater 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 obj An object whose field to get and set
     * @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(T obj, V x,
                                    BinaryOperator<V> accumulatorFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSet(obj, prev, next));
        return next;
    }

    private static final class AtomicReferenceFieldUpdaterImpl<T,V>
        extends AtomicReferenceFieldUpdater<T,V> {
        private static final Unsafe U = Unsafe.getUnsafe();
        private final long offset;
        
if field is protected, the subclass constructing updater, else
the same as tclass

/**
         * if field is protected, the subclass constructing updater, else
         * the same as tclass
         */
        private final Class<?> cclass;
        
class holding the field 
/** class holding the field */
        private final Class<T> tclass;
        
field value type 
/** field value type */
        private final Class<V> vclass;

        /*
         * Internal type checks within all update methods contain
         * internal inlined optimizations checking for the common
         * cases where the class is final (in which case a simple
         * getClass comparison suffices) or is of type Object (in
         * which case no check is needed because all objects are
         * instances of Object). The Object case is handled simply by
         * setting vclass to null in constructor.  The targetCheck and
         * updateCheck methods are invoked when these faster
         * screenings fail.
         */

        AtomicReferenceFieldUpdaterImpl(final Class<T> tclass,
                                        final Class<V> vclass,
                                        final String fieldName,
                                        final Class<?> caller) {
            final Field field;
            final Class<?> fieldClass;
            final int modifiers;
            try {
                field = AccessController.doPrivileged(
                    new PrivilegedExceptionAction<Field>() {
                        public Field run() throws NoSuchFieldException {
                            return tclass.getDeclaredField(fieldName);
                        }
                    });
                modifiers = field.getModifiers();
                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
                    caller, tclass, null, modifiers);
                ClassLoader cl = tclass.getClassLoader();
                ClassLoader ccl = caller.getClassLoader();
                if ((ccl != null) && (ccl != cl) &&
                    ((cl == null) || !isAncestor(cl, ccl))) {
                    sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
                }
                fieldClass = field.getType();
            } catch (PrivilegedActionException pae) {
                throw new RuntimeException(pae.getException());
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }

            if (vclass != fieldClass)
                throw new ClassCastException();
            if (vclass.isPrimitive())
                throw new IllegalArgumentException("Must be reference type");

            if (!Modifier.isVolatile(modifiers))
                throw new IllegalArgumentException("Must be volatile type");

            // Access to protected field members is restricted to receivers only
            // of the accessing class, or one of its subclasses, and the
            // accessing class must in turn be a subclass (or package sibling)
            // of the protected member's defining class.
            // If the updater refers to a protected field of a declaring class
            // outside the current package, the receiver argument will be
            // narrowed to the type of the accessing class.
            this.cclass = (Modifier.isProtected(modifiers) &&
                           tclass.isAssignableFrom(caller) &&
                           !isSamePackage(tclass, caller))
                          ? caller : tclass;
            this.tclass = tclass;
            this.vclass = vclass;
            this.offset = U.objectFieldOffset(field);
        }

        
Returns true if the second classloader can be found in the first
classloader's delegation chain.
Equivalent to the inaccessible: first.isAncestor(second).

/**
         * Returns true if the second classloader can be found in the first
         * classloader's delegation chain.
         * Equivalent to the inaccessible: first.isAncestor(second).
         */
        private static boolean isAncestor(ClassLoader first, ClassLoader second) {
            ClassLoader acl = first;
            do {
                acl = acl.getParent();
                if (second == acl) {
                    return true;
                }
            } while (acl != null);
            return false;
        }

        
Returns true if the two classes have the same class loader and
package qualifier

/**
         * Returns true if the two classes have the same class loader and
         * package qualifier
         */
        private static boolean isSamePackage(Class<?> class1, Class<?> class2) {
            return class1.getClassLoader() == class2.getClassLoader()
                   && Objects.equals(class1.getPackageName(), class2.getPackageName());
        }

        
Checks that target argument is instance of cclass.  On
failure, throws cause.

/**
         * Checks that target argument is instance of cclass.  On
         * failure, throws cause.
         */
        private final void accessCheck(T obj) {
            if (!cclass.isInstance(obj))
                throwAccessCheckException(obj);
        }

        
Throws access exception if accessCheck failed due to
protected access, else ClassCastException.

/**
         * Throws access exception if accessCheck failed due to
         * protected access, else ClassCastException.
         */
        private final void throwAccessCheckException(T obj) {
            if (cclass == tclass)
                throw new ClassCastException();
            else
                throw new RuntimeException(
                    new IllegalAccessException(
                        "Class " +
                        cclass.getName() +
                        " can not access a protected member of class " +
                        tclass.getName() +
                        " using an instance of " +
                        obj.getClass().getName()));
        }

        private final void valueCheck(V v) {
            if (v != null && !(vclass.isInstance(v)))
                throwCCE();
        }

        static void throwCCE() {
            throw new ClassCastException();
        }

        public final boolean compareAndSet(T obj, V expect, V update) {
            accessCheck(obj);
            valueCheck(update);
            return U.compareAndSetObject(obj, offset, expect, update);
        }

        public final boolean weakCompareAndSet(T obj, V expect, V update) {
            // same implementation as strong form for now
            accessCheck(obj);
            valueCheck(update);
            return U.compareAndSetObject(obj, offset, expect, update);
        }

        public final void set(T obj, V newValue) {
            accessCheck(obj);
            valueCheck(newValue);
            U.putObjectVolatile(obj, offset, newValue);
        }

        public final void lazySet(T obj, V newValue) {
            accessCheck(obj);
            valueCheck(newValue);
            U.putObjectRelease(obj, offset, newValue);
        }

        @SuppressWarnings("unchecked")
        public final V get(T obj) {
            accessCheck(obj);
            return (V)U.getObjectVolatile(obj, offset);
        }

        @SuppressWarnings("unchecked")
        public final V getAndSet(T obj, V newValue) {
            accessCheck(obj);
            valueCheck(newValue);
            return (V)U.getAndSetObject(obj, offset, newValue);
        }
    }
}