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/*

 * 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/licenses/publicdomain

 */

package java.util.concurrent.atomic;

import sun.misc.Unsafe;

import java.lang.reflect.*;

/**

 * A reflection-based utility that enables atomic updates to

 * designated <tt>volatile</tt> 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>

 * 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.

 *

 * @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.

     *

     * @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.

     * @return the updater

     * @throws IllegalArgumentException if the field is not a volatile reference type.

     * @throws RuntimeException with a nested reflection-based

     * exception if the class does not hold field or is the wrong type.

     */

    public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) {

        return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,

                                                        vclass,

                                                        fieldName);

    }

    /**

     * 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 <tt>==</tt> the

     * expected value. This method is guaranteed to be atomic with respect to

     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, 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 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 <tt>==</tt> the

     * expected value. This method is guaranteed to be atomic with respect to

     * other calls to <tt>compareAndSet</tt> and <tt>set</tt>, 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 <tt>compareAndSet</tt>.

     *

     * @param obj An object whose field to conditionally set

     * @param expect the expected value

     * @param update the new value

     * @return 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

     * <tt>compareAndSet</tt>.

     *

     * @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.

     *

     * @param obj An object whose field to set

     * @param newValue the new value

     * @since 1.6

     */

    public abstract void lazySet(T obj, V newValue);

    /**

     * Gets 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.

     *

     * @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) {

        for (;;) {

            V current = get(obj);

            if (compareAndSet(obj, current, newValue))

                return current;

        }

    }

    private static final class AtomicReferenceFieldUpdaterImpl<T,V>

        extends AtomicReferenceFieldUpdater<T,V> {

        private static final Unsafe unsafe = Unsafe.getUnsafe();

        private final long offset;

        private final Class<T> tclass;

        private final Class<V> vclass;

        private final Class cclass;

        /*

         * 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(Class<T> tclass,

                                        Class<V> vclass,

                                        String fieldName) {

            Field field = null;

            Class fieldClass = null;

            Class caller = null;

            int modifiers = 0;

            try {

                field = tclass.getDeclaredField(fieldName);

                caller = sun.reflect.Reflection.getCallerClass(3);

                modifiers = field.getModifiers();

                sun.reflect.misc.ReflectUtil.ensureMemberAccess(

                    caller, tclass, null, modifiers);

                sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);

                fieldClass = field.getType();

            } catch (Exception ex) {

                throw new RuntimeException(ex);

            }

            if (vclass != fieldClass)

                throw new ClassCastException();

            if (!Modifier.isVolatile(modifiers))

                throw new IllegalArgumentException("Must be volatile type");

            this.cclass = (Modifier.isProtected(modifiers) &&

                           caller != tclass) ? caller : null;

            this.tclass = tclass;

            if (vclass == Object.class)

                this.vclass = null;

            else

                this.vclass = vclass;

            offset = unsafe.objectFieldOffset(field);

        }

        void targetCheck(T obj) {

            if (!tclass.isInstance(obj))

                throw new ClassCastException();

            if (cclass != null)

                ensureProtectedAccess(obj);

        }

        void updateCheck(T obj, V update) {

            if (!tclass.isInstance(obj) ||

                (update != null && vclass != null && !vclass.isInstance(update)))

                throw new ClassCastException();

            if (cclass != null)

                ensureProtectedAccess(obj);

        }

        public boolean compareAndSet(T obj, V expect, V update) {

            if (obj == null || obj.getClass() != tclass || cclass != null ||

                (update != null && vclass != null &&

                 vclass != update.getClass()))

                updateCheck(obj, update);

            return unsafe.compareAndSwapObject(obj, offset, expect, update);

        }

        public boolean weakCompareAndSet(T obj, V expect, V update) {

            // same implementation as strong form for now

            if (obj == null || obj.getClass() != tclass || cclass != null ||

                (update != null && vclass != null &&

                 vclass != update.getClass()))

                updateCheck(obj, update);

            return unsafe.compareAndSwapObject(obj, offset, expect, update);

        }

        public void set(T obj, V newValue) {

            if (obj == null || obj.getClass() != tclass || cclass != null ||

                (newValue != null && vclass != null &&

                 vclass != newValue.getClass()))

                updateCheck(obj, newValue);

            unsafe.putObjectVolatile(obj, offset, newValue);

        }

        public void lazySet(T obj, V newValue) {

            if (obj == null || obj.getClass() != tclass || cclass != null ||

                (newValue != null && vclass != null &&

                 vclass != newValue.getClass()))

                updateCheck(obj, newValue);

            unsafe.putOrderedObject(obj, offset, newValue);

        }

        public V get(T obj) {

            if (obj == null || obj.getClass() != tclass || cclass != null)

                targetCheck(obj);

            return (V)unsafe.getObjectVolatile(obj, offset);

        }

        private void ensureProtectedAccess(T obj) {

            if (cclass.isInstance(obj)) {

                return;

            }

            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()

                )

            );

        }

    }

}