/*

  * 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

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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 sun.misc.Unsafe;

 import java.lang.reflect.*;

 /**

  * 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.

  *

  * @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 {@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 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 {@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>May <a href="package-summary.html#Spurious">fail spuriously</a>

      * and does not provide ordering guarantees, 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 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 {@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.

      *

      * @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()

                 )

             );

         }

     }

 }