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  * Copyright (c) 2010, 2011, Oracle and/or its affiliates. All rights reserved.

  * 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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 package java.lang;

 import java.util.WeakHashMap;

 import java.util.concurrent.atomic.AtomicInteger;

 /**

  * Lazily associate a computed value with (potentially) every type.

  * For example, if a dynamic language needs to construct a message dispatch

  * table for each class encountered at a message send call site,

  * it can use a {@code ClassValue} to cache information needed to

  * perform the message send quickly, for each class encountered.

  * @author John Rose, JSR 292 EG

  * @since 1.7

  */

 public abstract class ClassValue<T> {

     /**

      * Sole constructor.  (For invocation by subclass constructors, typically

      * implicit.)

      */

     protected ClassValue() {

     }

     /**

      * Computes the given class's derived value for this {@code ClassValue}.

      * <p>

      * This method will be invoked within the first thread that accesses

      * the value with the {@link #get get} method.

      * <p>

      * Normally, this method is invoked at most once per class,

      * but it may be invoked again if there has been a call to

      * {@link #remove remove}.

      * <p>

      * If this method throws an exception, the corresponding call to {@code get}

      * will terminate abnormally with that exception, and no class value will be recorded.

      *

      * @param type the type whose class value must be computed

      * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface

      * @see #get

      * @see #remove

      */

     protected abstract T computeValue(Class<?> type);

     /**

      * Returns the value for the given class.

      * If no value has yet been computed, it is obtained by

      * an invocation of the {@link #computeValue computeValue} method.

      * <p>

      * The actual installation of the value on the class

      * is performed atomically.

      * At that point, if several racing threads have

      * computed values, one is chosen, and returned to

      * all the racing threads.

      * <p>

      * The {@code type} parameter is typically a class, but it may be any type,

      * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.

      * <p>

      * In the absence of {@code remove} calls, a class value has a simple

      * state diagram:  uninitialized and initialized.

      * When {@code remove} calls are made,

      * the rules for value observation are more complex.

      * See the documentation for {@link #remove remove} for more information.

      *

      * @param type the type whose class value must be computed or retrieved

      * @return the current value associated with this {@code ClassValue}, for the given class or interface

      * @throws NullPointerException if the argument is null

      * @see #remove

      * @see #computeValue

      */

     public T get(Class<?> type) {

         ClassValueMap map = getMap(type);

         if (map != null) {

             Object x = map.get(this);

             if (x != null) {

                 return (T) map.unmaskNull(x);

             }

         }

         return setComputedValue(type);

     }

     /**

      * Removes the associated value for the given class.

      * If this value is subsequently {@linkplain #get read} for the same class,

      * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.

      * This may result in an additional invocation of the

      * {@code computeValue} method for the given class.

      * <p>

      * In order to explain the interaction between {@code get} and {@code remove} calls,

      * we must model the state transitions of a class value to take into account

      * the alternation between uninitialized and initialized states.

      * To do this, number these states sequentially from zero, and note that

      * uninitialized (or removed) states are numbered with even numbers,

      * while initialized (or re-initialized) states have odd numbers.

      * <p>

      * When a thread {@code T} removes a class value in state {@code 2N},

      * nothing happens, since the class value is already uninitialized.

      * Otherwise, the state is advanced atomically to {@code 2N+1}.

      * <p>

      * When a thread {@code T} queries a class value in state {@code 2N},

      * the thread first attempts to initialize the class value to state {@code 2N+1}

      * by invoking {@code computeValue} and installing the resulting value.

      * <p>

      * When {@code T} attempts to install the newly computed value,

      * if the state is still at {@code 2N}, the class value will be initialized

      * with the computed value, advancing it to state {@code 2N+1}.

      * <p>

      * Otherwise, whether the new state is even or odd,

      * {@code T} will discard the newly computed value

      * and retry the {@code get} operation.

      * <p>

      * Discarding and retrying is an important proviso,

      * since otherwise {@code T} could potentially install

      * a disastrously stale value.  For example:

      * <ul>

      * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}

      * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it

      * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time

      * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}

      * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}

      * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work

      * <li> the previous actions of {@code T2} are repeated several times

      * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...

      * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>

      * </ul>

      * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly

      * observe the time-dependent states as it computes {@code V1}, etc.

      * This does not remove the threat of a stale value, since there is a window of time

      * between the return of {@code computeValue} in {@code T} and the installation

      * of the the new value.  No user synchronization is possible during this time.

      *

      * @param type the type whose class value must be removed

      * @throws NullPointerException if the argument is null

      */

     public void remove(Class<?> type) {

         ClassValueMap map = getMap(type);

         if (map != null) {

             synchronized (map) {

                 map.remove(this);

             }

         }

     }

     /// Implementation...

     // FIXME: Use a data structure here similar that of ThreadLocal (7030453).

     private static final AtomicInteger STORE_BARRIER = new AtomicInteger();

     /** Slow path for {@link #get}. */

     private T setComputedValue(Class<?> type) {

         ClassValueMap map = getMap(type);

         if (map == null) {

             map = initializeMap(type);

         }

         T value = computeValue(type);

         STORE_BARRIER.lazySet(0);

         // All stores pending from computeValue are completed.

         synchronized (map) {

             // Warm up the table with a null entry.

             map.preInitializeEntry(this);

         }

         STORE_BARRIER.lazySet(0);

         // All stores pending from table expansion are completed.

         synchronized (map) {

             value = (T) map.initializeEntry(this, value);

             // One might fear a possible race condition here

             // if the code for map.put has flushed the write

             // to map.table[*] before the writes to the Map.Entry

             // are done.  This is not possible, since we have

             // warmed up the table with an empty entry.

         }

         return value;

     }

     // Replace this map by a per-class slot.

     private static final WeakHashMap<Class<?>, ClassValueMap> ROOT

         = new WeakHashMap<Class<?>, ClassValueMap>();

     private static ClassValueMap getMap(Class<?> type) {

         type.getClass();  // test for null

         return ROOT.get(type);

     }

     private static ClassValueMap initializeMap(Class<?> type) {

         synchronized (ClassValue.class) {

             ClassValueMap map = ROOT.get(type);

             if (map == null)

                 ROOT.put(type, map = new ClassValueMap());

             return map;

         }

     }

     static class ClassValueMap extends WeakHashMap<ClassValue, Object> {

         /** Make sure this table contains an Entry for the given key, even if it is empty. */

         void preInitializeEntry(ClassValue key) {

             if (!this.containsKey(key))

                 this.put(key, null);

         }

         /** Make sure this table contains a non-empty Entry for the given key. */

         Object initializeEntry(ClassValue key, Object value) {

             Object prior = this.get(key);

             if (prior != null) {

                 return unmaskNull(prior);

             }

             this.put(key, maskNull(value));

             return value;

         }

         Object maskNull(Object x) {

             return x == null ? this : x;

         }

         Object unmaskNull(Object x) {

             return x == this ? null : x;

         }

     }

 }