1 /*

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

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.  Oracle designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Oracle in the LICENSE file that accompanied this code.

    10  *

    11  * This code is distributed in the hope that it will be useful, but WITHOUT

    12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    14  * version 2 for more details (a copy is included in the LICENSE file that

    15  * accompanied this code).

    16  *

    17  * You should have received a copy of the GNU General Public License version

    18  * 2 along with this work; if not, write to the Free Software Foundation,

    19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    20  *

    21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    22  * or visit www.oracle.com if you need additional information or have any

    23  * questions.

    24  */

    25 

    26 package java.dyn;

    27 

    28 import java.util.WeakHashMap;

    29 import java.util.concurrent.atomic.AtomicInteger;

    30 import java.util.concurrent.atomic.AtomicReference;

    31 

    32 /**

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

    34  * @author John Rose, JSR 292 EG

    35  */

    36 public abstract class ClassValue<T> {

    37     /**

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

    39      * <p>

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

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

    42      * <p>

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

    44      * but it may be invoked again in case of subsequent invocations

    45      * of {@link #remove} followed by {@link #get}.

    46      *

    47      * @return the computed value for this thread-local

    48      */

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

    50 

    51     /**

    52      * Creates a new class value.

    53      */

    54     protected ClassValue() {

    55     }

    56 

    57     /**

    58      * Returns the value for the given class.

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

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

    61      * <p>

    62      * The actual installation of the value on the class

    63      * is performed while the class's synchronization lock

    64      * is held.  At that point, if racing threads have

    65      * computed values, one is chosen, and returned to

    66      * all the racing threads.

    67      *

    68      * @return the current thread's value of this thread-local

    69      */

    70     public T get(Class<?> type) {

    71         ClassValueMap map = getMap(type);

    72         if (map != null) {

    73             Object x = map.get(this);

    74             if (x != null) {

    75                 return (T) map.unmaskNull(x);

    76             }

    77         }

    78         return setComputedValue(type);

    79     }

    80 

    81     /**

    82      * Removes the associated value for the given class.

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

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

    85      * This may result in an additional invocation of the

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

    87      */

    88     public void remove(Class<?> type) {

    89         ClassValueMap map = getMap(type);

    90         if (map != null) {

    91             synchronized (map) {

    92                 map.remove(this);

    93             }

    94         }

    95     }

    96 

    97     /// Implementation...

    98 

    99     /** The hash code for this type is based on the identity of the object,

   100      *  and is well-dispersed for power-of-two tables.

   101      */

   102     public final int hashCode() { return hashCode; }

   103     private final int hashCode = HASH_CODES.getAndAdd(0x61c88647);

   104     private static final AtomicInteger HASH_CODES = new AtomicInteger();

   105 

   106     private static final AtomicInteger STORE_BARRIER = new AtomicInteger();

   107 

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

   109     private T setComputedValue(Class<?> type) {

   110         ClassValueMap map = getMap(type);

   111         if (map == null) {

   112             map = initializeMap(type);

   113         }

   114         T value = computeValue(type);

   115         STORE_BARRIER.lazySet(0);

   116         // All stores pending from computeValue are completed.

   117         synchronized (map) {

   118             // Warm up the table with a null entry.

   119             map.preInitializeEntry(this);

   120         }

   121         // All stores pending from table expansion are completed.

   122         synchronized (map) {

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

   124             // One might fear a possible race condition here

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

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

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

   128             // warmed up the table with an empty entry.

   129         }

   130         return value;

   131     }

   132 

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

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

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

   136 

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

   138         return ROOT.get(type);

   139     }

   140 

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

   142         synchronized (ClassValue.class) {

   143             ClassValueMap map = ROOT.get(type);

   144             if (map == null)

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

   146             return map;

   147         }

   148     }

   149 

   150     static class ClassValueMap extends WeakHashMap<ClassValue, Object> {

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

   152         void preInitializeEntry(ClassValue key) {

   153             if (!this.containsKey(key))

   154                 this.put(key, null);

   155         }

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

   157         Object initializeEntry(ClassValue key, Object value) {

   158             Object prior = this.get(key);

   159             if (prior != null) {

   160                 return unmaskNull(prior);

   161             }

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

   163             return value;

   164         }

   165 

   166         Object maskNull(Object x) {

   167             return x == null ? this : x;

   168         }

   169         Object unmaskNull(Object x) {

   170             return x == this ? null : x;

   171         }

   172     }

   173 }