jaroslav@1981: /*
jaroslav@1981:  * Copyright (c) 2010, 2011, Oracle and/or its affiliates. All rights reserved.
jaroslav@1981:  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
jaroslav@1981:  *
jaroslav@1981:  * This code is free software; you can redistribute it and/or modify it
jaroslav@1981:  * under the terms of the GNU General Public License version 2 only, as
jaroslav@1981:  * published by the Free Software Foundation.  Oracle designates this
jaroslav@1981:  * particular file as subject to the "Classpath" exception as provided
jaroslav@1981:  * by Oracle in the LICENSE file that accompanied this code.
jaroslav@1981:  *
jaroslav@1981:  * This code is distributed in the hope that it will be useful, but WITHOUT
jaroslav@1981:  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
jaroslav@1981:  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
jaroslav@1981:  * version 2 for more details (a copy is included in the LICENSE file that
jaroslav@1981:  * accompanied this code).
jaroslav@1981:  *
jaroslav@1981:  * You should have received a copy of the GNU General Public License version
jaroslav@1981:  * 2 along with this work; if not, write to the Free Software Foundation,
jaroslav@1981:  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
jaroslav@1981:  *
jaroslav@1981:  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
jaroslav@1981:  * or visit www.oracle.com if you need additional information or have any
jaroslav@1981:  * questions.
jaroslav@1981:  */
jaroslav@1981: 
jaroslav@1981: package java.lang;
jaroslav@1981: 
jaroslav@1981: import java.util.WeakHashMap;
jaroslav@1981: import java.util.concurrent.atomic.AtomicInteger;
jaroslav@1981: 
jaroslav@1981: /**
jaroslav@1981:  * Lazily associate a computed value with (potentially) every type.
jaroslav@1981:  * For example, if a dynamic language needs to construct a message dispatch
jaroslav@1981:  * table for each class encountered at a message send call site,
jaroslav@1981:  * it can use a {@code ClassValue} to cache information needed to
jaroslav@1981:  * perform the message send quickly, for each class encountered.
jaroslav@1981:  * @author John Rose, JSR 292 EG
jaroslav@1981:  * @since 1.7
jaroslav@1981:  */
jaroslav@1981: public abstract class ClassValue<T> {
jaroslav@1981:     /**
jaroslav@1981:      * Sole constructor.  (For invocation by subclass constructors, typically
jaroslav@1981:      * implicit.)
jaroslav@1981:      */
jaroslav@1981:     protected ClassValue() {
jaroslav@1981:     }
jaroslav@1981: 
jaroslav@1981:     /**
jaroslav@1981:      * Computes the given class's derived value for this {@code ClassValue}.
jaroslav@1981:      * <p>
jaroslav@1981:      * This method will be invoked within the first thread that accesses
jaroslav@1981:      * the value with the {@link #get get} method.
jaroslav@1981:      * <p>
jaroslav@1981:      * Normally, this method is invoked at most once per class,
jaroslav@1981:      * but it may be invoked again if there has been a call to
jaroslav@1981:      * {@link #remove remove}.
jaroslav@1981:      * <p>
jaroslav@1981:      * If this method throws an exception, the corresponding call to {@code get}
jaroslav@1981:      * will terminate abnormally with that exception, and no class value will be recorded.
jaroslav@1981:      *
jaroslav@1981:      * @param type the type whose class value must be computed
jaroslav@1981:      * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
jaroslav@1981:      * @see #get
jaroslav@1981:      * @see #remove
jaroslav@1981:      */
jaroslav@1981:     protected abstract T computeValue(Class<?> type);
jaroslav@1981: 
jaroslav@1981:     /**
jaroslav@1981:      * Returns the value for the given class.
jaroslav@1981:      * If no value has yet been computed, it is obtained by
jaroslav@1981:      * an invocation of the {@link #computeValue computeValue} method.
jaroslav@1981:      * <p>
jaroslav@1981:      * The actual installation of the value on the class
jaroslav@1981:      * is performed atomically.
jaroslav@1981:      * At that point, if several racing threads have
jaroslav@1981:      * computed values, one is chosen, and returned to
jaroslav@1981:      * all the racing threads.
jaroslav@1981:      * <p>
jaroslav@1981:      * The {@code type} parameter is typically a class, but it may be any type,
jaroslav@1981:      * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
jaroslav@1981:      * <p>
jaroslav@1981:      * In the absence of {@code remove} calls, a class value has a simple
jaroslav@1981:      * state diagram:  uninitialized and initialized.
jaroslav@1981:      * When {@code remove} calls are made,
jaroslav@1981:      * the rules for value observation are more complex.
jaroslav@1981:      * See the documentation for {@link #remove remove} for more information.
jaroslav@1981:      *
jaroslav@1981:      * @param type the type whose class value must be computed or retrieved
jaroslav@1981:      * @return the current value associated with this {@code ClassValue}, for the given class or interface
jaroslav@1981:      * @throws NullPointerException if the argument is null
jaroslav@1981:      * @see #remove
jaroslav@1981:      * @see #computeValue
jaroslav@1981:      */
jaroslav@1981:     public T get(Class<?> type) {
jaroslav@1981:         ClassValueMap map = getMap(type);
jaroslav@1981:         if (map != null) {
jaroslav@1981:             Object x = map.get(this);
jaroslav@1981:             if (x != null) {
jaroslav@1981:                 return (T) map.unmaskNull(x);
jaroslav@1981:             }
jaroslav@1981:         }
jaroslav@1981:         return setComputedValue(type);
jaroslav@1981:     }
jaroslav@1981: 
jaroslav@1981:     /**
jaroslav@1981:      * Removes the associated value for the given class.
jaroslav@1981:      * If this value is subsequently {@linkplain #get read} for the same class,
jaroslav@1981:      * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
jaroslav@1981:      * This may result in an additional invocation of the
jaroslav@1981:      * {@code computeValue} method for the given class.
jaroslav@1981:      * <p>
jaroslav@1981:      * In order to explain the interaction between {@code get} and {@code remove} calls,
jaroslav@1981:      * we must model the state transitions of a class value to take into account
jaroslav@1981:      * the alternation between uninitialized and initialized states.
jaroslav@1981:      * To do this, number these states sequentially from zero, and note that
jaroslav@1981:      * uninitialized (or removed) states are numbered with even numbers,
jaroslav@1981:      * while initialized (or re-initialized) states have odd numbers.
jaroslav@1981:      * <p>
jaroslav@1981:      * When a thread {@code T} removes a class value in state {@code 2N},
jaroslav@1981:      * nothing happens, since the class value is already uninitialized.
jaroslav@1981:      * Otherwise, the state is advanced atomically to {@code 2N+1}.
jaroslav@1981:      * <p>
jaroslav@1981:      * When a thread {@code T} queries a class value in state {@code 2N},
jaroslav@1981:      * the thread first attempts to initialize the class value to state {@code 2N+1}
jaroslav@1981:      * by invoking {@code computeValue} and installing the resulting value.
jaroslav@1981:      * <p>
jaroslav@1981:      * When {@code T} attempts to install the newly computed value,
jaroslav@1981:      * if the state is still at {@code 2N}, the class value will be initialized
jaroslav@1981:      * with the computed value, advancing it to state {@code 2N+1}.
jaroslav@1981:      * <p>
jaroslav@1981:      * Otherwise, whether the new state is even or odd,
jaroslav@1981:      * {@code T} will discard the newly computed value
jaroslav@1981:      * and retry the {@code get} operation.
jaroslav@1981:      * <p>
jaroslav@1981:      * Discarding and retrying is an important proviso,
jaroslav@1981:      * since otherwise {@code T} could potentially install
jaroslav@1981:      * a disastrously stale value.  For example:
jaroslav@1981:      * <ul>
jaroslav@1981:      * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
jaroslav@1981:      * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
jaroslav@1981:      * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
jaroslav@1981:      * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
jaroslav@1981:      * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
jaroslav@1981:      * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
jaroslav@1981:      * <li> the previous actions of {@code T2} are repeated several times
jaroslav@1981:      * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
jaroslav@1981:      * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
jaroslav@1981:      * </ul>
jaroslav@1981:      * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
jaroslav@1981:      * observe the time-dependent states as it computes {@code V1}, etc.
jaroslav@1981:      * This does not remove the threat of a stale value, since there is a window of time
jaroslav@1981:      * between the return of {@code computeValue} in {@code T} and the installation
jaroslav@1981:      * of the the new value.  No user synchronization is possible during this time.
jaroslav@1981:      *
jaroslav@1981:      * @param type the type whose class value must be removed
jaroslav@1981:      * @throws NullPointerException if the argument is null
jaroslav@1981:      */
jaroslav@1981:     public void remove(Class<?> type) {
jaroslav@1981:         ClassValueMap map = getMap(type);
jaroslav@1981:         if (map != null) {
jaroslav@1981:             synchronized (map) {
jaroslav@1981:                 map.remove(this);
jaroslav@1981:             }
jaroslav@1981:         }
jaroslav@1981:     }
jaroslav@1981: 
jaroslav@1981:     /// Implementation...
jaroslav@1981:     // FIXME: Use a data structure here similar that of ThreadLocal (7030453).
jaroslav@1981: 
jaroslav@1981:     private static final AtomicInteger STORE_BARRIER = new AtomicInteger();
jaroslav@1981: 
jaroslav@1981:     /** Slow path for {@link #get}. */
jaroslav@1981:     private T setComputedValue(Class<?> type) {
jaroslav@1981:         ClassValueMap map = getMap(type);
jaroslav@1981:         if (map == null) {
jaroslav@1981:             map = initializeMap(type);
jaroslav@1981:         }
jaroslav@1981:         T value = computeValue(type);
jaroslav@1981:         STORE_BARRIER.lazySet(0);
jaroslav@1981:         // All stores pending from computeValue are completed.
jaroslav@1981:         synchronized (map) {
jaroslav@1981:             // Warm up the table with a null entry.
jaroslav@1981:             map.preInitializeEntry(this);
jaroslav@1981:         }
jaroslav@1981:         STORE_BARRIER.lazySet(0);
jaroslav@1981:         // All stores pending from table expansion are completed.
jaroslav@1981:         synchronized (map) {
jaroslav@1981:             value = (T) map.initializeEntry(this, value);
jaroslav@1981:             // One might fear a possible race condition here
jaroslav@1981:             // if the code for map.put has flushed the write
jaroslav@1981:             // to map.table[*] before the writes to the Map.Entry
jaroslav@1981:             // are done.  This is not possible, since we have
jaroslav@1981:             // warmed up the table with an empty entry.
jaroslav@1981:         }
jaroslav@1981:         return value;
jaroslav@1981:     }
jaroslav@1981: 
jaroslav@1981:     // Replace this map by a per-class slot.
jaroslav@1981:     private static final WeakHashMap<Class<?>, ClassValueMap> ROOT
jaroslav@1981:         = new WeakHashMap<Class<?>, ClassValueMap>();
jaroslav@1981: 
jaroslav@1981:     private static ClassValueMap getMap(Class<?> type) {
jaroslav@1981:         type.getClass();  // test for null
jaroslav@1981:         return ROOT.get(type);
jaroslav@1981:     }
jaroslav@1981: 
jaroslav@1981:     private static ClassValueMap initializeMap(Class<?> type) {
jaroslav@1981:         synchronized (ClassValue.class) {
jaroslav@1981:             ClassValueMap map = ROOT.get(type);
jaroslav@1981:             if (map == null)
jaroslav@1981:                 ROOT.put(type, map = new ClassValueMap());
jaroslav@1981:             return map;
jaroslav@1981:         }
jaroslav@1981:     }
jaroslav@1981: 
jaroslav@1981:     static class ClassValueMap extends WeakHashMap<ClassValue, Object> {
jaroslav@1981:         /** Make sure this table contains an Entry for the given key, even if it is empty. */
jaroslav@1981:         void preInitializeEntry(ClassValue key) {
jaroslav@1981:             if (!this.containsKey(key))
jaroslav@1981:                 this.put(key, null);
jaroslav@1981:         }
jaroslav@1981:         /** Make sure this table contains a non-empty Entry for the given key. */
jaroslav@1981:         Object initializeEntry(ClassValue key, Object value) {
jaroslav@1981:             Object prior = this.get(key);
jaroslav@1981:             if (prior != null) {
jaroslav@1981:                 return unmaskNull(prior);
jaroslav@1981:             }
jaroslav@1981:             this.put(key, maskNull(value));
jaroslav@1981:             return value;
jaroslav@1981:         }
jaroslav@1981: 
jaroslav@1981:         Object maskNull(Object x) {
jaroslav@1981:             return x == null ? this : x;
jaroslav@1981:         }
jaroslav@1981:         Object unmaskNull(Object x) {
jaroslav@1981:             return x == this ? null : x;
jaroslav@1981:         }
jaroslav@1981:     }
jaroslav@1981: }