ClassValue.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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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> {
    /**
     * 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 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) {
        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;
        }
    }
}