# HG changeset patch # User Jaroslav Tulach # Date 1484630109 -3600 # Node ID 84dba41e2e9229c16b4afd358e6d77783503256b # Parent cee8ecc45816e5e3d1b14f0e9f70b5626512071f# Parent 189dcebe46c623353aeeb3ab0966a72b80b158c0 Bringing in ClassValue implementation diff -r cee8ecc45816 -r 84dba41e2e92 emul/src/main/java/java/lang/ClassValue.java --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/emul/src/main/java/java/lang/ClassValue.java Tue Jan 17 06:15:09 2017 +0100 @@ -0,0 +1,239 @@ +/* + * 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 + * or visit www.oracle.com if you need additional information or have any + * questions. + */ + +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 { + /** + * Sole constructor. (For invocation by subclass constructors, typically + * implicit.) + */ + protected ClassValue() { + } + + /** + * Computes the given class's derived value for this {@code ClassValue}. + *

+ * This method will be invoked within the first thread that accesses + * the value with the {@link #get get} method. + *

+ * 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}. + *

+ * 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. + *

+ * 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. + *

+ * 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}. + *

+ * 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. + *

+ * 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. + *

+ * 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}. + *

+ * 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. + *

+ * 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}. + *

+ * Otherwise, whether the new state is even or odd, + * {@code T} will discard the newly computed value + * and retry the {@code get} operation. + *

+ * Discarding and retrying is an important proviso, + * since otherwise {@code T} could potentially install + * a disastrously stale value. For example: + *

+ * 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, ClassValueMap> ROOT + = new WeakHashMap, 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 { + /** 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; + } + } +}