1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/emul/src/main/java/java/lang/ClassValue.java Tue Jan 17 06:11:57 2017 +0100
1.3 @@ -0,0 +1,239 @@
1.4 +/*
1.5 + * Copyright (c) 2010, 2011, Oracle and/or its affiliates. All rights reserved.
1.6 + * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
1.7 + *
1.8 + * This code is free software; you can redistribute it and/or modify it
1.9 + * under the terms of the GNU General Public License version 2 only, as
1.10 + * published by the Free Software Foundation. Oracle designates this
1.11 + * particular file as subject to the "Classpath" exception as provided
1.12 + * by Oracle in the LICENSE file that accompanied this code.
1.13 + *
1.14 + * This code is distributed in the hope that it will be useful, but WITHOUT
1.15 + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
1.16 + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
1.17 + * version 2 for more details (a copy is included in the LICENSE file that
1.18 + * accompanied this code).
1.19 + *
1.20 + * You should have received a copy of the GNU General Public License version
1.21 + * 2 along with this work; if not, write to the Free Software Foundation,
1.22 + * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
1.23 + *
1.24 + * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
1.25 + * or visit www.oracle.com if you need additional information or have any
1.26 + * questions.
1.27 + */
1.28 +
1.29 +package java.lang;
1.30 +
1.31 +import java.util.WeakHashMap;
1.32 +import java.util.concurrent.atomic.AtomicInteger;
1.33 +
1.34 +/**
1.35 + * Lazily associate a computed value with (potentially) every type.
1.36 + * For example, if a dynamic language needs to construct a message dispatch
1.37 + * table for each class encountered at a message send call site,
1.38 + * it can use a {@code ClassValue} to cache information needed to
1.39 + * perform the message send quickly, for each class encountered.
1.40 + * @author John Rose, JSR 292 EG
1.41 + * @since 1.7
1.42 + */
1.43 +public abstract class ClassValue<T> {
1.44 + /**
1.45 + * Sole constructor. (For invocation by subclass constructors, typically
1.46 + * implicit.)
1.47 + */
1.48 + protected ClassValue() {
1.49 + }
1.50 +
1.51 + /**
1.52 + * Computes the given class's derived value for this {@code ClassValue}.
1.53 + * <p>
1.54 + * This method will be invoked within the first thread that accesses
1.55 + * the value with the {@link #get get} method.
1.56 + * <p>
1.57 + * Normally, this method is invoked at most once per class,
1.58 + * but it may be invoked again if there has been a call to
1.59 + * {@link #remove remove}.
1.60 + * <p>
1.61 + * If this method throws an exception, the corresponding call to {@code get}
1.62 + * will terminate abnormally with that exception, and no class value will be recorded.
1.63 + *
1.64 + * @param type the type whose class value must be computed
1.65 + * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
1.66 + * @see #get
1.67 + * @see #remove
1.68 + */
1.69 + protected abstract T computeValue(Class<?> type);
1.70 +
1.71 + /**
1.72 + * Returns the value for the given class.
1.73 + * If no value has yet been computed, it is obtained by
1.74 + * an invocation of the {@link #computeValue computeValue} method.
1.75 + * <p>
1.76 + * The actual installation of the value on the class
1.77 + * is performed atomically.
1.78 + * At that point, if several racing threads have
1.79 + * computed values, one is chosen, and returned to
1.80 + * all the racing threads.
1.81 + * <p>
1.82 + * The {@code type} parameter is typically a class, but it may be any type,
1.83 + * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
1.84 + * <p>
1.85 + * In the absence of {@code remove} calls, a class value has a simple
1.86 + * state diagram: uninitialized and initialized.
1.87 + * When {@code remove} calls are made,
1.88 + * the rules for value observation are more complex.
1.89 + * See the documentation for {@link #remove remove} for more information.
1.90 + *
1.91 + * @param type the type whose class value must be computed or retrieved
1.92 + * @return the current value associated with this {@code ClassValue}, for the given class or interface
1.93 + * @throws NullPointerException if the argument is null
1.94 + * @see #remove
1.95 + * @see #computeValue
1.96 + */
1.97 + public T get(Class<?> type) {
1.98 + ClassValueMap map = getMap(type);
1.99 + if (map != null) {
1.100 + Object x = map.get(this);
1.101 + if (x != null) {
1.102 + return (T) map.unmaskNull(x);
1.103 + }
1.104 + }
1.105 + return setComputedValue(type);
1.106 + }
1.107 +
1.108 + /**
1.109 + * Removes the associated value for the given class.
1.110 + * If this value is subsequently {@linkplain #get read} for the same class,
1.111 + * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
1.112 + * This may result in an additional invocation of the
1.113 + * {@code computeValue} method for the given class.
1.114 + * <p>
1.115 + * In order to explain the interaction between {@code get} and {@code remove} calls,
1.116 + * we must model the state transitions of a class value to take into account
1.117 + * the alternation between uninitialized and initialized states.
1.118 + * To do this, number these states sequentially from zero, and note that
1.119 + * uninitialized (or removed) states are numbered with even numbers,
1.120 + * while initialized (or re-initialized) states have odd numbers.
1.121 + * <p>
1.122 + * When a thread {@code T} removes a class value in state {@code 2N},
1.123 + * nothing happens, since the class value is already uninitialized.
1.124 + * Otherwise, the state is advanced atomically to {@code 2N+1}.
1.125 + * <p>
1.126 + * When a thread {@code T} queries a class value in state {@code 2N},
1.127 + * the thread first attempts to initialize the class value to state {@code 2N+1}
1.128 + * by invoking {@code computeValue} and installing the resulting value.
1.129 + * <p>
1.130 + * When {@code T} attempts to install the newly computed value,
1.131 + * if the state is still at {@code 2N}, the class value will be initialized
1.132 + * with the computed value, advancing it to state {@code 2N+1}.
1.133 + * <p>
1.134 + * Otherwise, whether the new state is even or odd,
1.135 + * {@code T} will discard the newly computed value
1.136 + * and retry the {@code get} operation.
1.137 + * <p>
1.138 + * Discarding and retrying is an important proviso,
1.139 + * since otherwise {@code T} could potentially install
1.140 + * a disastrously stale value. For example:
1.141 + * <ul>
1.142 + * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
1.143 + * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
1.144 + * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
1.145 + * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
1.146 + * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
1.147 + * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
1.148 + * <li> the previous actions of {@code T2} are repeated several times
1.149 + * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
1.150 + * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
1.151 + * </ul>
1.152 + * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
1.153 + * observe the time-dependent states as it computes {@code V1}, etc.
1.154 + * This does not remove the threat of a stale value, since there is a window of time
1.155 + * between the return of {@code computeValue} in {@code T} and the installation
1.156 + * of the the new value. No user synchronization is possible during this time.
1.157 + *
1.158 + * @param type the type whose class value must be removed
1.159 + * @throws NullPointerException if the argument is null
1.160 + */
1.161 + public void remove(Class<?> type) {
1.162 + ClassValueMap map = getMap(type);
1.163 + if (map != null) {
1.164 + synchronized (map) {
1.165 + map.remove(this);
1.166 + }
1.167 + }
1.168 + }
1.169 +
1.170 + /// Implementation...
1.171 + // FIXME: Use a data structure here similar that of ThreadLocal (7030453).
1.172 +
1.173 + private static final AtomicInteger STORE_BARRIER = new AtomicInteger();
1.174 +
1.175 + /** Slow path for {@link #get}. */
1.176 + private T setComputedValue(Class<?> type) {
1.177 + ClassValueMap map = getMap(type);
1.178 + if (map == null) {
1.179 + map = initializeMap(type);
1.180 + }
1.181 + T value = computeValue(type);
1.182 + STORE_BARRIER.lazySet(0);
1.183 + // All stores pending from computeValue are completed.
1.184 + synchronized (map) {
1.185 + // Warm up the table with a null entry.
1.186 + map.preInitializeEntry(this);
1.187 + }
1.188 + STORE_BARRIER.lazySet(0);
1.189 + // All stores pending from table expansion are completed.
1.190 + synchronized (map) {
1.191 + value = (T) map.initializeEntry(this, value);
1.192 + // One might fear a possible race condition here
1.193 + // if the code for map.put has flushed the write
1.194 + // to map.table[*] before the writes to the Map.Entry
1.195 + // are done. This is not possible, since we have
1.196 + // warmed up the table with an empty entry.
1.197 + }
1.198 + return value;
1.199 + }
1.200 +
1.201 + // Replace this map by a per-class slot.
1.202 + private static final WeakHashMap<Class<?>, ClassValueMap> ROOT
1.203 + = new WeakHashMap<Class<?>, ClassValueMap>();
1.204 +
1.205 + private static ClassValueMap getMap(Class<?> type) {
1.206 + type.getClass(); // test for null
1.207 + return ROOT.get(type);
1.208 + }
1.209 +
1.210 + private static ClassValueMap initializeMap(Class<?> type) {
1.211 + synchronized (ClassValue.class) {
1.212 + ClassValueMap map = ROOT.get(type);
1.213 + if (map == null)
1.214 + ROOT.put(type, map = new ClassValueMap());
1.215 + return map;
1.216 + }
1.217 + }
1.218 +
1.219 + static class ClassValueMap extends WeakHashMap<ClassValue, Object> {
1.220 + /** Make sure this table contains an Entry for the given key, even if it is empty. */
1.221 + void preInitializeEntry(ClassValue key) {
1.222 + if (!this.containsKey(key))
1.223 + this.put(key, null);
1.224 + }
1.225 + /** Make sure this table contains a non-empty Entry for the given key. */
1.226 + Object initializeEntry(ClassValue key, Object value) {
1.227 + Object prior = this.get(key);
1.228 + if (prior != null) {
1.229 + return unmaskNull(prior);
1.230 + }
1.231 + this.put(key, maskNull(value));
1.232 + return value;
1.233 + }
1.234 +
1.235 + Object maskNull(Object x) {
1.236 + return x == null ? this : x;
1.237 + }
1.238 + Object unmaskNull(Object x) {
1.239 + return x == this ? null : x;
1.240 + }
1.241 + }
1.242 +}