1.1 --- a/rt/emul/mini/src/main/java/java/lang/ClassValue.java Tue Jan 17 06:16:06 2017 +0100
1.2 +++ /dev/null Thu Jan 01 00:00:00 1970 +0000
1.3 @@ -1,239 +0,0 @@
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 -}