1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/rt/emul/compact/src/main/java/java/lang/invoke/MutableCallSite.java Sun Aug 17 20:09:05 2014 +0200
1.3 @@ -0,0 +1,283 @@
1.4 +/*
1.5 + * Copyright (c) 2008, 2013, 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.invoke;
1.30 +
1.31 +import java.util.concurrent.atomic.AtomicInteger;
1.32 +
1.33 +/**
1.34 + * A {@code MutableCallSite} is a {@link CallSite} whose target variable
1.35 + * behaves like an ordinary field.
1.36 + * An {@code invokedynamic} instruction linked to a {@code MutableCallSite} delegates
1.37 + * all calls to the site's current target.
1.38 + * The {@linkplain CallSite#dynamicInvoker dynamic invoker} of a mutable call site
1.39 + * also delegates each call to the site's current target.
1.40 + * <p>
1.41 + * Here is an example of a mutable call site which introduces a
1.42 + * state variable into a method handle chain.
1.43 + * <!-- JavaDocExamplesTest.testMutableCallSite -->
1.44 + * <blockquote><pre>{@code
1.45 +MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class));
1.46 +MethodHandle MH_name = name.dynamicInvoker();
1.47 +MethodType MT_str1 = MethodType.methodType(String.class);
1.48 +MethodHandle MH_upcase = MethodHandles.lookup()
1.49 + .findVirtual(String.class, "toUpperCase", MT_str1);
1.50 +MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase);
1.51 +name.setTarget(MethodHandles.constant(String.class, "Rocky"));
1.52 +assertEquals("ROCKY", (String) worker1.invokeExact());
1.53 +name.setTarget(MethodHandles.constant(String.class, "Fred"));
1.54 +assertEquals("FRED", (String) worker1.invokeExact());
1.55 +// (mutation can be continued indefinitely)
1.56 + * }</pre></blockquote>
1.57 + * <p>
1.58 + * The same call site may be used in several places at once.
1.59 + * <blockquote><pre>{@code
1.60 +MethodType MT_str2 = MethodType.methodType(String.class, String.class);
1.61 +MethodHandle MH_cat = lookup().findVirtual(String.class,
1.62 + "concat", methodType(String.class, String.class));
1.63 +MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?");
1.64 +MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear);
1.65 +assertEquals("Fred, dear?", (String) worker2.invokeExact());
1.66 +name.setTarget(MethodHandles.constant(String.class, "Wilma"));
1.67 +assertEquals("WILMA", (String) worker1.invokeExact());
1.68 +assertEquals("Wilma, dear?", (String) worker2.invokeExact());
1.69 + * }</pre></blockquote>
1.70 + * <p>
1.71 + * <em>Non-synchronization of target values:</em>
1.72 + * A write to a mutable call site's target does not force other threads
1.73 + * to become aware of the updated value. Threads which do not perform
1.74 + * suitable synchronization actions relative to the updated call site
1.75 + * may cache the old target value and delay their use of the new target
1.76 + * value indefinitely.
1.77 + * (This is a normal consequence of the Java Memory Model as applied
1.78 + * to object fields.)
1.79 + * <p>
1.80 + * The {@link #syncAll syncAll} operation provides a way to force threads
1.81 + * to accept a new target value, even if there is no other synchronization.
1.82 + * <p>
1.83 + * For target values which will be frequently updated, consider using
1.84 + * a {@linkplain VolatileCallSite volatile call site} instead.
1.85 + * @author John Rose, JSR 292 EG
1.86 + */
1.87 +public class MutableCallSite extends CallSite {
1.88 + /**
1.89 + * Creates a blank call site object with the given method type.
1.90 + * The initial target is set to a method handle of the given type
1.91 + * which will throw an {@link IllegalStateException} if called.
1.92 + * <p>
1.93 + * The type of the call site is permanently set to the given type.
1.94 + * <p>
1.95 + * Before this {@code CallSite} object is returned from a bootstrap method,
1.96 + * or invoked in some other manner,
1.97 + * it is usually provided with a more useful target method,
1.98 + * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.
1.99 + * @param type the method type that this call site will have
1.100 + * @throws NullPointerException if the proposed type is null
1.101 + */
1.102 + public MutableCallSite(MethodType type) {
1.103 + super(type);
1.104 + }
1.105 +
1.106 + /**
1.107 + * Creates a call site object with an initial target method handle.
1.108 + * The type of the call site is permanently set to the initial target's type.
1.109 + * @param target the method handle that will be the initial target of the call site
1.110 + * @throws NullPointerException if the proposed target is null
1.111 + */
1.112 + public MutableCallSite(MethodHandle target) {
1.113 + super(target);
1.114 + }
1.115 +
1.116 + /**
1.117 + * Returns the target method of the call site, which behaves
1.118 + * like a normal field of the {@code MutableCallSite}.
1.119 + * <p>
1.120 + * The interactions of {@code getTarget} with memory are the same
1.121 + * as of a read from an ordinary variable, such as an array element or a
1.122 + * non-volatile, non-final field.
1.123 + * <p>
1.124 + * In particular, the current thread may choose to reuse the result
1.125 + * of a previous read of the target from memory, and may fail to see
1.126 + * a recent update to the target by another thread.
1.127 + *
1.128 + * @return the linkage state of this call site, a method handle which can change over time
1.129 + * @see #setTarget
1.130 + */
1.131 + @Override public final MethodHandle getTarget() {
1.132 + return target;
1.133 + }
1.134 +
1.135 + /**
1.136 + * Updates the target method of this call site, as a normal variable.
1.137 + * The type of the new target must agree with the type of the old target.
1.138 + * <p>
1.139 + * The interactions with memory are the same
1.140 + * as of a write to an ordinary variable, such as an array element or a
1.141 + * non-volatile, non-final field.
1.142 + * <p>
1.143 + * In particular, unrelated threads may fail to see the updated target
1.144 + * until they perform a read from memory.
1.145 + * Stronger guarantees can be created by putting appropriate operations
1.146 + * into the bootstrap method and/or the target methods used
1.147 + * at any given call site.
1.148 + *
1.149 + * @param newTarget the new target
1.150 + * @throws NullPointerException if the proposed new target is null
1.151 + * @throws WrongMethodTypeException if the proposed new target
1.152 + * has a method type that differs from the previous target
1.153 + * @see #getTarget
1.154 + */
1.155 + @Override public void setTarget(MethodHandle newTarget) {
1.156 + checkTargetChange(this.target, newTarget);
1.157 + setTargetNormal(newTarget);
1.158 + }
1.159 +
1.160 + /**
1.161 + * {@inheritDoc}
1.162 + */
1.163 + @Override
1.164 + public final MethodHandle dynamicInvoker() {
1.165 + return makeDynamicInvoker();
1.166 + }
1.167 +
1.168 + /**
1.169 + * Performs a synchronization operation on each call site in the given array,
1.170 + * forcing all other threads to throw away any cached values previously
1.171 + * loaded from the target of any of the call sites.
1.172 + * <p>
1.173 + * This operation does not reverse any calls that have already started
1.174 + * on an old target value.
1.175 + * (Java supports {@linkplain java.lang.Object#wait() forward time travel} only.)
1.176 + * <p>
1.177 + * The overall effect is to force all future readers of each call site's target
1.178 + * to accept the most recently stored value.
1.179 + * ("Most recently" is reckoned relative to the {@code syncAll} itself.)
1.180 + * Conversely, the {@code syncAll} call may block until all readers have
1.181 + * (somehow) decached all previous versions of each call site's target.
1.182 + * <p>
1.183 + * To avoid race conditions, calls to {@code setTarget} and {@code syncAll}
1.184 + * should generally be performed under some sort of mutual exclusion.
1.185 + * Note that reader threads may observe an updated target as early
1.186 + * as the {@code setTarget} call that install the value
1.187 + * (and before the {@code syncAll} that confirms the value).
1.188 + * On the other hand, reader threads may observe previous versions of
1.189 + * the target until the {@code syncAll} call returns
1.190 + * (and after the {@code setTarget} that attempts to convey the updated version).
1.191 + * <p>
1.192 + * This operation is likely to be expensive and should be used sparingly.
1.193 + * If possible, it should be buffered for batch processing on sets of call sites.
1.194 + * <p>
1.195 + * If {@code sites} contains a null element,
1.196 + * a {@code NullPointerException} will be raised.
1.197 + * In this case, some non-null elements in the array may be
1.198 + * processed before the method returns abnormally.
1.199 + * Which elements these are (if any) is implementation-dependent.
1.200 + *
1.201 + * <h1>Java Memory Model details</h1>
1.202 + * In terms of the Java Memory Model, this operation performs a synchronization
1.203 + * action which is comparable in effect to the writing of a volatile variable
1.204 + * by the current thread, and an eventual volatile read by every other thread
1.205 + * that may access one of the affected call sites.
1.206 + * <p>
1.207 + * The following effects are apparent, for each individual call site {@code S}:
1.208 + * <ul>
1.209 + * <li>A new volatile variable {@code V} is created, and written by the current thread.
1.210 + * As defined by the JMM, this write is a global synchronization event.
1.211 + * <li>As is normal with thread-local ordering of write events,
1.212 + * every action already performed by the current thread is
1.213 + * taken to happen before the volatile write to {@code V}.
1.214 + * (In some implementations, this means that the current thread
1.215 + * performs a global release operation.)
1.216 + * <li>Specifically, the write to the current target of {@code S} is
1.217 + * taken to happen before the volatile write to {@code V}.
1.218 + * <li>The volatile write to {@code V} is placed
1.219 + * (in an implementation specific manner)
1.220 + * in the global synchronization order.
1.221 + * <li>Consider an arbitrary thread {@code T} (other than the current thread).
1.222 + * If {@code T} executes a synchronization action {@code A}
1.223 + * after the volatile write to {@code V} (in the global synchronization order),
1.224 + * it is therefore required to see either the current target
1.225 + * of {@code S}, or a later write to that target,
1.226 + * if it executes a read on the target of {@code S}.
1.227 + * (This constraint is called "synchronization-order consistency".)
1.228 + * <li>The JMM specifically allows optimizing compilers to elide
1.229 + * reads or writes of variables that are known to be useless.
1.230 + * Such elided reads and writes have no effect on the happens-before
1.231 + * relation. Regardless of this fact, the volatile {@code V}
1.232 + * will not be elided, even though its written value is
1.233 + * indeterminate and its read value is not used.
1.234 + * </ul>
1.235 + * Because of the last point, the implementation behaves as if a
1.236 + * volatile read of {@code V} were performed by {@code T}
1.237 + * immediately after its action {@code A}. In the local ordering
1.238 + * of actions in {@code T}, this read happens before any future
1.239 + * read of the target of {@code S}. It is as if the
1.240 + * implementation arbitrarily picked a read of {@code S}'s target
1.241 + * by {@code T}, and forced a read of {@code V} to precede it,
1.242 + * thereby ensuring communication of the new target value.
1.243 + * <p>
1.244 + * As long as the constraints of the Java Memory Model are obeyed,
1.245 + * implementations may delay the completion of a {@code syncAll}
1.246 + * operation while other threads ({@code T} above) continue to
1.247 + * use previous values of {@code S}'s target.
1.248 + * However, implementations are (as always) encouraged to avoid
1.249 + * livelock, and to eventually require all threads to take account
1.250 + * of the updated target.
1.251 + *
1.252 + * <p style="font-size:smaller;">
1.253 + * <em>Discussion:</em>
1.254 + * For performance reasons, {@code syncAll} is not a virtual method
1.255 + * on a single call site, but rather applies to a set of call sites.
1.256 + * Some implementations may incur a large fixed overhead cost
1.257 + * for processing one or more synchronization operations,
1.258 + * but a small incremental cost for each additional call site.
1.259 + * In any case, this operation is likely to be costly, since
1.260 + * other threads may have to be somehow interrupted
1.261 + * in order to make them notice the updated target value.
1.262 + * However, it may be observed that a single call to synchronize
1.263 + * several sites has the same formal effect as many calls,
1.264 + * each on just one of the sites.
1.265 + *
1.266 + * <p style="font-size:smaller;">
1.267 + * <em>Implementation Note:</em>
1.268 + * Simple implementations of {@code MutableCallSite} may use
1.269 + * a volatile variable for the target of a mutable call site.
1.270 + * In such an implementation, the {@code syncAll} method can be a no-op,
1.271 + * and yet it will conform to the JMM behavior documented above.
1.272 + *
1.273 + * @param sites an array of call sites to be synchronized
1.274 + * @throws NullPointerException if the {@code sites} array reference is null
1.275 + * or the array contains a null
1.276 + */
1.277 + public static void syncAll(MutableCallSite[] sites) {
1.278 + if (sites.length == 0) return;
1.279 + STORE_BARRIER.lazySet(0);
1.280 + for (int i = 0; i < sites.length; i++) {
1.281 + sites[i].getClass(); // trigger NPE on first null
1.282 + }
1.283 + // FIXME: NYI
1.284 + }
1.285 + private static final AtomicInteger STORE_BARRIER = new AtomicInteger();
1.286 +}