1 /* 2 * Copyright (c) 2008, 2011, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. Oracle designates this 8 * particular file as subject to the "Classpath" exception as provided 9 * by Oracle in the LICENSE file that accompanied this code. 10 * 11 * This code is distributed in the hope that it will be useful, but WITHOUT 12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 14 * version 2 for more details (a copy is included in the LICENSE file that 15 * accompanied this code). 16 * 17 * You should have received a copy of the GNU General Public License version 18 * 2 along with this work; if not, write to the Free Software Foundation, 19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 20 * 21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 22 * or visit www.oracle.com if you need additional information or have any 23 * questions. 24 */ 25 26 package java.lang.invoke; 27 28 import java.util.concurrent.atomic.AtomicInteger; 29 30 /** 31 * A {@code MutableCallSite} is a {@link CallSite} whose target variable 32 * behaves like an ordinary field. 33 * An {@code invokedynamic} instruction linked to a {@code MutableCallSite} delegates 34 * all calls to the site's current target. 35 * The {@linkplain CallSite#dynamicInvoker dynamic invoker} of a mutable call site 36 * also delegates each call to the site's current target. 37 * <p> 38 * Here is an example of a mutable call site which introduces a 39 * state variable into a method handle chain. 40 * <!-- JavaDocExamplesTest.testMutableCallSite --> 41 * <blockquote><pre> 42 MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class)); 43 MethodHandle MH_name = name.dynamicInvoker(); 44 MethodType MT_str1 = MethodType.methodType(String.class); 45 MethodHandle MH_upcase = MethodHandles.lookup() 46 .findVirtual(String.class, "toUpperCase", MT_str1); 47 MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase); 48 name.setTarget(MethodHandles.constant(String.class, "Rocky")); 49 assertEquals("ROCKY", (String) worker1.invokeExact()); 50 name.setTarget(MethodHandles.constant(String.class, "Fred")); 51 assertEquals("FRED", (String) worker1.invokeExact()); 52 // (mutation can be continued indefinitely) 53 * </pre></blockquote> 54 * <p> 55 * The same call site may be used in several places at once. 56 * <blockquote><pre> 57 MethodType MT_str2 = MethodType.methodType(String.class, String.class); 58 MethodHandle MH_cat = lookup().findVirtual(String.class, 59 "concat", methodType(String.class, String.class)); 60 MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?"); 61 MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear); 62 assertEquals("Fred, dear?", (String) worker2.invokeExact()); 63 name.setTarget(MethodHandles.constant(String.class, "Wilma")); 64 assertEquals("WILMA", (String) worker1.invokeExact()); 65 assertEquals("Wilma, dear?", (String) worker2.invokeExact()); 66 * </pre></blockquote> 67 * <p> 68 * <em>Non-synchronization of target values:</em> 69 * A write to a mutable call site's target does not force other threads 70 * to become aware of the updated value. Threads which do not perform 71 * suitable synchronization actions relative to the updated call site 72 * may cache the old target value and delay their use of the new target 73 * value indefinitely. 74 * (This is a normal consequence of the Java Memory Model as applied 75 * to object fields.) 76 * <p> 77 * The {@link #syncAll syncAll} operation provides a way to force threads 78 * to accept a new target value, even if there is no other synchronization. 79 * <p> 80 * For target values which will be frequently updated, consider using 81 * a {@linkplain VolatileCallSite volatile call site} instead. 82 * @author John Rose, JSR 292 EG 83 */ 84 public class MutableCallSite extends CallSite { 85 /** 86 * Creates a blank call site object with the given method type. 87 * The initial target is set to a method handle of the given type 88 * which will throw an {@link IllegalStateException} if called. 89 * <p> 90 * The type of the call site is permanently set to the given type. 91 * <p> 92 * Before this {@code CallSite} object is returned from a bootstrap method, 93 * or invoked in some other manner, 94 * it is usually provided with a more useful target method, 95 * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}. 96 * @param type the method type that this call site will have 97 * @throws NullPointerException if the proposed type is null 98 */ 99 public MutableCallSite(MethodType type) { 100 super(type); 101 } 102 103 /** 104 * Creates a call site object with an initial target method handle. 105 * The type of the call site is permanently set to the initial target's type. 106 * @param target the method handle that will be the initial target of the call site 107 * @throws NullPointerException if the proposed target is null 108 */ 109 public MutableCallSite(MethodHandle target) { 110 super(target); 111 } 112 113 /** 114 * Returns the target method of the call site, which behaves 115 * like a normal field of the {@code MutableCallSite}. 116 * <p> 117 * The interactions of {@code getTarget} with memory are the same 118 * as of a read from an ordinary variable, such as an array element or a 119 * non-volatile, non-final field. 120 * <p> 121 * In particular, the current thread may choose to reuse the result 122 * of a previous read of the target from memory, and may fail to see 123 * a recent update to the target by another thread. 124 * 125 * @return the linkage state of this call site, a method handle which can change over time 126 * @see #setTarget 127 */ 128 @Override public final MethodHandle getTarget() { 129 return target; 130 } 131 132 /** 133 * Updates the target method of this call site, as a normal variable. 134 * The type of the new target must agree with the type of the old target. 135 * <p> 136 * The interactions with memory are the same 137 * as of a write to an ordinary variable, such as an array element or a 138 * non-volatile, non-final field. 139 * <p> 140 * In particular, unrelated threads may fail to see the updated target 141 * until they perform a read from memory. 142 * Stronger guarantees can be created by putting appropriate operations 143 * into the bootstrap method and/or the target methods used 144 * at any given call site. 145 * 146 * @param newTarget the new target 147 * @throws NullPointerException if the proposed new target is null 148 * @throws WrongMethodTypeException if the proposed new target 149 * has a method type that differs from the previous target 150 * @see #getTarget 151 */ 152 @Override public void setTarget(MethodHandle newTarget) { 153 checkTargetChange(this.target, newTarget); 154 setTargetNormal(newTarget); 155 } 156 157 /** 158 * {@inheritDoc} 159 */ 160 @Override 161 public final MethodHandle dynamicInvoker() { 162 return makeDynamicInvoker(); 163 } 164 165 /** 166 * Performs a synchronization operation on each call site in the given array, 167 * forcing all other threads to throw away any cached values previously 168 * loaded from the target of any of the call sites. 169 * <p> 170 * This operation does not reverse any calls that have already started 171 * on an old target value. 172 * (Java supports {@linkplain java.lang.Object#wait() forward time travel} only.) 173 * <p> 174 * The overall effect is to force all future readers of each call site's target 175 * to accept the most recently stored value. 176 * ("Most recently" is reckoned relative to the {@code syncAll} itself.) 177 * Conversely, the {@code syncAll} call may block until all readers have 178 * (somehow) decached all previous versions of each call site's target. 179 * <p> 180 * To avoid race conditions, calls to {@code setTarget} and {@code syncAll} 181 * should generally be performed under some sort of mutual exclusion. 182 * Note that reader threads may observe an updated target as early 183 * as the {@code setTarget} call that install the value 184 * (and before the {@code syncAll} that confirms the value). 185 * On the other hand, reader threads may observe previous versions of 186 * the target until the {@code syncAll} call returns 187 * (and after the {@code setTarget} that attempts to convey the updated version). 188 * <p> 189 * This operation is likely to be expensive and should be used sparingly. 190 * If possible, it should be buffered for batch processing on sets of call sites. 191 * <p> 192 * If {@code sites} contains a null element, 193 * a {@code NullPointerException} will be raised. 194 * In this case, some non-null elements in the array may be 195 * processed before the method returns abnormally. 196 * Which elements these are (if any) is implementation-dependent. 197 * 198 * <h1>Java Memory Model details</h1> 199 * In terms of the Java Memory Model, this operation performs a synchronization 200 * action which is comparable in effect to the writing of a volatile variable 201 * by the current thread, and an eventual volatile read by every other thread 202 * that may access one of the affected call sites. 203 * <p> 204 * The following effects are apparent, for each individual call site {@code S}: 205 * <ul> 206 * <li>A new volatile variable {@code V} is created, and written by the current thread. 207 * As defined by the JMM, this write is a global synchronization event. 208 * <li>As is normal with thread-local ordering of write events, 209 * every action already performed by the current thread is 210 * taken to happen before the volatile write to {@code V}. 211 * (In some implementations, this means that the current thread 212 * performs a global release operation.) 213 * <li>Specifically, the write to the current target of {@code S} is 214 * taken to happen before the volatile write to {@code V}. 215 * <li>The volatile write to {@code V} is placed 216 * (in an implementation specific manner) 217 * in the global synchronization order. 218 * <li>Consider an arbitrary thread {@code T} (other than the current thread). 219 * If {@code T} executes a synchronization action {@code A} 220 * after the volatile write to {@code V} (in the global synchronization order), 221 * it is therefore required to see either the current target 222 * of {@code S}, or a later write to that target, 223 * if it executes a read on the target of {@code S}. 224 * (This constraint is called "synchronization-order consistency".) 225 * <li>The JMM specifically allows optimizing compilers to elide 226 * reads or writes of variables that are known to be useless. 227 * Such elided reads and writes have no effect on the happens-before 228 * relation. Regardless of this fact, the volatile {@code V} 229 * will not be elided, even though its written value is 230 * indeterminate and its read value is not used. 231 * </ul> 232 * Because of the last point, the implementation behaves as if a 233 * volatile read of {@code V} were performed by {@code T} 234 * immediately after its action {@code A}. In the local ordering 235 * of actions in {@code T}, this read happens before any future 236 * read of the target of {@code S}. It is as if the 237 * implementation arbitrarily picked a read of {@code S}'s target 238 * by {@code T}, and forced a read of {@code V} to precede it, 239 * thereby ensuring communication of the new target value. 240 * <p> 241 * As long as the constraints of the Java Memory Model are obeyed, 242 * implementations may delay the completion of a {@code syncAll} 243 * operation while other threads ({@code T} above) continue to 244 * use previous values of {@code S}'s target. 245 * However, implementations are (as always) encouraged to avoid 246 * livelock, and to eventually require all threads to take account 247 * of the updated target. 248 * 249 * <p style="font-size:smaller;"> 250 * <em>Discussion:</em> 251 * For performance reasons, {@code syncAll} is not a virtual method 252 * on a single call site, but rather applies to a set of call sites. 253 * Some implementations may incur a large fixed overhead cost 254 * for processing one or more synchronization operations, 255 * but a small incremental cost for each additional call site. 256 * In any case, this operation is likely to be costly, since 257 * other threads may have to be somehow interrupted 258 * in order to make them notice the updated target value. 259 * However, it may be observed that a single call to synchronize 260 * several sites has the same formal effect as many calls, 261 * each on just one of the sites. 262 * 263 * <p style="font-size:smaller;"> 264 * <em>Implementation Note:</em> 265 * Simple implementations of {@code MutableCallSite} may use 266 * a volatile variable for the target of a mutable call site. 267 * In such an implementation, the {@code syncAll} method can be a no-op, 268 * and yet it will conform to the JMM behavior documented above. 269 * 270 * @param sites an array of call sites to be synchronized 271 * @throws NullPointerException if the {@code sites} array reference is null 272 * or the array contains a null 273 */ 274 public static void syncAll(MutableCallSite[] sites) { 275 if (sites.length == 0) return; 276 STORE_BARRIER.lazySet(0); 277 for (int i = 0; i < sites.length; i++) { 278 sites[i].getClass(); // trigger NPE on first null 279 } 280 // FIXME: NYI 281 } 282 private static final AtomicInteger STORE_BARRIER = new AtomicInteger(); 283 }