1 /* 2 * Copyright (c) 2010, 2013, 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 /** 29 * <p> 30 * A {@code SwitchPoint} is an object which can publish state transitions to other threads. 31 * A switch point is initially in the <em>valid</em> state, but may at any time be 32 * changed to the <em>invalid</em> state. Invalidation cannot be reversed. 33 * A switch point can combine a <em>guarded pair</em> of method handles into a 34 * <em>guarded delegator</em>. 35 * The guarded delegator is a method handle which delegates to one of the old method handles. 36 * The state of the switch point determines which of the two gets the delegation. 37 * <p> 38 * A single switch point may be used to control any number of method handles. 39 * (Indirectly, therefore, it can control any number of call sites.) 40 * This is done by using the single switch point as a factory for combining 41 * any number of guarded method handle pairs into guarded delegators. 42 * <p> 43 * When a guarded delegator is created from a guarded pair, the pair 44 * is wrapped in a new method handle {@code M}, 45 * which is permanently associated with the switch point that created it. 46 * Each pair consists of a target {@code T} and a fallback {@code F}. 47 * While the switch point is valid, invocations to {@code M} are delegated to {@code T}. 48 * After it is invalidated, invocations are delegated to {@code F}. 49 * <p> 50 * Invalidation is global and immediate, as if the switch point contained a 51 * volatile boolean variable consulted on every call to {@code M}. 52 * The invalidation is also permanent, which means the switch point 53 * can change state only once. 54 * The switch point will always delegate to {@code F} after being invalidated. 55 * At that point {@code guardWithTest} may ignore {@code T} and return {@code F}. 56 * <p> 57 * Here is an example of a switch point in action: 58 * <pre>{@code 59 * MethodHandle MH_strcat = MethodHandles.lookup() 60 * .findVirtual(String.class, "concat", MethodType.methodType(String.class, String.class)); 61 * SwitchPoint spt = new SwitchPoint(); 62 * assert(!spt.hasBeenInvalidated()); 63 * // the following steps may be repeated to re-use the same switch point: 64 * MethodHandle worker1 = MH_strcat; 65 * MethodHandle worker2 = MethodHandles.permuteArguments(MH_strcat, MH_strcat.type(), 1, 0); 66 * MethodHandle worker = spt.guardWithTest(worker1, worker2); 67 * assertEquals("method", (String) worker.invokeExact("met", "hod")); 68 * SwitchPoint.invalidateAll(new SwitchPoint[]{ spt }); 69 * assert(spt.hasBeenInvalidated()); 70 * assertEquals("hodmet", (String) worker.invokeExact("met", "hod")); 71 * }</pre> 72 * <p style="font-size:smaller;"> 73 * <em>Discussion:</em> 74 * Switch points are useful without subclassing. They may also be subclassed. 75 * This may be useful in order to associate application-specific invalidation logic 76 * with the switch point. 77 * Notice that there is no permanent association between a switch point and 78 * the method handles it produces and consumes. 79 * The garbage collector may collect method handles produced or consumed 80 * by a switch point independently of the lifetime of the switch point itself. 81 * <p style="font-size:smaller;"> 82 * <em>Implementation Note:</em> 83 * A switch point behaves as if implemented on top of {@link MutableCallSite}, 84 * approximately as follows: 85 * <pre>{@code 86 * public class SwitchPoint { 87 * private static final MethodHandle 88 * K_true = MethodHandles.constant(boolean.class, true), 89 * K_false = MethodHandles.constant(boolean.class, false); 90 * private final MutableCallSite mcs; 91 * private final MethodHandle mcsInvoker; 92 * public SwitchPoint() { 93 * this.mcs = new MutableCallSite(K_true); 94 * this.mcsInvoker = mcs.dynamicInvoker(); 95 * } 96 * public MethodHandle guardWithTest( 97 * MethodHandle target, MethodHandle fallback) { 98 * // Note: mcsInvoker is of type ()boolean. 99 * // Target and fallback may take any arguments, but must have the same type. 100 * return MethodHandles.guardWithTest(this.mcsInvoker, target, fallback); 101 * } 102 * public static void invalidateAll(SwitchPoint[] spts) { 103 * List<MutableCallSite> mcss = new ArrayList<>(); 104 * for (SwitchPoint spt : spts) mcss.add(spt.mcs); 105 * for (MutableCallSite mcs : mcss) mcs.setTarget(K_false); 106 * MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0])); 107 * } 108 * } 109 * }</pre> 110 * @author Remi Forax, JSR 292 EG 111 * @since 1.7 112 */ 113 public class SwitchPoint { 114 private static final MethodHandle 115 K_true = MethodHandles.constant(boolean.class, true), 116 K_false = MethodHandles.constant(boolean.class, false); 117 118 private final MutableCallSite mcs; 119 private final MethodHandle mcsInvoker; 120 121 /** 122 * Creates a new switch point. 123 */ SwitchPoint()124 public SwitchPoint() { 125 this.mcs = new MutableCallSite(K_true); 126 this.mcsInvoker = mcs.dynamicInvoker(); 127 } 128 129 /** 130 * Determines if this switch point has been invalidated yet. 131 * 132 * <p style="font-size:smaller;"> 133 * <em>Discussion:</em> 134 * Because of the one-way nature of invalidation, once a switch point begins 135 * to return true for {@code hasBeenInvalidated}, 136 * it will always do so in the future. 137 * On the other hand, a valid switch point visible to other threads may 138 * be invalidated at any moment, due to a request by another thread. 139 * <p style="font-size:smaller;"> 140 * Since invalidation is a global and immediate operation, 141 * the execution of this query, on a valid switchpoint, 142 * must be internally sequenced with any 143 * other threads that could cause invalidation. 144 * This query may therefore be expensive. 145 * The recommended way to build a boolean-valued method handle 146 * which queries the invalidation state of a switch point {@code s} is 147 * to call {@code s.guardWithTest} on 148 * {@link MethodHandles#constant constant} true and false method handles. 149 * 150 * @return true if this switch point has been invalidated 151 */ hasBeenInvalidated()152 public boolean hasBeenInvalidated() { 153 return (mcs.getTarget() != K_true); 154 } 155 156 /** 157 * Returns a method handle which always delegates either to the target or the fallback. 158 * The method handle will delegate to the target exactly as long as the switch point is valid. 159 * After that, it will permanently delegate to the fallback. 160 * <p> 161 * The target and fallback must be of exactly the same method type, 162 * and the resulting combined method handle will also be of this type. 163 * 164 * @param target the method handle selected by the switch point as long as it is valid 165 * @param fallback the method handle selected by the switch point after it is invalidated 166 * @return a combined method handle which always calls either the target or fallback 167 * @throws NullPointerException if either argument is null 168 * @throws IllegalArgumentException if the two method types do not match 169 * @see MethodHandles#guardWithTest 170 */ guardWithTest(MethodHandle target, MethodHandle fallback)171 public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) { 172 if (mcs.getTarget() == K_false) 173 return fallback; // already invalid 174 return MethodHandles.guardWithTest(mcsInvoker, target, fallback); 175 } 176 177 /** 178 * Sets all of the given switch points into the invalid state. 179 * After this call executes, no thread will observe any of the 180 * switch points to be in a valid state. 181 * <p> 182 * This operation is likely to be expensive and should be used sparingly. 183 * If possible, it should be buffered for batch processing on sets of switch points. 184 * <p> 185 * If {@code switchPoints} contains a null element, 186 * a {@code NullPointerException} will be raised. 187 * In this case, some non-null elements in the array may be 188 * processed before the method returns abnormally. 189 * Which elements these are (if any) is implementation-dependent. 190 * 191 * <p style="font-size:smaller;"> 192 * <em>Discussion:</em> 193 * For performance reasons, {@code invalidateAll} is not a virtual method 194 * on a single switch point, but rather applies to a set of switch points. 195 * Some implementations may incur a large fixed overhead cost 196 * for processing one or more invalidation operations, 197 * but a small incremental cost for each additional invalidation. 198 * In any case, this operation is likely to be costly, since 199 * other threads may have to be somehow interrupted 200 * in order to make them notice the updated switch point state. 201 * However, it may be observed that a single call to invalidate 202 * several switch points has the same formal effect as many calls, 203 * each on just one of the switch points. 204 * 205 * <p style="font-size:smaller;"> 206 * <em>Implementation Note:</em> 207 * Simple implementations of {@code SwitchPoint} may use 208 * a private {@link MutableCallSite} to publish the state of a switch point. 209 * In such an implementation, the {@code invalidateAll} method can 210 * simply change the call site's target, and issue one call to 211 * {@linkplain MutableCallSite#syncAll synchronize} all the 212 * private call sites. 213 * 214 * @param switchPoints an array of call sites to be synchronized 215 * @throws NullPointerException if the {@code switchPoints} array reference is null 216 * or the array contains a null 217 */ invalidateAll(SwitchPoint[] switchPoints)218 public static void invalidateAll(SwitchPoint[] switchPoints) { 219 if (switchPoints.length == 0) return; 220 MutableCallSite[] sites = new MutableCallSite[switchPoints.length]; 221 for (int i = 0; i < switchPoints.length; i++) { 222 SwitchPoint spt = switchPoints[i]; 223 if (spt == null) break; // MSC.syncAll will trigger a NPE 224 sites[i] = spt.mcs; 225 spt.mcs.setTarget(K_false); 226 } 227 MutableCallSite.syncAll(sites); 228 } 229 } 230