1 /*
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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 * <blockquote><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></blockquote>
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 * <blockquote><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></blockquote>
110 * @author Remi Forax, JSR 292 EG
111 */
112 public class SwitchPoint {
113 private static final MethodHandle
114 K_true = MethodHandles.constant(boolean.class, true),
115 K_false = MethodHandles.constant(boolean.class, false);
116
117 private final MutableCallSite mcs;
118 private final MethodHandle mcsInvoker;
119
120 /**
121 * Creates a new switch point.
122 */
123 public SwitchPoint() {
124 this.mcs = new MutableCallSite(K_true);
125 this.mcsInvoker = mcs.dynamicInvoker();
126 }
127
128 /**
129 * Determines if this switch point has been invalidated yet.
130 *
131 * <p style="font-size:smaller;">
132 * <em>Discussion:</em>
133 * Because of the one-way nature of invalidation, once a switch point begins
134 * to return true for {@code hasBeenInvalidated},
135 * it will always do so in the future.
136 * On the other hand, a valid switch point visible to other threads may
137 * be invalidated at any moment, due to a request by another thread.
138 * <p style="font-size:smaller;">
139 * Since invalidation is a global and immediate operation,
140 * the execution of this query, on a valid switchpoint,
141 * must be internally sequenced with any
142 * other threads that could cause invalidation.
143 * This query may therefore be expensive.
144 * The recommended way to build a boolean-valued method handle
145 * which queries the invalidation state of a switch point {@code s} is
146 * to call {@code s.guardWithTest} on
147 * {@link MethodHandles#constant constant} true and false method handles.
148 *
149 * @return true if this switch point has been invalidated
150 */
151 public boolean hasBeenInvalidated() {
152 return (mcs.getTarget() != K_true);
153 }
154
155 /**
156 * Returns a method handle which always delegates either to the target or the fallback.
157 * The method handle will delegate to the target exactly as long as the switch point is valid.
158 * After that, it will permanently delegate to the fallback.
159 * <p>
160 * The target and fallback must be of exactly the same method type,
161 * and the resulting combined method handle will also be of this type.
162 *
163 * @param target the method handle selected by the switch point as long as it is valid
164 * @param fallback the method handle selected by the switch point after it is invalidated
165 * @return a combined method handle which always calls either the target or fallback
166 * @throws NullPointerException if either argument is null
167 * @throws IllegalArgumentException if the two method types do not match
168 * @see MethodHandles#guardWithTest
169 */
170 public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) {
171 if (mcs.getTarget() == K_false)
172 return fallback; // already invalid
173 return MethodHandles.guardWithTest(mcsInvoker, target, fallback);
174 }
175
176 /**
177 * Sets all of the given switch points into the invalid state.
178 * After this call executes, no thread will observe any of the
179 * switch points to be in a valid state.
180 * <p>
181 * This operation is likely to be expensive and should be used sparingly.
182 * If possible, it should be buffered for batch processing on sets of switch points.
183 * <p>
184 * If {@code switchPoints} contains a null element,
185 * a {@code NullPointerException} will be raised.
186 * In this case, some non-null elements in the array may be
187 * processed before the method returns abnormally.
188 * Which elements these are (if any) is implementation-dependent.
189 *
190 * <p style="font-size:smaller;">
191 * <em>Discussion:</em>
192 * For performance reasons, {@code invalidateAll} is not a virtual method
193 * on a single switch point, but rather applies to a set of switch points.
194 * Some implementations may incur a large fixed overhead cost
195 * for processing one or more invalidation operations,
196 * but a small incremental cost for each additional invalidation.
197 * In any case, this operation is likely to be costly, since
198 * other threads may have to be somehow interrupted
199 * in order to make them notice the updated switch point state.
200 * However, it may be observed that a single call to invalidate
201 * several switch points has the same formal effect as many calls,
202 * each on just one of the switch points.
203 *
204 * <p style="font-size:smaller;">
205 * <em>Implementation Note:</em>
206 * Simple implementations of {@code SwitchPoint} may use
207 * a private {@link MutableCallSite} to publish the state of a switch point.
208 * In such an implementation, the {@code invalidateAll} method can
209 * simply change the call site's target, and issue one call to
210 * {@linkplain MutableCallSite#syncAll synchronize} all the
211 * private call sites.
212 *
213 * @param switchPoints an array of call sites to be synchronized
214 * @throws NullPointerException if the {@code switchPoints} array reference is null
215 * or the array contains a null
216 */
217 public static void invalidateAll(SwitchPoint[] switchPoints) {
218 if (switchPoints.length == 0) return;
219 MutableCallSite[] sites = new MutableCallSite[switchPoints.length];
220 for (int i = 0; i < switchPoints.length; i++) {
221 SwitchPoint spt = switchPoints[i];
222 if (spt == null) break; // MSC.syncAll will trigger a NPE
223 sites[i] = spt.mcs;
224 spt.mcs.setTarget(K_false);
225 }
226 MutableCallSite.syncAll(sites);
227 }
228 }