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25
26package 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 */
113public 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     */
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     */
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     */
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     */
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