+ * A {@code SwitchPoint} is an object which can publish state transitions to other threads. + * A switch point is initially in the valid state, but may at any time be + * changed to the invalid state. Invalidation cannot be reversed. + * A switch point can combine a guarded pair of method handles into a + * guarded delegator. + * The guarded delegator is a method handle which delegates to one of the old method handles. + * The state of the switch point determines which of the two gets the delegation. + *
+ * A single switch point may be used to control any number of method handles. + * (Indirectly, therefore, it can control any number of call sites.) + * This is done by using the single switch point as a factory for combining + * any number of guarded method handle pairs into guarded delegators. + *
+ * When a guarded delegator is created from a guarded pair, the pair + * is wrapped in a new method handle {@code M}, + * which is permanently associated with the switch point that created it. + * Each pair consists of a target {@code T} and a fallback {@code F}. + * While the switch point is valid, invocations to {@code M} are delegated to {@code T}. + * After it is invalidated, invocations are delegated to {@code F}. + *
+ * Invalidation is global and immediate, as if the switch point contained a + * volatile boolean variable consulted on every call to {@code M}. + * The invalidation is also permanent, which means the switch point + * can change state only once. + * The switch point will always delegate to {@code F} after being invalidated. + * At that point {@code guardWithTest} may ignore {@code T} and return {@code F}. + *
+ * Here is an example of a switch point in action: + *
{@code
+MethodHandle MH_strcat = MethodHandles.lookup()
+ .findVirtual(String.class, "concat", MethodType.methodType(String.class, String.class));
+SwitchPoint spt = new SwitchPoint();
+assert(!spt.hasBeenInvalidated());
+// the following steps may be repeated to re-use the same switch point:
+MethodHandle worker1 = MH_strcat;
+MethodHandle worker2 = MethodHandles.permuteArguments(MH_strcat, MH_strcat.type(), 1, 0);
+MethodHandle worker = spt.guardWithTest(worker1, worker2);
+assertEquals("method", (String) worker.invokeExact("met", "hod"));
+SwitchPoint.invalidateAll(new SwitchPoint[]{ spt });
+assert(spt.hasBeenInvalidated());
+assertEquals("hodmet", (String) worker.invokeExact("met", "hod"));
+ * }+ * Discussion: + * Switch points are useful without subclassing. They may also be subclassed. + * This may be useful in order to associate application-specific invalidation logic + * with the switch point. + * Notice that there is no permanent association between a switch point and + * the method handles it produces and consumes. + * The garbage collector may collect method handles produced or consumed + * by a switch point independently of the lifetime of the switch point itself. + *
+ * Implementation Note: + * A switch point behaves as if implemented on top of {@link MutableCallSite}, + * approximately as follows: + *
{@code
+public class SwitchPoint {
+ private static final MethodHandle
+ K_true = MethodHandles.constant(boolean.class, true),
+ K_false = MethodHandles.constant(boolean.class, false);
+ private final MutableCallSite mcs;
+ private final MethodHandle mcsInvoker;
+ public SwitchPoint() {
+ this.mcs = new MutableCallSite(K_true);
+ this.mcsInvoker = mcs.dynamicInvoker();
+ }
+ public MethodHandle guardWithTest(
+ MethodHandle target, MethodHandle fallback) {
+ // Note: mcsInvoker is of type ()boolean.
+ // Target and fallback may take any arguments, but must have the same type.
+ return MethodHandles.guardWithTest(this.mcsInvoker, target, fallback);
+ }
+ public static void invalidateAll(SwitchPoint[] spts) {
+ List<MutableCallSite> mcss = new ArrayList<>();
+ for (SwitchPoint spt : spts) mcss.add(spt.mcs);
+ for (MutableCallSite mcs : mcss) mcs.setTarget(K_false);
+ MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0]));
+ }
+}
+ * }+ * Discussion: + * Because of the one-way nature of invalidation, once a switch point begins + * to return true for {@code hasBeenInvalidated}, + * it will always do so in the future. + * On the other hand, a valid switch point visible to other threads may + * be invalidated at any moment, due to a request by another thread. + *
+ * Since invalidation is a global and immediate operation, + * the execution of this query, on a valid switchpoint, + * must be internally sequenced with any + * other threads that could cause invalidation. + * This query may therefore be expensive. + * The recommended way to build a boolean-valued method handle + * which queries the invalidation state of a switch point {@code s} is + * to call {@code s.guardWithTest} on + * {@link MethodHandles#constant constant} true and false method handles. + * + * @return true if this switch point has been invalidated + */ + public boolean hasBeenInvalidated() { + return (mcs.getTarget() != K_true); + } + + /** + * Returns a method handle which always delegates either to the target or the fallback. + * The method handle will delegate to the target exactly as long as the switch point is valid. + * After that, it will permanently delegate to the fallback. + *
+ * The target and fallback must be of exactly the same method type, + * and the resulting combined method handle will also be of this type. + * + * @param target the method handle selected by the switch point as long as it is valid + * @param fallback the method handle selected by the switch point after it is invalidated + * @return a combined method handle which always calls either the target or fallback + * @throws NullPointerException if either argument is null + * @throws IllegalArgumentException if the two method types do not match + * @see MethodHandles#guardWithTest + */ + public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) { + if (mcs.getTarget() == K_false) + return fallback; // already invalid + return MethodHandles.guardWithTest(mcsInvoker, target, fallback); + } + + /** + * Sets all of the given switch points into the invalid state. + * After this call executes, no thread will observe any of the + * switch points to be in a valid state. + *
+ * This operation is likely to be expensive and should be used sparingly. + * If possible, it should be buffered for batch processing on sets of switch points. + *
+ * If {@code switchPoints} contains a null element, + * a {@code NullPointerException} will be raised. + * In this case, some non-null elements in the array may be + * processed before the method returns abnormally. + * Which elements these are (if any) is implementation-dependent. + * + *
+ * Discussion: + * For performance reasons, {@code invalidateAll} is not a virtual method + * on a single switch point, but rather applies to a set of switch points. + * Some implementations may incur a large fixed overhead cost + * for processing one or more invalidation operations, + * but a small incremental cost for each additional invalidation. + * In any case, this operation is likely to be costly, since + * other threads may have to be somehow interrupted + * in order to make them notice the updated switch point state. + * However, it may be observed that a single call to invalidate + * several switch points has the same formal effect as many calls, + * each on just one of the switch points. + * + *
+ * Implementation Note: + * Simple implementations of {@code SwitchPoint} may use + * a private {@link MutableCallSite} to publish the state of a switch point. + * In such an implementation, the {@code invalidateAll} method can + * simply change the call site's target, and issue one call to + * {@linkplain MutableCallSite#syncAll synchronize} all the + * private call sites. + * + * @param switchPoints an array of call sites to be synchronized + * @throws NullPointerException if the {@code switchPoints} array reference is null + * or the array contains a null + */ + public static void invalidateAll(SwitchPoint[] switchPoints) { + if (switchPoints.length == 0) return; + MutableCallSite[] sites = new MutableCallSite[switchPoints.length]; + for (int i = 0; i < switchPoints.length; i++) { + SwitchPoint spt = switchPoints[i]; + if (spt == null) break; // MSC.syncAll will trigger a NPE + sites[i] = spt.mcs; + spt.mcs.setTarget(K_false); + } + MutableCallSite.syncAll(sites); + } +}