- * 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); - } -}