| /* |
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| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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| * |
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| * accompanied this code). |
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| |
| package java.lang.invoke; |
| |
| /** |
| * <p> |
| * A {@code SwitchPoint} is an object which can publish state transitions to other threads. |
| * A switch point is initially in the <em>valid</em> state, but may at any time be |
| * changed to the <em>invalid</em> state. Invalidation cannot be reversed. |
| * A switch point can combine a <em>guarded pair</em> of method handles into a |
| * <em>guarded delegator</em>. |
| * 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. |
| * <p> |
| * 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. |
| * <p> |
| * 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}. |
| * <p> |
| * 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}. |
| * <p> |
| * Here is an example of a switch point in action: |
| * <blockquote><pre> |
| 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")); |
| * </pre></blockquote> |
| * <p style="font-size:smaller;"> |
| * <em>Discussion:</em> |
| * 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. |
| * <p style="font-size:smaller;"> |
| * <em>Implementation Note:</em> |
| * A switch point behaves as if implemented on top of {@link MutableCallSite}, |
| * approximately as follows: |
| * <blockquote><pre> |
| 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])); |
| } |
| } |
| * </pre></blockquote> |
| * @author Remi Forax, JSR 292 EG |
| */ |
| 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; |
| |
| /** |
| * Creates a new switch point. |
| */ |
| public SwitchPoint() { |
| this.mcs = new MutableCallSite(K_true); |
| this.mcsInvoker = mcs.dynamicInvoker(); |
| } |
| |
| /** |
| * Determines if this switch point has been invalidated yet. |
| * |
| * <p style="font-size:smaller;"> |
| * <em>Discussion:</em> |
| * 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. |
| * <p style="font-size:smaller;"> |
| * 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. |
| * <p> |
| * 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. |
| * <p> |
| * 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. |
| * <p> |
| * 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. |
| * |
| * <p style="font-size:smaller;"> |
| * <em>Discussion:</em> |
| * 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. |
| * |
| * <p style="font-size:smaller;"> |
| * <em>Implementation Note:</em> |
| * 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); |
| } |
| } |
