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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>{@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"));

  * }</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>{@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]));

   }

 }

  * }</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);

     }

 }