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   25   
   26   package 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    * <blockquote><pre>
   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></blockquote>
   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    * <blockquote><pre>
   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&lt;MutableCallSite&gt; mcss = new ArrayList&lt;&gt;();
  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></blockquote>
  110    * @author Remi Forax, JSR 292 EG
  111    */
  112   public class SwitchPoint {
  113       private static final MethodHandle
  114           K_true  = MethodHandles.constant(boolean.class, true),
  115           K_false = MethodHandles.constant(boolean.class, false);
  116   
  117       private final MutableCallSite mcs;
  118       private final MethodHandle mcsInvoker;
  119   
  120       /**
  121        * Creates a new switch point.
  122        */
  123       public SwitchPoint() {
  124           this.mcs = new MutableCallSite(K_true);
  125           this.mcsInvoker = mcs.dynamicInvoker();
  126       }
  127   
  128       /**
  129        * Determines if this switch point has been invalidated yet.
  130        *
  131        * <p style="font-size:smaller;">
  132        * <em>Discussion:</em>
  133        * Because of the one-way nature of invalidation, once a switch point begins
  134        * to return true for {@code hasBeenInvalidated},
  135        * it will always do so in the future.
  136        * On the other hand, a valid switch point visible to other threads may
  137        * invalidated at any moment, due to a request by another thread.
  138        * <p style="font-size:smaller;">
  139        * Since invalidation is a global and immediate operation,
  140        * the execution of this query, on a valid switchpoint,
  141        * must be internally sequenced with any
  142        * other threads that could cause invalidation.
  143        * This query may therefore be expensive.
  144        * The recommended way to build a boolean-valued method handle
  145        * which queries the invalidation state of a switch point {@code s} is
  146        * to call {@code s.guardWithTest} on
  147        * {@link MethodHandles#constant constant} true and false method handles.
  148        *
  149        * @return true if this switch point has been invalidated
  150        */
  151       public boolean hasBeenInvalidated() {
  152           return (mcs.getTarget() != K_true);
  153       }
  154   
  155       /**
  156        * Returns a method handle which always delegates either to the target or the fallback.
  157        * The method handle will delegate to the target exactly as long as the switch point is valid.
  158        * After that, it will permanently delegate to the fallback.
  159        * <p>
  160        * The target and fallback must be of exactly the same method type,
  161        * and the resulting combined method handle will also be of this type.
  162        *
  163        * @param target the method handle selected by the switch point as long as it is valid
  164        * @param fallback the method handle selected by the switch point after it is invalidated
  165        * @return a combined method handle which always calls either the target or fallback
  166        * @throws NullPointerException if either argument is null
  167        * @throws IllegalArgumentException if the two method types do not match
  168        * @see MethodHandles#guardWithTest
  169        */
  170       public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) {
  171           if (mcs.getTarget() == K_false)
  172               return fallback;  // already invalid
  173           return MethodHandles.guardWithTest(mcsInvoker, target, fallback);
  174       }
  175   
  176       /**
  177        * Sets all of the given switch points into the invalid state.
  178        * After this call executes, no thread will observe any of the
  179        * switch points to be in a valid state.
  180        * <p>
  181        * This operation is likely to be expensive and should be used sparingly.
  182        * If possible, it should be buffered for batch processing on sets of switch points.
  183        * <p>
  184        * If {@code switchPoints} contains a null element,
  185        * a {@code NullPointerException} will be raised.
  186        * In this case, some non-null elements in the array may be
  187        * processed before the method returns abnormally.
  188        * Which elements these are (if any) is implementation-dependent.
  189        *
  190        * <p style="font-size:smaller;">
  191        * <em>Discussion:</em>
  192        * For performance reasons, {@code invalidateAll} is not a virtual method
  193        * on a single switch point, but rather applies to a set of switch points.
  194        * Some implementations may incur a large fixed overhead cost
  195        * for processing one or more invalidation operations,
  196        * but a small incremental cost for each additional invalidation.
  197        * In any case, this operation is likely to be costly, since
  198        * other threads may have to be somehow interrupted
  199        * in order to make them notice the updated switch point state.
  200        * However, it may be observed that a single call to invalidate
  201        * several switch points has the same formal effect as many calls,
  202        * each on just one of the switch points.
  203        *
  204        * <p style="font-size:smaller;">
  205        * <em>Implementation Note:</em>
  206        * Simple implementations of {@code SwitchPoint} may use
  207        * a private {@link MutableCallSite} to publish the state of a switch point.
  208        * In such an implementation, the {@code invalidateAll} method can
  209        * simply change the call site's target, and issue one call to
  210        * {@linkplain MutableCallSite#syncAll synchronize} all the
  211        * private call sites.
  212        *
  213        * @param switchPoints an array of call sites to be synchronized
  214        * @throws NullPointerException if the {@code switchPoints} array reference is null
  215        *                              or the array contains a null
  216        */
  217       public static void invalidateAll(SwitchPoint[] switchPoints) {
  218           if (switchPoints.length == 0)  return;
  219           MutableCallSite[] sites = new MutableCallSite[switchPoints.length];
  220           for (int i = 0; i < switchPoints.length; i++) {
  221               SwitchPoint spt = switchPoints[i];
  222               if (spt == null)  break;  // MSC.syncAll will trigger a NPE
  223               sites[i] = spt.mcs;
  224               spt.mcs.setTarget(K_false);
  225           }
  226           MutableCallSite.syncAll(sites);
  227       }
  228   }

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