1 /*

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     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

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     7  * published by the Free Software Foundation.  Oracle designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Oracle in the LICENSE file that accompanied this code.

    10  *

    11  * This code is distributed in the hope that it will be useful, but WITHOUT

    12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    14  * version 2 for more details (a copy is included in the LICENSE file that

    15  * accompanied this code).

    16  *

    17  * You should have received a copy of the GNU General Public License version

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    20  *

    21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    22  * or visit www.oracle.com if you need additional information or have any

    23  * questions.

    24  */

    25 

    26 package java.dyn;

    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 MethodType MT_str2 = MethodType.methodType(String.class, String.class);

    60 MethodHandle MH_strcat = MethodHandles.lookup()

    61     .findVirtual(String.class, "concat", MT_str2);

    62 SwitchPoint spt = new SwitchPoint();

    63 // the following steps may be repeated to re-use the same switch point:

    64 MethodHandle worker1 = strcat;

    65 MethodHandle worker2 = MethodHandles.permuteArguments(strcat, MT_str2, 1, 0);

    66 MethodHandle worker = spt.guardWithTest(worker1, worker2);

    67 assertEquals("method", (String) worker.invokeExact("met", "hod"));

    68 SwitchPoint.invalidateAll(new SwitchPoint[]{ spt });

    69 assertEquals("hodmet", (String) worker.invokeExact("met", "hod"));

    70  * </pre></blockquote>

    71  * <p style="font-size:smaller;">

    72  * <em>Discussion:</em>

    73  * Switch points are useful without subclassing.  They may also be subclassed.

    74  * This may be useful in order to associate application-specific invalidation logic

    75  * with the switch point.

    76  * <p style="font-size:smaller;">

    77  * <em>Implementation Note:</em>

    78  * A switch point behaves as if implemented on top of {@link MutableCallSite},

    79  * approximately as follows:

    80  * <blockquote><pre>

    81 public class SwitchPoint {

    82   private static final MethodHandle

    83     K_true  = MethodHandles.constant(boolean.class, true),

    84     K_false = MethodHandles.constant(boolean.class, false);

    85   private final MutableCallSite mcs;

    86   private final MethodHandle mcsInvoker;

    87   public SwitchPoint() {

    88     this.mcs = new MutableCallSite(K_true);

    89     this.mcsInvoker = mcs.dynamicInvoker();

    90   }

    91   public MethodHandle guardWithTest(

    92                 MethodHandle target, MethodHandle fallback) {

    93     // Note:  mcsInvoker is of type ()boolean.

    94     // Target and fallback may take any arguments, but must have the same type.

    95     return MethodHandles.guardWithTest(this.mcsInvoker, target, fallback);

    96   }

    97   public static void invalidateAll(SwitchPoint[] spts) {

    98     List<MutableCallSite> mcss = new ArrayList<>();

    99     for (SwitchPoint spt : spts)  mcss.add(spt.mcs);

   100     for (MutableCallSite mcs : mcss)  mcs.setTarget(K_false);

   101     MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0]));

   102   }

   103 }

   104  * </pre></blockquote>

   105  * @author Remi Forax, JSR 292 EG

   106  */

   107 public class SwitchPoint {

   108     private static final MethodHandle

   109         K_true  = MethodHandles.constant(boolean.class, true),

   110         K_false = MethodHandles.constant(boolean.class, false);

   111 

   112     private final MutableCallSite mcs;

   113     private final MethodHandle mcsInvoker;

   114 

   115     /**

   116      * Creates a new switch point.

   117      */

   118     public SwitchPoint() {

   119         this.mcs = new MutableCallSite(K_true);

   120         this.mcsInvoker = mcs.dynamicInvoker();

   121     }

   122 

   123     /**

   124      * Returns a method handle which always delegates either to the target or the fallback.

   125      * The method handle will delegate to the target exactly as long as the switch point is valid.

   126      * After that, it will permanently delegate to the fallback.

   127      * <p>

   128      * The target and fallback must be of exactly the same method type,

   129      * and the resulting combined method handle will also be of this type.

   130      *

   131      * @param target the method handle selected by the switch point as long as it is valid

   132      * @param fallback the method handle selected by the switch point after it is invalidated

   133      * @return a combined method handle which always calls either the target or fallback

   134      * @throws NullPointerException if either argument is null

   135      * @see MethodHandles#guardWithTest

   136      */

   137     public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) {

   138         if (mcs.getTarget() == K_false)

   139             return fallback;  // already invalid

   140         return MethodHandles.guardWithTest(mcsInvoker, target, fallback);

   141     }

   142 

   143     /**

   144      * Sets all of the given switch points into the invalid state.

   145      * After this call executes, no thread will observe any of the

   146      * switch points to be in a valid state.

   147      * <p>

   148      * This operation is likely to be expensive and should be used sparingly.

   149      * If possible, it should be buffered for batch processing on sets of switch points.

   150      * <p>

   151      * If {@code switchPoints} contains a null element,

   152      * a {@code NullPointerException} will be raised.

   153      * In this case, some non-null elements in the array may be

   154      * processed before the method returns abnormally.

   155      * Which elements these are (if any) is implementation-dependent.

   156      *

   157      * <p style="font-size:smaller;">

   158      * <em>Discussion:</em>

   159      * For performance reasons, {@code invalidateAll} is not a virtual method

   160      * on a single switch point, but rather applies to a set of switch points.

   161      * Some implementations may incur a large fixed overhead cost

   162      * for processing one or more invalidation operations,

   163      * but a small incremental cost for each additional invalidation.

   164      * In any case, this operation is likely to be costly, since

   165      * other threads may have to be somehow interrupted

   166      * in order to make them notice the updated switch point state.

   167      * However, it may be observed that a single call to invalidate

   168      * several switch points has the same formal effect as many calls,

   169      * each on just one of the switch points.

   170      *

   171      * <p style="font-size:smaller;">

   172      * <em>Implementation Note:</em>

   173      * Simple implementations of {@code SwitchPoint} may use

   174      * a private {@link MutableCallSite} to publish the state of a switch point.

   175      * In such an implementation, the {@code invalidateAll} method can

   176      * simply change the call site's target, and issue one call to

   177      * {@linkplain MutableCallSite#syncAll synchronize} all the

   178      * private call sites.

   179      *

   180      * @param switchPoints an array of call sites to be synchronized

   181      * @throws NullPointerException if the {@code switchPoints} array reference is null

   182      *                              or the array contains a null

   183      */

   184     public static void invalidateAll(SwitchPoint[] switchPoints) {

   185         if (switchPoints.length == 0)  return;

   186         MutableCallSite[] sites = new MutableCallSite[switchPoints.length];

   187         for (int i = 0; i < switchPoints.length; i++) {

   188             SwitchPoint spt = switchPoints[i];

   189             if (spt == null)  break;  // MSC.syncAll will trigger a NPE

   190             sites[i] = spt.mcs;

   191             spt.mcs.setTarget(K_false);

   192         }

   193         MutableCallSite.syncAll(sites);

   194     }

   195 }