1 /*

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

     4  *

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     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  *

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    19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    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.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>{@code

    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>{@code

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

   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      * be 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 }