1 /*

     2  * Copyright (c) 2008, 2011, Oracle and/or its affiliates. All rights reserved.

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

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

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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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    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 import java.util.concurrent.atomic.AtomicInteger;

    29 

    30 /**

    31  * A {@code MutableCallSite} is a {@link CallSite} whose target variable

    32  * behaves like an ordinary field.

    33  * An {@code invokedynamic} instruction linked to a {@code MutableCallSite} delegates

    34  * all calls to the site's current target.

    35  * The {@linkplain CallSite#dynamicInvoker dynamic invoker} of a mutable call site

    36  * also delegates each call to the site's current target.

    37  * <p>

    38  * Here is an example of a mutable call site which introduces a

    39  * state variable into a method handle chain.

    40  * <blockquote><pre>

    41 MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class));

    42 MethodHandle MH_name = name.dynamicInvoker();

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

    44 MethodHandle MH_upcase = MethodHandles.lookup()

    45     .findVirtual(String.class, "toUpperCase", MT_str2);

    46 MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase);

    47 name.setTarget(MethodHandles.constant(String.class, "Rocky"));

    48 assertEquals("ROCKY", (String) worker1.invokeExact());

    49 name.setTarget(MethodHandles.constant(String.class, "Fred"));

    50 assertEquals("FRED", (String) worker1.invokeExact());

    51 // (mutation can be continued indefinitely)

    52  * </pre></blockquote>

    53  * <p>

    54  * The same call site may be used in several places at once.

    55  * <blockquote><pre>

    56 MethodHandle MH_dear = MethodHandles.lookup()

    57     .findVirtual(String.class, "concat", MT_str2).bindTo(", dear?");

    58 MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear);

    59 assertEquals("Fred, dear?", (String) worker2.invokeExact());

    60 name.setTarget(MethodHandles.constant(String.class, "Wilma"));

    61 assertEquals("WILMA", (String) worker1.invokeExact());

    62 assertEquals("Wilma, dear?", (String) worker2.invokeExact());

    63  * </pre></blockquote>

    64  * <p>

    65  * <em>Non-synchronization of target values:</em>

    66  * A write to a mutable call site's target does not force other threads

    67  * to become aware of the updated value.  Threads which do not perform

    68  * suitable synchronization actions relative to the updated call site

    69  * may cache the old target value and delay their use of the new target

    70  * value indefinitely.

    71  * (This is a normal consequence of the Java Memory Model as applied

    72  * to object fields.)

    73  * <p>

    74  * The {@link #syncAll syncAll} operation provides a way to force threads

    75  * to accept a new target value, even if there is no other synchronization.

    76  * <p>

    77  * For target values which will be frequently updated, consider using

    78  * a {@linkplain VolatileCallSite volatile call site} instead.

    79  * @author John Rose, JSR 292 EG

    80  */

    81 public class MutableCallSite extends CallSite {

    82     /**

    83      * Creates a blank call site object with the given method type.

    84      * The initial target is set to a method handle of the given type

    85      * which will throw an {@link IllegalStateException} if called.

    86      * <p>

    87      * The type of the call site is permanently set to the given type.

    88      * <p>

    89      * Before this {@code CallSite} object is returned from a bootstrap method,

    90      * or invoked in some other manner,

    91      * it is usually provided with a more useful target method,

    92      * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.

    93      * @param type the method type that this call site will have

    94      * @throws NullPointerException if the proposed type is null

    95      */

    96     public MutableCallSite(MethodType type) {

    97         super(type);

    98     }

    99 

   100     /**

   101      * Creates a call site object with an initial target method handle.

   102      * The type of the call site is permanently set to the initial target's type.

   103      * @param target the method handle that will be the initial target of the call site

   104      * @throws NullPointerException if the proposed target is null

   105      */

   106     public MutableCallSite(MethodHandle target) {

   107         super(target);

   108     }

   109 

   110     /**

   111      * Returns the target method of the call site, which behaves

   112      * like a normal field of the {@code MutableCallSite}.

   113      * <p>

   114      * The interactions of {@code getTarget} with memory are the same

   115      * as of a read from an ordinary variable, such as an array element or a

   116      * non-volatile, non-final field.

   117      * <p>

   118      * In particular, the current thread may choose to reuse the result

   119      * of a previous read of the target from memory, and may fail to see

   120      * a recent update to the target by another thread.

   121      *

   122      * @return the linkage state of this call site, a method handle which can change over time

   123      * @see #setTarget

   124      */

   125     @Override public final MethodHandle getTarget() {

   126         return target;

   127     }

   128 

   129     /**

   130      * Updates the target method of this call site, as a normal variable.

   131      * The type of the new target must agree with the type of the old target.

   132      * <p>

   133      * The interactions with memory are the same

   134      * as of a write to an ordinary variable, such as an array element or a

   135      * non-volatile, non-final field.

   136      * <p>

   137      * In particular, unrelated threads may fail to see the updated target

   138      * until they perform a read from memory.

   139      * Stronger guarantees can be created by putting appropriate operations

   140      * into the bootstrap method and/or the target methods used

   141      * at any given call site.

   142      *

   143      * @param newTarget the new target

   144      * @throws NullPointerException if the proposed new target is null

   145      * @throws WrongMethodTypeException if the proposed new target

   146      *         has a method type that differs from the previous target

   147      * @see #getTarget

   148      */

   149     @Override public void setTarget(MethodHandle newTarget) {

   150         checkTargetChange(this.target, newTarget);

   151         setTargetNormal(newTarget);

   152     }

   153 

   154     /**

   155      * {@inheritDoc}

   156      */

   157     @Override

   158     public final MethodHandle dynamicInvoker() {

   159         return makeDynamicInvoker();

   160     }

   161 

   162     /**

   163      * Performs a synchronization operation on each call site in the given array,

   164      * forcing all other threads to throw away any cached values previously

   165      * loaded from the target of any of the call sites.

   166      * <p>

   167      * This operation does not reverse any calls that have already started

   168      * on an old target value.

   169      * (Java supports {@linkplain java.lang.Object#wait() forward time travel} only.)

   170      * <p>

   171      * The overall effect is to force all future readers of each call site's target

   172      * to accept the most recently stored value.

   173      * ("Most recently" is reckoned relative to the {@code syncAll} itself.)

   174      * Conversely, the {@code syncAll} call may block until all readers have

   175      * (somehow) decached all previous versions of each call site's target.

   176      * <p>

   177      * To avoid race conditions, calls to {@code setTarget} and {@code syncAll}

   178      * should generally be performed under some sort of mutual exclusion.

   179      * Note that reader threads may observe an updated target as early

   180      * as the {@code setTarget} call that install the value

   181      * (and before the {@code syncAll} that confirms the value).

   182      * On the other hand, reader threads may observe previous versions of

   183      * the target until the {@code syncAll} call returns

   184      * (and after the {@code setTarget} that attempts to convey the updated version).

   185      * <p>

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

   187      * If possible, it should be buffered for batch processing on sets of call sites.

   188      * <p>

   189      * If {@code sites} contains a null element,

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

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

   192      * processed before the method returns abnormally.

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

   194      *

   195      * <h3>Java Memory Model details</h3>

   196      * In terms of the Java Memory Model, this operation performs a synchronization

   197      * action which is comparable in effect to the writing of a volatile variable

   198      * by the current thread, and an eventual volatile read by every other thread

   199      * that may access one of the affected call sites.

   200      * <p>

   201      * The following effects are apparent, for each individual call site {@code S}:

   202      * <ul>

   203      * <li>A new volatile variable {@code V} is created, and written by the current thread.

   204      *     As defined by the JMM, this write is a global synchronization event.

   205      * <li>As is normal with thread-local ordering of write events,

   206      *     every action already performed by the current thread is

   207      *     taken to happen before the volatile write to {@code V}.

   208      *     (In some implementations, this means that the current thread

   209      *     performs a global release operation.)

   210      * <li>Specifically, the write to the current target of {@code S} is

   211      *     taken to happen before the volatile write to {@code V}.

   212      * <li>The volatile write to {@code V} is placed

   213      *     (in an implementation specific manner)

   214      *     in the global synchronization order.

   215      * <li>Consider an arbitrary thread {@code T} (other than the current thread).

   216      *     If {@code T} executes a synchronization action {@code A}

   217      *     after the volatile write to {@code V} (in the global synchronization order),

   218      *     it is therefore required to see either the current target

   219      *     of {@code S}, or a later write to that target,

   220      *     if it executes a read on the target of {@code S}.

   221      *     (This constraint is called "synchronization-order consistency".)

   222      * <li>The JMM specifically allows optimizing compilers to elide

   223      *     reads or writes of variables that are known to be useless.

   224      *     Such elided reads and writes have no effect on the happens-before

   225      *     relation.  Regardless of this fact, the volatile {@code V}

   226      *     will not be elided, even though its written value is

   227      *     indeterminate and its read value is not used.

   228      * </ul>

   229      * Because of the last point, the implementation behaves as if a

   230      * volatile read of {@code V} were performed by {@code T}

   231      * immediately after its action {@code A}.  In the local ordering

   232      * of actions in {@code T}, this read happens before any future

   233      * read of the target of {@code S}.  It is as if the

   234      * implementation arbitrarily picked a read of {@code S}'s target

   235      * by {@code T}, and forced a read of {@code V} to precede it,

   236      * thereby ensuring communication of the new target value.

   237      * <p>

   238      * As long as the constraints of the Java Memory Model are obeyed,

   239      * implementations may delay the completion of a {@code syncAll}

   240      * operation while other threads ({@code T} above) continue to

   241      * use previous values of {@code S}'s target.

   242      * However, implementations are (as always) encouraged to avoid

   243      * livelock, and to eventually require all threads to take account

   244      * of the updated target.

   245      *

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

   247      * <em>Discussion:</em>

   248      * For performance reasons, {@code syncAll} is not a virtual method

   249      * on a single call site, but rather applies to a set of call sites.

   250      * Some implementations may incur a large fixed overhead cost

   251      * for processing one or more synchronization operations,

   252      * but a small incremental cost for each additional call site.

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

   254      * other threads may have to be somehow interrupted

   255      * in order to make them notice the updated target value.

   256      * However, it may be observed that a single call to synchronize

   257      * several sites has the same formal effect as many calls,

   258      * each on just one of the sites.

   259      *

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

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

   262      * Simple implementations of {@code MutableCallSite} may use

   263      * a volatile variable for the target of a mutable call site.

   264      * In such an implementation, the {@code syncAll} method can be a no-op,

   265      * and yet it will conform to the JMM behavior documented above.

   266      *

   267      * @param sites an array of call sites to be synchronized

   268      * @throws NullPointerException if the {@code sites} array reference is null

   269      *                              or the array contains a null

   270      */

   271     public static void syncAll(MutableCallSite[] sites) {

   272         if (sites.length == 0)  return;

   273         STORE_BARRIER.lazySet(0);

   274         for (int i = 0; i < sites.length; i++) {

   275             sites[i].getClass();  // trigger NPE on first null

   276         }

   277         // FIXME: NYI

   278     }

   279     private static final AtomicInteger STORE_BARRIER = new AtomicInteger();

   280 }