1 /*

     2  * Copyright (c) 2010, 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

     6  * under the terms of the GNU General Public License version 2 only, as

     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

    18  * 2 along with this work; if not, write to the Free Software Foundation,

    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  * A {@code VolatileCallSite} is a {@link CallSite} whose target acts like a volatile variable.

    30  * An {@code invokedynamic} instruction linked to a {@code VolatileCallSite} sees updates

    31  * to its call site target immediately, even if the update occurs in another thread.

    32  * There may be a performance penalty for such tight coupling between threads.

    33  * <p>

    34  * Unlike {@code MutableCallSite}, there is no

    35  * {@linkplain MutableCallSite#syncAll syncAll operation} on volatile

    36  * call sites, since every write to a volatile variable is implicitly

    37  * synchronized with reader threads.

    38  * <p>

    39  * In other respects, a {@code VolatileCallSite} is interchangeable

    40  * with {@code MutableCallSite}.

    41  * @see MutableCallSite

    42  * @author John Rose, JSR 292 EG

    43  */

    44 public class VolatileCallSite extends CallSite {

    45     /**

    46      * Creates a call site with a volatile binding to its target.

    47      * The initial target is set to a method handle

    48      * of the given type which will throw an {@code IllegalStateException} if called.

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

    50      * @throws NullPointerException if the proposed type is null

    51      */

    52     public VolatileCallSite(MethodType type) {

    53         super(type);

    54     }

    55 

    56     /**

    57      * Creates a call site with a volatile binding to its target.

    58      * The target is set to the given value.

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

    60      * @throws NullPointerException if the proposed target is null

    61      */

    62     public VolatileCallSite(MethodHandle target) {

    63         super(target);

    64     }

    65 

    66     /**

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

    68      * like a {@code volatile} field of the {@code VolatileCallSite}.

    69      * <p>

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

    71      * as of a read from a {@code volatile} field.

    72      * <p>

    73      * In particular, the current thread is required to issue a fresh

    74      * read of the target from memory, and must not fail to see

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

    76      *

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

    78      * @see #setTarget

    79      */

    80     @Override public final MethodHandle getTarget() {

    81         return getTargetVolatile();

    82     }

    83 

    84     /**

    85      * Updates the target method of this call site, as a volatile variable.

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

    87      * <p>

    88      * The interactions with memory are the same as of a write to a volatile field.

    89      * In particular, any threads is guaranteed to see the updated target

    90      * the next time it calls {@code getTarget}.

    91      * @param newTarget the new target

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

    93      * @throws WrongMethodTypeException if the proposed new target

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

    95      * @see #getTarget

    96      */

    97     @Override public void setTarget(MethodHandle newTarget) {

    98         checkTargetChange(getTargetVolatile(), newTarget);

    99         setTargetVolatile(newTarget);

   100     }

   101 

   102     /**

   103      * {@inheritDoc}

   104      */

   105     @Override

   106     public final MethodHandle dynamicInvoker() {

   107         return makeDynamicInvoker();

   108     }

   109 }