1 /*

     2  * Copyright (c) 2009, 2010, 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.dyn;

    27 

    28 /**

    29  * An interface for an object to provide a target {@linkplain MethodHandle method handle} to a {@code invokedynamic} instruction.

    30  * There are many function-like objects in various Java APIs.

    31  * This interface provides a standard way for such function-like objects to be bound

    32  * to a dynamic call site, by providing a view of their behavior in the form of a low-level method handle.

    33  * <p>

    34  * The type {@link MethodHandle} is a concrete class whose implementation

    35  * hierarchy (if any) may be tightly coupled to the underlying JVM implementation.

    36  * It cannot also serve as a base type for user-defined functional APIs.

    37  * For this reason, {@code MethodHandle} cannot be subclassed to add new

    38  * behavior to method handles.  But this interface can be used to provide

    39  * a link between a user-defined function and the {@code invokedynamic}

    40  * instruction and the method handle API.

    41  */

    42 public interface MethodHandleProvider {

    43     /** Produce a method handle which will serve as a behavioral proxy for the current object.

    44      *  The type and invocation behavior of the proxy method handle are user-defined,

    45      *  and should have some relation to the intended meaning of the original object itself.

    46      *  <p>

    47      *  The current object may have a changeable behavior.

    48      *  For example, {@link CallSite} has a {@code setTarget} method which changes its invocation.

    49      *  In such a case, it is <em>incorrect</em> for {@code asMethodHandle} to return

    50      *  a method handle whose behavior may diverge from that of the current object.

    51      *  Rather, the returned method handle must stably and permanently access

    52      *  the behavior of the current object, even if that behavior is changeable.

    53      *  <p>

    54      *  The reference identity of the proxy method handle is not guaranteed to

    55      *  have any particular relation to the reference identity of the object.

    56      *  In particular, several objects with the same intended meaning could

    57      *  share a common method handle, or the same object could return different

    58      *  method handles at different times.  In the latter case, the different

    59      *  method handles should have the same type and invocation behavior,

    60      *  and be usable from any thread at any time.

    61      *  In particular, if a MethodHandleProvider is bound to an <code>invokedynamic</code>

    62      *  call site, the proxy method handle extracted at the time of binding

    63      *  will be used for an unlimited time, until the call site is rebound.

    64      *  <p>

    65      *  The type {@link MethodHandle} itself implements {@code MethodHandleProvider}, and

    66      *  for this method simply returns {@code this}.

    67      */

    68     public MethodHandle asMethodHandle();

    69 

    70     /** Produce a method handle of a given type which will serve as a behavioral proxy for the current object.

    71      *  As for the no-argument version {@link #asMethodHandle()}, the invocation behavior of the

    72      *  proxy method handle is user-defined.  But the type must be the given type,

    73      *  or else a {@link WrongMethodTypeException} must be thrown.

    74      *  <p>

    75      *  If the current object somehow represents a variadic or overloaded behavior,

    76      *  the method handle returned for a given type might represent only a subset of

    77      *  the current object's repertoire of behaviors, which correspond to that type.

    78      */

    79     public MethodHandle asMethodHandle(MethodType type) throws WrongMethodTypeException;

    80 }