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

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package java.lang.invoke;

import java.util.Objects;

import java.util.concurrent.atomic.AtomicInteger;

/**

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

 * behaves like an ordinary field.

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

 * all calls to the site's current target.

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

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

 * <p>

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

 * state variable into a method handle chain.

 * <!-- JavaDocExamplesTest.testMutableCallSite -->

 * <blockquote><pre>{@code

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

MethodHandle MH_name = name.dynamicInvoker();

MethodType MT_str1 = MethodType.methodType(String.class);

MethodHandle MH_upcase = MethodHandles.lookup()

    .findVirtual(String.class, "toUpperCase", MT_str1);

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

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

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

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

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

// (mutation can be continued indefinitely)

 * }</pre></blockquote>

 * <p>

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

 * <blockquote><pre>{@code

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

MethodHandle MH_cat = lookup().findVirtual(String.class,

  "concat", methodType(String.class, String.class));

MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?");

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

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

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

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

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

 * }</pre></blockquote>

 * <p>

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

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

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

 * suitable synchronization actions relative to the updated call site

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

 * value indefinitely.

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

 * to object fields.)

 * <p>

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

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

 * <p>

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

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

 * @author John Rose, JSR 292 EG

 */

public class MutableCallSite extends CallSite {

    /**

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

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

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

     * <p>

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

     * <p>

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

     * or invoked in some other manner,

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

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

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

     * @throws NullPointerException if the proposed type is null

     */

    public MutableCallSite(MethodType type) {

        super(type);

    }

    /**

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

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

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

     * @throws NullPointerException if the proposed target is null

     */

    public MutableCallSite(MethodHandle target) {

        super(target);

    }

    /**

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

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

     * <p>

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

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

     * non-volatile, non-final field.

     * <p>

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

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

     * a recent update to the target by another thread.

     *

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

     * @see #setTarget

     */

    @Override public final MethodHandle getTarget() {

        return target;

    }

    /**

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

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

     * <p>

     * The interactions with memory are the same

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

     * non-volatile, non-final field.

     * <p>

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

     * until they perform a read from memory.

     * Stronger guarantees can be created by putting appropriate operations

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

     * at any given call site.

     *

     * @param newTarget the new target

     * @throws NullPointerException if the proposed new target is null

     * @throws WrongMethodTypeException if the proposed new target

     *         has a method type that differs from the previous target

     * @see #getTarget

     */

    @Override public void setTarget(MethodHandle newTarget) {

        checkTargetChange(this.target, newTarget);

        setTargetNormal(newTarget);

    }

    /**

     * {@inheritDoc}

     */

    @Override

    public final MethodHandle dynamicInvoker() {

        return makeDynamicInvoker();

    }

    /**

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

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

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

     * <p>

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

     * on an old target value.

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

     * <p>

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

     * to accept the most recently stored value.

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

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

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

     * <p>

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

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

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

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

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

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

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

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

     * <p>

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

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

     * <p>

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

     * a {@code NullPointerException} will be raised.

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

     * processed before the method returns abnormally.

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

     *

     * <h1>Java Memory Model details</h1>

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

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

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

     * that may access one of the affected call sites.

     * <p>

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

     * <ul>

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

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

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

     *     every action already performed by the current thread is

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

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

     *     performs a global release operation.)

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

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

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

     *     (in an implementation specific manner)

     *     in the global synchronization order.

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

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

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

     *     it is therefore required to see either the current target

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

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

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

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

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

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

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

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

     *     indeterminate and its read value is not used.

     * </ul>

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

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

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

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

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

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

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

     * thereby ensuring communication of the new target value.

     * <p>

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

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

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

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

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

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

     * of the updated target.

     *

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

     * <em>Discussion:</em>

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

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

     * Some implementations may incur a large fixed overhead cost

     * for processing one or more synchronization operations,

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

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

     * other threads may have to be somehow interrupted

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

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

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

     * each on just one of the sites.

     *

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

     * <em>Implementation Note:</em>

     * Simple implementations of {@code MutableCallSite} may use

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

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

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

     *

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

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

     *                              or the array contains a null

     */

    public static void syncAll(MutableCallSite[] sites) {

        if (sites.length == 0)  return;

        STORE_BARRIER.lazySet(0);

        for (MutableCallSite site : sites) {

            Objects.requireNonNull(site); // trigger NPE on first null

        }

        // FIXME: NYI

    }

    private static final AtomicInteger STORE_BARRIER = new AtomicInteger();

}