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package java.lang.invoke;
// Android-changed: not used.
// import java.util.concurrent.atomic.AtomicInteger;
// Android-changed: removed references to MutableCallSite.syncAll().
/**
* 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.
*
* Here is an example of a mutable call site which introduces a
* state variable into a method handle chain.
*
*
{@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)
* }
*
* The same call site may be used in several places at once.
*
{@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());
* }
*
* Non-synchronization of target values:
* 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.)
*
* 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.
*
* The type of the call site is permanently set to the given type.
*
* 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}.
*
* 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.
*
* 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.
*
* 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.
*
* 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();
}
// Android-changed: not exposing incomplete implementation.
// /**
// * 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.
// *
// * 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.)
// *
// * 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.
// *
// * 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).
// *
// * 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.
// *
// * 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.
// *
// *
Java Memory Model details
// * 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.
// *
// * The following effects are apparent, for each individual call site {@code S}:
// *
// * - 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.
// *
- 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.)
// *
- Specifically, the write to the current target of {@code S} is
// * taken to happen before the volatile write to {@code V}.
// *
- The volatile write to {@code V} is placed
// * (in an implementation specific manner)
// * in the global synchronization order.
// *
- 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".)
// *
- 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.
// *
// * 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.
// *
// * 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.
// *
// *
// * Discussion:
// * 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.
// *
// *
// * Implementation Note:
// * 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 (int i = 0; i < sites.length; i++) {
// sites[i].getClass(); // trigger NPE on first null
// }
// // FIXME: NYI
// }
// private static final AtomicInteger STORE_BARRIER = new AtomicInteger();
}