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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();
}