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package java.lang.invoke;

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 (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();
}