MutableCallSite.java revision 16889:8ac762a3d4a4 
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25
26package java.lang.invoke;
27
28import java.util.Objects;
29import java.util.concurrent.atomic.AtomicInteger;
30
31/**
32 * A {@code MutableCallSite} is a {@link CallSite} whose target variable
33 * behaves like an ordinary field.
34 * An {@code invokedynamic} instruction linked to a {@code MutableCallSite} delegates
35 * all calls to the site's current target.
36 * The {@linkplain CallSite#dynamicInvoker dynamic invoker} of a mutable call site
37 * also delegates each call to the site's current target.
38 * <p>
39 * Here is an example of a mutable call site which introduces a
40 * state variable into a method handle chain.
41 * <!-- JavaDocExamplesTest.testMutableCallSite -->
42 * <blockquote><pre>{@code
43MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class));
44MethodHandle MH_name = name.dynamicInvoker();
45MethodType MT_str1 = MethodType.methodType(String.class);
46MethodHandle MH_upcase = MethodHandles.lookup()
47    .findVirtual(String.class, "toUpperCase", MT_str1);
48MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase);
49name.setTarget(MethodHandles.constant(String.class, "Rocky"));
50assertEquals("ROCKY", (String) worker1.invokeExact());
51name.setTarget(MethodHandles.constant(String.class, "Fred"));
52assertEquals("FRED", (String) worker1.invokeExact());
53// (mutation can be continued indefinitely)
54 * }</pre></blockquote>
55 * <p>
56 * The same call site may be used in several places at once.
57 * <blockquote><pre>{@code
58MethodType MT_str2 = MethodType.methodType(String.class, String.class);
59MethodHandle MH_cat = lookup().findVirtual(String.class,
60  "concat", methodType(String.class, String.class));
61MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?");
62MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear);
63assertEquals("Fred, dear?", (String) worker2.invokeExact());
64name.setTarget(MethodHandles.constant(String.class, "Wilma"));
65assertEquals("WILMA", (String) worker1.invokeExact());
66assertEquals("Wilma, dear?", (String) worker2.invokeExact());
67 * }</pre></blockquote>
68 * <p>
69 * <em>Non-synchronization of target values:</em>
70 * A write to a mutable call site's target does not force other threads
71 * to become aware of the updated value.  Threads which do not perform
72 * suitable synchronization actions relative to the updated call site
73 * may cache the old target value and delay their use of the new target
74 * value indefinitely.
75 * (This is a normal consequence of the Java Memory Model as applied
76 * to object fields.)
77 * <p>
78 * The {@link #syncAll syncAll} operation provides a way to force threads
79 * to accept a new target value, even if there is no other synchronization.
80 * <p>
81 * For target values which will be frequently updated, consider using
82 * a {@linkplain VolatileCallSite volatile call site} instead.
83 * @author John Rose, JSR 292 EG
84 * @since 1.7
85 */
86public class MutableCallSite extends CallSite {
87    /**
88     * Creates a blank call site object with the given method type.
89     * The initial target is set to a method handle of the given type
90     * which will throw an {@link IllegalStateException} if called.
91     * <p>
92     * The type of the call site is permanently set to the given type.
93     * <p>
94     * Before this {@code CallSite} object is returned from a bootstrap method,
95     * or invoked in some other manner,
96     * it is usually provided with a more useful target method,
97     * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.
98     * @param type the method type that this call site will have
99     * @throws NullPointerException if the proposed type is null
100     */
101    public MutableCallSite(MethodType type) {
102        super(type);
103    }
104
105    /**
106     * Creates a call site object with an initial target method handle.
107     * The type of the call site is permanently set to the initial target's type.
108     * @param target the method handle that will be the initial target of the call site
109     * @throws NullPointerException if the proposed target is null
110     */
111    public MutableCallSite(MethodHandle target) {
112        super(target);
113    }
114
115    /**
116     * Returns the target method of the call site, which behaves
117     * like a normal field of the {@code MutableCallSite}.
118     * <p>
119     * The interactions of {@code getTarget} with memory are the same
120     * as of a read from an ordinary variable, such as an array element or a
121     * non-volatile, non-final field.
122     * <p>
123     * In particular, the current thread may choose to reuse the result
124     * of a previous read of the target from memory, and may fail to see
125     * a recent update to the target by another thread.
126     *
127     * @return the linkage state of this call site, a method handle which can change over time
128     * @see #setTarget
129     */
130    @Override public final MethodHandle getTarget() {
131        return target;
132    }
133
134    /**
135     * Updates the target method of this call site, as a normal variable.
136     * The type of the new target must agree with the type of the old target.
137     * <p>
138     * The interactions with memory are the same
139     * as of a write to an ordinary variable, such as an array element or a
140     * non-volatile, non-final field.
141     * <p>
142     * In particular, unrelated threads may fail to see the updated target
143     * until they perform a read from memory.
144     * Stronger guarantees can be created by putting appropriate operations
145     * into the bootstrap method and/or the target methods used
146     * at any given call site.
147     *
148     * @param newTarget the new target
149     * @throws NullPointerException if the proposed new target is null
150     * @throws WrongMethodTypeException if the proposed new target
151     *         has a method type that differs from the previous target
152     * @see #getTarget
153     */
154    @Override public void setTarget(MethodHandle newTarget) {
155        checkTargetChange(this.target, newTarget);
156        setTargetNormal(newTarget);
157    }
158
159    /**
160     * {@inheritDoc}
161     */
162    @Override
163    public final MethodHandle dynamicInvoker() {
164        return makeDynamicInvoker();
165    }
166
167    /**
168     * Performs a synchronization operation on each call site in the given array,
169     * forcing all other threads to throw away any cached values previously
170     * loaded from the target of any of the call sites.
171     * <p>
172     * This operation does not reverse any calls that have already started
173     * on an old target value.
174     * (Java supports {@linkplain java.lang.Object#wait() forward time travel} only.)
175     * <p>
176     * The overall effect is to force all future readers of each call site's target
177     * to accept the most recently stored value.
178     * ("Most recently" is reckoned relative to the {@code syncAll} itself.)
179     * Conversely, the {@code syncAll} call may block until all readers have
180     * (somehow) decached all previous versions of each call site's target.
181     * <p>
182     * To avoid race conditions, calls to {@code setTarget} and {@code syncAll}
183     * should generally be performed under some sort of mutual exclusion.
184     * Note that reader threads may observe an updated target as early
185     * as the {@code setTarget} call that install the value
186     * (and before the {@code syncAll} that confirms the value).
187     * On the other hand, reader threads may observe previous versions of
188     * the target until the {@code syncAll} call returns
189     * (and after the {@code setTarget} that attempts to convey the updated version).
190     * <p>
191     * This operation is likely to be expensive and should be used sparingly.
192     * If possible, it should be buffered for batch processing on sets of call sites.
193     * <p>
194     * If {@code sites} contains a null element,
195     * a {@code NullPointerException} will be raised.
196     * In this case, some non-null elements in the array may be
197     * processed before the method returns abnormally.
198     * Which elements these are (if any) is implementation-dependent.
199     *
200     * <h1>Java Memory Model details</h1>
201     * In terms of the Java Memory Model, this operation performs a synchronization
202     * action which is comparable in effect to the writing of a volatile variable
203     * by the current thread, and an eventual volatile read by every other thread
204     * that may access one of the affected call sites.
205     * <p>
206     * The following effects are apparent, for each individual call site {@code S}:
207     * <ul>
208     * <li>A new volatile variable {@code V} is created, and written by the current thread.
209     *     As defined by the JMM, this write is a global synchronization event.
210     * <li>As is normal with thread-local ordering of write events,
211     *     every action already performed by the current thread is
212     *     taken to happen before the volatile write to {@code V}.
213     *     (In some implementations, this means that the current thread
214     *     performs a global release operation.)
215     * <li>Specifically, the write to the current target of {@code S} is
216     *     taken to happen before the volatile write to {@code V}.
217     * <li>The volatile write to {@code V} is placed
218     *     (in an implementation specific manner)
219     *     in the global synchronization order.
220     * <li>Consider an arbitrary thread {@code T} (other than the current thread).
221     *     If {@code T} executes a synchronization action {@code A}
222     *     after the volatile write to {@code V} (in the global synchronization order),
223     *     it is therefore required to see either the current target
224     *     of {@code S}, or a later write to that target,
225     *     if it executes a read on the target of {@code S}.
226     *     (This constraint is called "synchronization-order consistency".)
227     * <li>The JMM specifically allows optimizing compilers to elide
228     *     reads or writes of variables that are known to be useless.
229     *     Such elided reads and writes have no effect on the happens-before
230     *     relation.  Regardless of this fact, the volatile {@code V}
231     *     will not be elided, even though its written value is
232     *     indeterminate and its read value is not used.
233     * </ul>
234     * Because of the last point, the implementation behaves as if a
235     * volatile read of {@code V} were performed by {@code T}
236     * immediately after its action {@code A}.  In the local ordering
237     * of actions in {@code T}, this read happens before any future
238     * read of the target of {@code S}.  It is as if the
239     * implementation arbitrarily picked a read of {@code S}'s target
240     * by {@code T}, and forced a read of {@code V} to precede it,
241     * thereby ensuring communication of the new target value.
242     * <p>
243     * As long as the constraints of the Java Memory Model are obeyed,
244     * implementations may delay the completion of a {@code syncAll}
245     * operation while other threads ({@code T} above) continue to
246     * use previous values of {@code S}'s target.
247     * However, implementations are (as always) encouraged to avoid
248     * livelock, and to eventually require all threads to take account
249     * of the updated target.
250     *
251     * <p style="font-size:smaller;">
252     * <em>Discussion:</em>
253     * For performance reasons, {@code syncAll} is not a virtual method
254     * on a single call site, but rather applies to a set of call sites.
255     * Some implementations may incur a large fixed overhead cost
256     * for processing one or more synchronization operations,
257     * but a small incremental cost for each additional call site.
258     * In any case, this operation is likely to be costly, since
259     * other threads may have to be somehow interrupted
260     * in order to make them notice the updated target value.
261     * However, it may be observed that a single call to synchronize
262     * several sites has the same formal effect as many calls,
263     * each on just one of the sites.
264     *
265     * <p style="font-size:smaller;">
266     * <em>Implementation Note:</em>
267     * Simple implementations of {@code MutableCallSite} may use
268     * a volatile variable for the target of a mutable call site.
269     * In such an implementation, the {@code syncAll} method can be a no-op,
270     * and yet it will conform to the JMM behavior documented above.
271     *
272     * @param sites an array of call sites to be synchronized
273     * @throws NullPointerException if the {@code sites} array reference is null
274     *                              or the array contains a null
275     */
276    public static void syncAll(MutableCallSite[] sites) {
277        if (sites.length == 0)  return;
278        STORE_BARRIER.lazySet(0);
279        for (MutableCallSite site : sites) {
280            Objects.requireNonNull(site); // trigger NPE on first null
281        }
282        // FIXME: NYI
283    }
284    private static final AtomicInteger STORE_BARRIER = new AtomicInteger();
285}
286