assets/app/src/main/java/java/lang/invoke/SwitchPoint.java
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package java.lang.invoke;
/**
* <p>
* A {@code SwitchPoint} is an object which can publish state transitions to other threads.
* A switch point is initially in the <em>valid</em> state, but may at any time be
* changed to the <em>invalid</em> state. Invalidation cannot be reversed.
* A switch point can combine a <em>guarded pair</em> of method handles into a
* <em>guarded delegator</em>.
* The guarded delegator is a method handle which delegates to one of the old method handles.
* The state of the switch point determines which of the two gets the delegation.
* <p>
* A single switch point may be used to control any number of method handles.
* (Indirectly, therefore, it can control any number of call sites.)
* This is done by using the single switch point as a factory for combining
* any number of guarded method handle pairs into guarded delegators.
* <p>
* When a guarded delegator is created from a guarded pair, the pair
* is wrapped in a new method handle {@code M},
* which is permanently associated with the switch point that created it.
* Each pair consists of a target {@code T} and a fallback {@code F}.
* While the switch point is valid, invocations to {@code M} are delegated to {@code T}.
* After it is invalidated, invocations are delegated to {@code F}.
* <p>
* Invalidation is global and immediate, as if the switch point contained a
* volatile boolean variable consulted on every call to {@code M}.
* The invalidation is also permanent, which means the switch point
* can change state only once.
* The switch point will always delegate to {@code F} after being invalidated.
* At that point {@code guardWithTest} may ignore {@code T} and return {@code F}.
* <p>
* Here is an example of a switch point in action:
* <pre>{@code
* MethodHandle MH_strcat = MethodHandles.lookup()
* .findVirtual(String.class, "concat", MethodType.methodType(String.class, String.class));
* SwitchPoint spt = new SwitchPoint();
* assert(!spt.hasBeenInvalidated());
* // the following steps may be repeated to re-use the same switch point:
* MethodHandle worker1 = MH_strcat;
* MethodHandle worker2 = MethodHandles.permuteArguments(MH_strcat, MH_strcat.type(), 1, 0);
* MethodHandle worker = spt.guardWithTest(worker1, worker2);
* assertEquals("method", (String) worker.invokeExact("met", "hod"));
* SwitchPoint.invalidateAll(new SwitchPoint[]{ spt });
* assert(spt.hasBeenInvalidated());
* assertEquals("hodmet", (String) worker.invokeExact("met", "hod"));
* }</pre>
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* Switch points are useful without subclassing. They may also be subclassed.
* This may be useful in order to associate application-specific invalidation logic
* with the switch point.
* Notice that there is no permanent association between a switch point and
* the method handles it produces and consumes.
* The garbage collector may collect method handles produced or consumed
* by a switch point independently of the lifetime of the switch point itself.
* <p style="font-size:smaller;">
* <em>Implementation Note:</em>
* A switch point behaves as if implemented on top of {@link MutableCallSite},
* approximately as follows:
* <pre>{@code
* public class SwitchPoint {
* private static final MethodHandle
* K_true = MethodHandles.constant(boolean.class, true),
* K_false = MethodHandles.constant(boolean.class, false);
* private final MutableCallSite mcs;
* private final MethodHandle mcsInvoker;
* public SwitchPoint() {
* this.mcs = new MutableCallSite(K_true);
* this.mcsInvoker = mcs.dynamicInvoker();
* }
* public MethodHandle guardWithTest(
* MethodHandle target, MethodHandle fallback) {
* // Note: mcsInvoker is of type ()boolean.
* // Target and fallback may take any arguments, but must have the same type.
* return MethodHandles.guardWithTest(this.mcsInvoker, target, fallback);
* }
* public static void invalidateAll(SwitchPoint[] spts) {
* List<MutableCallSite> mcss = new ArrayList<>();
* for (SwitchPoint spt : spts) mcss.add(spt.mcs);
* for (MutableCallSite mcs : mcss) mcs.setTarget(K_false);
* MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0]));
* }
* }
* }</pre>
* @author Remi Forax, JSR 292 EG
*/
public class SwitchPoint {
private static final MethodHandle
K_true = MethodHandles.constant(boolean.class, true),
K_false = MethodHandles.constant(boolean.class, false);
private final MutableCallSite mcs;
private final MethodHandle mcsInvoker;
/**
* Creates a new switch point.
*/
public SwitchPoint() {
this.mcs = new MutableCallSite(K_true);
this.mcsInvoker = mcs.dynamicInvoker();
}
/**
* Determines if this switch point has been invalidated yet.
*
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* Because of the one-way nature of invalidation, once a switch point begins
* to return true for {@code hasBeenInvalidated},
* it will always do so in the future.
* On the other hand, a valid switch point visible to other threads may
* be invalidated at any moment, due to a request by another thread.
* <p style="font-size:smaller;">
* Since invalidation is a global and immediate operation,
* the execution of this query, on a valid switchpoint,
* must be internally sequenced with any
* other threads that could cause invalidation.
* This query may therefore be expensive.
* The recommended way to build a boolean-valued method handle
* which queries the invalidation state of a switch point {@code s} is
* to call {@code s.guardWithTest} on
* {@link MethodHandles#constant constant} true and false method handles.
*
* @return true if this switch point has been invalidated
*/
public boolean hasBeenInvalidated() {
return (mcs.getTarget() != K_true);
}
/**
* Returns a method handle which always delegates either to the target or the fallback.
* The method handle will delegate to the target exactly as long as the switch point is valid.
* After that, it will permanently delegate to the fallback.
* <p>
* The target and fallback must be of exactly the same method type,
* and the resulting combined method handle will also be of this type.
*
* @param target the method handle selected by the switch point as long as it is valid
* @param fallback the method handle selected by the switch point after it is invalidated
* @return a combined method handle which always calls either the target or fallback
* @throws NullPointerException if either argument is null
* @throws IllegalArgumentException if the two method types do not match
* @see MethodHandles#guardWithTest
*/
public MethodHandle guardWithTest(MethodHandle target, MethodHandle fallback) {
if (mcs.getTarget() == K_false)
return fallback; // already invalid
return MethodHandles.guardWithTest(mcsInvoker, target, fallback);
}
/**
* Sets all of the given switch points into the invalid state.
* After this call executes, no thread will observe any of the
* switch points to be in a valid state.
* <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 switch points.
* <p>
* If {@code switchPoints} 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.
*
* <p style="font-size:smaller;">
* <em>Discussion:</em>
* For performance reasons, {@code invalidateAll} is not a virtual method
* on a single switch point, but rather applies to a set of switch points.
* Some implementations may incur a large fixed overhead cost
* for processing one or more invalidation operations,
* but a small incremental cost for each additional invalidation.
* 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 switch point state.
* However, it may be observed that a single call to invalidate
* several switch points has the same formal effect as many calls,
* each on just one of the switch points.
*
* <p style="font-size:smaller;">
* <em>Implementation Note:</em>
* Simple implementations of {@code SwitchPoint} may use
* a private {@link MutableCallSite} to publish the state of a switch point.
* In such an implementation, the {@code invalidateAll} method can
* simply change the call site's target, and issue one call to
* {@linkplain MutableCallSite#syncAll synchronize} all the
* private call sites.
*
* @param switchPoints an array of call sites to be synchronized
* @throws NullPointerException if the {@code switchPoints} array reference is null
* or the array contains a null
*/
public static void invalidateAll(SwitchPoint[] switchPoints) {
if (switchPoints.length == 0) return;
MutableCallSite[] sites = new MutableCallSite[switchPoints.length];
for (int i = 0; i < switchPoints.length; i++) {
SwitchPoint spt = switchPoints[i];
if (spt == null) break; // MSC.syncAll will trigger a NPE
sites[i] = spt.mcs;
spt.mcs.setTarget(K_false);
}
// MutableCallSite.syncAll(sites);
}
}