jdk/src/java.base/share/classes/java/lang/invoke/SwitchPoint.java
changeset 25859 3317bb8137f4
parent 23010 6dadb192ad81
child 32276 2d5fcc89e099
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/java.base/share/classes/java/lang/invoke/SwitchPoint.java	Sun Aug 17 15:54:13 2014 +0100
@@ -0,0 +1,228 @@
+/*
+ * Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+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:
+ * <blockquote><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></blockquote>
+ * <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:
+ * <blockquote><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&lt;MutableCallSite&gt; mcss = new ArrayList&lt;&gt;();
+    for (SwitchPoint spt : spts)  mcss.add(spt.mcs);
+    for (MutableCallSite mcs : mcss)  mcs.setTarget(K_false);
+    MutableCallSite.syncAll(mcss.toArray(new MutableCallSite[0]));
+  }
+}
+ * }</pre></blockquote>
+ * @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);
+    }
+}