concurrent/atomic/AtomicReferenceFieldUpdater.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.function.BinaryOperator;
import java.util.function.UnaryOperator;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;

/**
 * A reflection-based utility that enables atomic updates to
 * designated {@code volatile} reference fields of designated
 * classes.  This class is designed for use in atomic data structures
 * in which several reference fields of the same node are
 * independently subject to atomic updates. For example, a tree node
 * might be declared as
 *
 * <pre> {@code
 * class Node {
 *   private volatile Node left, right;
 *
 *   private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
 *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
 *   private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
 *     AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
 *
 *   Node getLeft() { return left; }
 *   boolean compareAndSetLeft(Node expect, Node update) {
 *     return leftUpdater.compareAndSet(this, expect, update);
 *   }
 *   // ... and so on
 * }}</pre>
 *
 * <p>Note that the guarantees of the {@code compareAndSet}
 * method in this class are weaker than in other atomic classes.
 * Because this class cannot ensure that all uses of the field
 * are appropriate for purposes of atomic access, it can
 * guarantee atomicity only with respect to other invocations of
 * {@code compareAndSet} and {@code set} on the same updater.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <T> The type of the object holding the updatable field
 * @param <V> The type of the field
 */
public abstract class AtomicReferenceFieldUpdater<T,V> {

    /**
     * Creates and returns an updater for objects with the given field.
     * The Class arguments are needed to check that reflective types and
     * generic types match.
     *
     * @param tclass the class of the objects holding the field
     * @param vclass the class of the field
     * @param fieldName the name of the field to be updated
     * @param <U> the type of instances of tclass
     * @param <W> the type of instances of vclass
     * @return the updater
     * @throws ClassCastException if the field is of the wrong type
     * @throws IllegalArgumentException if the field is not volatile
     * @throws RuntimeException with a nested reflection-based
     * exception if the class does not hold field or is the wrong type,
     * or the field is inaccessible to the caller according to Java language
     * access control
     */
    @CallerSensitive
    public static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass,
                                                                    Class<W> vclass,
                                                                    String fieldName) {
        return new AtomicReferenceFieldUpdaterImpl<U,W>
            (tclass, vclass, fieldName, Reflection.getCallerClass());
    }

    /**
     * Protected do-nothing constructor for use by subclasses.
     */
    protected AtomicReferenceFieldUpdater() {
    }

    /**
     * Atomically sets the field of the given object managed by this updater
     * to the given updated value if the current value {@code ==} the
     * expected value. This method is guaranteed to be atomic with respect to
     * other calls to {@code compareAndSet} and {@code set}, but not
     * necessarily with respect to other changes in the field.
     *
     * @param obj An object whose field to conditionally set
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful
     */
    public abstract boolean compareAndSet(T obj, V expect, V update);

    /**
     * Atomically sets the field of the given object managed by this updater
     * to the given updated value if the current value {@code ==} the
     * expected value. This method is guaranteed to be atomic with respect to
     * other calls to {@code compareAndSet} and {@code set}, but not
     * necessarily with respect to other changes in the field.
     *
     * <p><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, so is
     * only rarely an appropriate alternative to {@code compareAndSet}.
     *
     * @param obj An object whose field to conditionally set
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful
     */
    public abstract boolean weakCompareAndSet(T obj, V expect, V update);

    /**
     * Sets the field of the given object managed by this updater to the
     * given updated value. This operation is guaranteed to act as a volatile
     * store with respect to subsequent invocations of {@code compareAndSet}.
     *
     * @param obj An object whose field to set
     * @param newValue the new value
     */
    public abstract void set(T obj, V newValue);

    /**
     * Eventually sets the field of the given object managed by this
     * updater to the given updated value.
     *
     * @param obj An object whose field to set
     * @param newValue the new value
     * @since 1.6
     */
    public abstract void lazySet(T obj, V newValue);

    /**
     * Gets the current value held in the field of the given object managed
     * by this updater.
     *
     * @param obj An object whose field to get
     * @return the current value
     */
    public abstract V get(T obj);

    /**
     * Atomically sets the field of the given object managed by this updater
     * to the given value and returns the old value.
     *
     * @param obj An object whose field to get and set
     * @param newValue the new value
     * @return the previous value
     */
    public V getAndSet(T obj, V newValue) {
        V prev;
        do {
            prev = get(obj);
        } while (!compareAndSet(obj, prev, newValue));
        return prev;
    }

    /**
     * Atomically updates the field of the given object managed by this updater
     * with the results of applying the given function, returning the previous
     * value. The function should be side-effect-free, since it may be
     * re-applied when attempted updates fail due to contention among threads.
     *
     * @param obj An object whose field to get and set
     * @param updateFunction a side-effect-free function
     * @return the previous value
     * @since 1.8
     */
    public final V getAndUpdate(T obj, UnaryOperator<V> updateFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = updateFunction.apply(prev);
        } while (!compareAndSet(obj, prev, next));
        return prev;
    }

    /**
     * Atomically updates the field of the given object managed by this updater
     * with the results of applying the given function, returning the updated
     * value. The function should be side-effect-free, since it may be
     * re-applied when attempted updates fail due to contention among threads.
     *
     * @param obj An object whose field to get and set
     * @param updateFunction a side-effect-free function
     * @return the updated value
     * @since 1.8
     */
    public final V updateAndGet(T obj, UnaryOperator<V> updateFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = updateFunction.apply(prev);
        } while (!compareAndSet(obj, prev, next));
        return next;
    }

    /**
     * Atomically updates the field of the given object managed by this
     * updater with the results of applying the given function to the
     * current and given values, returning the previous value. The
     * function should be side-effect-free, since it may be re-applied
     * when attempted updates fail due to contention among threads.  The
     * function is applied with the current value as its first argument,
     * and the given update as the second argument.
     *
     * @param obj An object whose field to get and set
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the previous value
     * @since 1.8
     */
    public final V getAndAccumulate(T obj, V x,
                                    BinaryOperator<V> accumulatorFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSet(obj, prev, next));
        return prev;
    }

    /**
     * Atomically updates the field of the given object managed by this
     * updater with the results of applying the given function to the
     * current and given values, returning the updated value. The
     * function should be side-effect-free, since it may be re-applied
     * when attempted updates fail due to contention among threads.  The
     * function is applied with the current value as its first argument,
     * and the given update as the second argument.
     *
     * @param obj An object whose field to get and set
     * @param x the update value
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @return the updated value
     * @since 1.8
     */
    public final V accumulateAndGet(T obj, V x,
                                    BinaryOperator<V> accumulatorFunction) {
        V prev, next;
        do {
            prev = get(obj);
            next = accumulatorFunction.apply(prev, x);
        } while (!compareAndSet(obj, prev, next));
        return next;
    }

    private static final class AtomicReferenceFieldUpdaterImpl<T,V>
        extends AtomicReferenceFieldUpdater<T,V> {
        private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
        private final long offset;
        /**
         * if field is protected, the subclass constructing updater, else
         * the same as tclass
         */
        private final Class<?> cclass;
        /** class holding the field */
        private final Class<T> tclass;
        /** field value type */
        private final Class<V> vclass;

        /*
         * Internal type checks within all update methods contain
         * internal inlined optimizations checking for the common
         * cases where the class is final (in which case a simple
         * getClass comparison suffices) or is of type Object (in
         * which case no check is needed because all objects are
         * instances of Object). The Object case is handled simply by
         * setting vclass to null in constructor.  The targetCheck and
         * updateCheck methods are invoked when these faster
         * screenings fail.
         */

        AtomicReferenceFieldUpdaterImpl(final Class<T> tclass,
                                        final Class<V> vclass,
                                        final String fieldName,
                                        final Class<?> caller) {
            final Field field;
            final Class<?> fieldClass;
            final int modifiers;
            try {
                field = AccessController.doPrivileged(
                    new PrivilegedExceptionAction<Field>() {
                        public Field run() throws NoSuchFieldException {
                            return tclass.getDeclaredField(fieldName);
                        }
                    });
                modifiers = field.getModifiers();
                sun.reflect.misc.ReflectUtil.ensureMemberAccess(
                    caller, tclass, null, modifiers);
                ClassLoader cl = tclass.getClassLoader();
                ClassLoader ccl = caller.getClassLoader();
                if ((ccl != null) && (ccl != cl) &&
                    ((cl == null) || !isAncestor(cl, ccl))) {
                    sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
                }
                fieldClass = field.getType();
            } catch (PrivilegedActionException pae) {
                throw new RuntimeException(pae.getException());
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }

            if (vclass != fieldClass)
                throw new ClassCastException();
            if (vclass.isPrimitive())
                throw new IllegalArgumentException("Must be reference type");

            if (!Modifier.isVolatile(modifiers))
                throw new IllegalArgumentException("Must be volatile type");

            this.cclass = (Modifier.isProtected(modifiers)) ? caller : tclass;
            this.tclass = tclass;
            this.vclass = vclass;
            this.offset = U.objectFieldOffset(field);
        }

        /**
         * Returns true if the second classloader can be found in the first
         * classloader's delegation chain.
         * Equivalent to the inaccessible: first.isAncestor(second).
         */
        private static boolean isAncestor(ClassLoader first, ClassLoader second) {
            ClassLoader acl = first;
            do {
                acl = acl.getParent();
                if (second == acl) {
                    return true;
                }
            } while (acl != null);
            return false;
        }

        /**
         * Checks that target argument is instance of cclass.  On
         * failure, throws cause.
         */
        private final void accessCheck(T obj) {
            if (!cclass.isInstance(obj))
                throwAccessCheckException(obj);
        }

        /**
         * Throws access exception if accessCheck failed due to
         * protected access, else ClassCastException.
         */
        private final void throwAccessCheckException(T obj) {
            if (cclass == tclass)
                throw new ClassCastException();
            else
                throw new RuntimeException(
                    new IllegalAccessException(
                        "Class " +
                        cclass.getName() +
                        " can not access a protected member of class " +
                        tclass.getName() +
                        " using an instance of " +
                        obj.getClass().getName()));
        }

        private final void valueCheck(V v) {
            if (v != null && !(vclass.isInstance(v)))
                throwCCE();
        }

        static void throwCCE() {
            throw new ClassCastException();
        }

        public final boolean compareAndSet(T obj, V expect, V update) {
            accessCheck(obj);
            valueCheck(update);
            return U.compareAndSwapObject(obj, offset, expect, update);
        }

        public final boolean weakCompareAndSet(T obj, V expect, V update) {
            // same implementation as strong form for now
            accessCheck(obj);
            valueCheck(update);
            return U.compareAndSwapObject(obj, offset, expect, update);
        }

        public final void set(T obj, V newValue) {
            accessCheck(obj);
            valueCheck(newValue);
            U.putObjectVolatile(obj, offset, newValue);
        }

        public final void lazySet(T obj, V newValue) {
            accessCheck(obj);
            valueCheck(newValue);
            U.putOrderedObject(obj, offset, newValue);
        }

        @SuppressWarnings("unchecked")
        public final V get(T obj) {
            accessCheck(obj);
            return (V)U.getObjectVolatile(obj, offset);
        }

        @SuppressWarnings("unchecked")
        public final V getAndSet(T obj, V newValue) {
            accessCheck(obj);
            valueCheck(newValue);
            return (V)U.getAndSetObject(obj, offset, newValue);
        }
    }
}
Revision:	1.1
Committed:	Sat Mar 26 06:22:51 2016 UTC (8 years, 2 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Log Message:	fork jdk8 maintenance branch for source and jtreg tests
#	User	Rev	Content
1	jsr166	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent.atomic;
8
9			import java.lang.reflect.Field;
10			import java.lang.reflect.Modifier;
11			import java.security.AccessController;
12			import java.security.PrivilegedActionException;
13			import java.security.PrivilegedExceptionAction;
14			import java.util.function.BinaryOperator;
15			import java.util.function.UnaryOperator;
16			import sun.reflect.CallerSensitive;
17			import sun.reflect.Reflection;
18
19			/**
20			* A reflection-based utility that enables atomic updates to
21			* designated {@code volatile} reference fields of designated
22			* classes. This class is designed for use in atomic data structures
23			* in which several reference fields of the same node are
24			* independently subject to atomic updates. For example, a tree node
25			* might be declared as
26			*
27			* <pre> {@code
28			* class Node {
29			* private volatile Node left, right;
30			*
31			* private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
32			* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
33			* private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
34			* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
35			*
36			* Node getLeft() { return left; }
37			* boolean compareAndSetLeft(Node expect, Node update) {
38			* return leftUpdater.compareAndSet(this, expect, update);
39			* }
40			* // ... and so on
41			* }}</pre>
42			*
43			* <p>Note that the guarantees of the {@code compareAndSet}
44			* method in this class are weaker than in other atomic classes.
45			* Because this class cannot ensure that all uses of the field
46			* are appropriate for purposes of atomic access, it can
47			* guarantee atomicity only with respect to other invocations of
48			* {@code compareAndSet} and {@code set} on the same updater.
49			*
50			* @since 1.5
51			* @author Doug Lea
52			* @param <T> The type of the object holding the updatable field
53			* @param <V> The type of the field
54			*/
55			public abstract class AtomicReferenceFieldUpdater<T,V> {
56
57			/**
58			* Creates and returns an updater for objects with the given field.
59			* The Class arguments are needed to check that reflective types and
60			* generic types match.
61			*
62			* @param tclass the class of the objects holding the field
63			* @param vclass the class of the field
64			* @param fieldName the name of the field to be updated
65			* @param <U> the type of instances of tclass
66			* @param <W> the type of instances of vclass
67			* @return the updater
68			* @throws ClassCastException if the field is of the wrong type
69			* @throws IllegalArgumentException if the field is not volatile
70			* @throws RuntimeException with a nested reflection-based
71			* exception if the class does not hold field or is the wrong type,
72			* or the field is inaccessible to the caller according to Java language
73			* access control
74			*/
75			@CallerSensitive
76			public static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass,
77			Class<W> vclass,
78			String fieldName) {
79			return new AtomicReferenceFieldUpdaterImpl<U,W>
80			(tclass, vclass, fieldName, Reflection.getCallerClass());
81			}
82
83			/**
84			* Protected do-nothing constructor for use by subclasses.
85			*/
86			protected AtomicReferenceFieldUpdater() {
87			}
88
89			/**
90			* Atomically sets the field of the given object managed by this updater
91			* to the given updated value if the current value {@code ==} the
92			* expected value. This method is guaranteed to be atomic with respect to
93			* other calls to {@code compareAndSet} and {@code set}, but not
94			* necessarily with respect to other changes in the field.
95			*
96			* @param obj An object whose field to conditionally set
97			* @param expect the expected value
98			* @param update the new value
99			* @return {@code true} if successful
100			*/
101			public abstract boolean compareAndSet(T obj, V expect, V update);
102
103			/**
104			* Atomically sets the field of the given object managed by this updater
105			* to the given updated value if the current value {@code ==} the
106			* expected value. This method is guaranteed to be atomic with respect to
107			* other calls to {@code compareAndSet} and {@code set}, but not
108			* necessarily with respect to other changes in the field.
109			*
110			* <p><a href="package-summary.html#weakCompareAndSet">May fail
111			* spuriously and does not provide ordering guarantees</a>, so is
112			* only rarely an appropriate alternative to {@code compareAndSet}.
113			*
114			* @param obj An object whose field to conditionally set
115			* @param expect the expected value
116			* @param update the new value
117			* @return {@code true} if successful
118			*/
119			public abstract boolean weakCompareAndSet(T obj, V expect, V update);
120
121			/**
122			* Sets the field of the given object managed by this updater to the
123			* given updated value. This operation is guaranteed to act as a volatile
124			* store with respect to subsequent invocations of {@code compareAndSet}.
125			*
126			* @param obj An object whose field to set
127			* @param newValue the new value
128			*/
129			public abstract void set(T obj, V newValue);
130
131			/**
132			* Eventually sets the field of the given object managed by this
133			* updater to the given updated value.
134			*
135			* @param obj An object whose field to set
136			* @param newValue the new value
137			* @since 1.6
138			*/
139			public abstract void lazySet(T obj, V newValue);
140
141			/**
142			* Gets the current value held in the field of the given object managed
143			* by this updater.
144			*
145			* @param obj An object whose field to get
146			* @return the current value
147			*/
148			public abstract V get(T obj);
149
150			/**
151			* Atomically sets the field of the given object managed by this updater
152			* to the given value and returns the old value.
153			*
154			* @param obj An object whose field to get and set
155			* @param newValue the new value
156			* @return the previous value
157			*/
158			public V getAndSet(T obj, V newValue) {
159			V prev;
160			do {
161			prev = get(obj);
162			} while (!compareAndSet(obj, prev, newValue));
163			return prev;
164			}
165
166			/**
167			* Atomically updates the field of the given object managed by this updater
168			* with the results of applying the given function, returning the previous
169			* value. The function should be side-effect-free, since it may be
170			* re-applied when attempted updates fail due to contention among threads.
171			*
172			* @param obj An object whose field to get and set
173			* @param updateFunction a side-effect-free function
174			* @return the previous value
175			* @since 1.8
176			*/
177			public final V getAndUpdate(T obj, UnaryOperator<V> updateFunction) {
178			V prev, next;
179			do {
180			prev = get(obj);
181			next = updateFunction.apply(prev);
182			} while (!compareAndSet(obj, prev, next));
183			return prev;
184			}
185
186			/**
187			* Atomically updates the field of the given object managed by this updater
188			* with the results of applying the given function, returning the updated
189			* value. The function should be side-effect-free, since it may be
190			* re-applied when attempted updates fail due to contention among threads.
191			*
192			* @param obj An object whose field to get and set
193			* @param updateFunction a side-effect-free function
194			* @return the updated value
195			* @since 1.8
196			*/
197			public final V updateAndGet(T obj, UnaryOperator<V> updateFunction) {
198			V prev, next;
199			do {
200			prev = get(obj);
201			next = updateFunction.apply(prev);
202			} while (!compareAndSet(obj, prev, next));
203			return next;
204			}
205
206			/**
207			* Atomically updates the field of the given object managed by this
208			* updater with the results of applying the given function to the
209			* current and given values, returning the previous value. The
210			* function should be side-effect-free, since it may be re-applied
211			* when attempted updates fail due to contention among threads. The
212			* function is applied with the current value as its first argument,
213			* and the given update as the second argument.
214			*
215			* @param obj An object whose field to get and set
216			* @param x the update value
217			* @param accumulatorFunction a side-effect-free function of two arguments
218			* @return the previous value
219			* @since 1.8
220			*/
221			public final V getAndAccumulate(T obj, V x,
222			BinaryOperator<V> accumulatorFunction) {
223			V prev, next;
224			do {
225			prev = get(obj);
226			next = accumulatorFunction.apply(prev, x);
227			} while (!compareAndSet(obj, prev, next));
228			return prev;
229			}
230
231			/**
232			* Atomically updates the field of the given object managed by this
233			* updater with the results of applying the given function to the
234			* current and given values, returning the updated value. The
235			* function should be side-effect-free, since it may be re-applied
236			* when attempted updates fail due to contention among threads. The
237			* function is applied with the current value as its first argument,
238			* and the given update as the second argument.
239			*
240			* @param obj An object whose field to get and set
241			* @param x the update value
242			* @param accumulatorFunction a side-effect-free function of two arguments
243			* @return the updated value
244			* @since 1.8
245			*/
246			public final V accumulateAndGet(T obj, V x,
247			BinaryOperator<V> accumulatorFunction) {
248			V prev, next;
249			do {
250			prev = get(obj);
251			next = accumulatorFunction.apply(prev, x);
252			} while (!compareAndSet(obj, prev, next));
253			return next;
254			}
255
256			private static final class AtomicReferenceFieldUpdaterImpl<T,V>
257			extends AtomicReferenceFieldUpdater<T,V> {
258			private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
259			private final long offset;
260			/**
261			* if field is protected, the subclass constructing updater, else
262			* the same as tclass
263			*/
264			private final Class<?> cclass;
265			/** class holding the field */
266			private final Class<T> tclass;
267			/** field value type */
268			private final Class<V> vclass;
269
270			/*
271			* Internal type checks within all update methods contain
272			* internal inlined optimizations checking for the common
273			* cases where the class is final (in which case a simple
274			* getClass comparison suffices) or is of type Object (in
275			* which case no check is needed because all objects are
276			* instances of Object). The Object case is handled simply by
277			* setting vclass to null in constructor. The targetCheck and
278			* updateCheck methods are invoked when these faster
279			* screenings fail.
280			*/
281
282			AtomicReferenceFieldUpdaterImpl(final Class<T> tclass,
283			final Class<V> vclass,
284			final String fieldName,
285			final Class<?> caller) {
286			final Field field;
287			final Class<?> fieldClass;
288			final int modifiers;
289			try {
290			field = AccessController.doPrivileged(
291			new PrivilegedExceptionAction<Field>() {
292			public Field run() throws NoSuchFieldException {
293			return tclass.getDeclaredField(fieldName);
294			}
295			});
296			modifiers = field.getModifiers();
297			sun.reflect.misc.ReflectUtil.ensureMemberAccess(
298			caller, tclass, null, modifiers);
299			ClassLoader cl = tclass.getClassLoader();
300			ClassLoader ccl = caller.getClassLoader();
301			if ((ccl != null) && (ccl != cl) &&
302			((cl == null) \|\| !isAncestor(cl, ccl))) {
303			sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
304			}
305			fieldClass = field.getType();
306			} catch (PrivilegedActionException pae) {
307			throw new RuntimeException(pae.getException());
308			} catch (Exception ex) {
309			throw new RuntimeException(ex);
310			}
311
312			if (vclass != fieldClass)
313			throw new ClassCastException();
314			if (vclass.isPrimitive())
315			throw new IllegalArgumentException("Must be reference type");
316
317			if (!Modifier.isVolatile(modifiers))
318			throw new IllegalArgumentException("Must be volatile type");
319
320			this.cclass = (Modifier.isProtected(modifiers)) ? caller : tclass;
321			this.tclass = tclass;
322			this.vclass = vclass;
323			this.offset = U.objectFieldOffset(field);
324			}
325
326			/**
327			* Returns true if the second classloader can be found in the first
328			* classloader's delegation chain.
329			* Equivalent to the inaccessible: first.isAncestor(second).
330			*/
331			private static boolean isAncestor(ClassLoader first, ClassLoader second) {
332			ClassLoader acl = first;
333			do {
334			acl = acl.getParent();
335			if (second == acl) {
336			return true;
337			}
338			} while (acl != null);
339			return false;
340			}
341
342			/**
343			* Checks that target argument is instance of cclass. On
344			* failure, throws cause.
345			*/
346			private final void accessCheck(T obj) {
347			if (!cclass.isInstance(obj))
348			throwAccessCheckException(obj);
349			}
350
351			/**
352			* Throws access exception if accessCheck failed due to
353			* protected access, else ClassCastException.
354			*/
355			private final void throwAccessCheckException(T obj) {
356			if (cclass == tclass)
357			throw new ClassCastException();
358			else
359			throw new RuntimeException(
360			new IllegalAccessException(
361			"Class " +
362			cclass.getName() +
363			" can not access a protected member of class " +
364			tclass.getName() +
365			" using an instance of " +
366			obj.getClass().getName()));
367			}
368
369			private final void valueCheck(V v) {
370			if (v != null && !(vclass.isInstance(v)))
371			throwCCE();
372			}
373
374			static void throwCCE() {
375			throw new ClassCastException();
376			}
377
378			public final boolean compareAndSet(T obj, V expect, V update) {
379			accessCheck(obj);
380			valueCheck(update);
381			return U.compareAndSwapObject(obj, offset, expect, update);
382			}
383
384			public final boolean weakCompareAndSet(T obj, V expect, V update) {
385			// same implementation as strong form for now
386			accessCheck(obj);
387			valueCheck(update);
388			return U.compareAndSwapObject(obj, offset, expect, update);
389			}
390
391			public final void set(T obj, V newValue) {
392			accessCheck(obj);
393			valueCheck(newValue);
394			U.putObjectVolatile(obj, offset, newValue);
395			}
396
397			public final void lazySet(T obj, V newValue) {
398			accessCheck(obj);
399			valueCheck(newValue);
400			U.putOrderedObject(obj, offset, newValue);
401			}
402
403			@SuppressWarnings("unchecked")
404			public final V get(T obj) {
405			accessCheck(obj);
406			return (V)U.getObjectVolatile(obj, offset);
407			}
408
409			@SuppressWarnings("unchecked")
410			public final V getAndSet(T obj, V newValue) {
411			accessCheck(obj);
412			valueCheck(newValue);
413			return (V)U.getAndSetObject(obj, offset, newValue);
414			}
415			}
416			}