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, 1 month ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Log Message:	fork jdk8 maintenance branch for source and jtreg tests
#	Content
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	}