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 sun.misc.Unsafe;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.security.AccessController;
import java.security.PrivilegedExceptionAction;
import java.security.PrivilegedActionException;

/**
 * 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.
     * @return the updater
     * @throws IllegalArgumentException if the field is not a volatile reference type.
     * @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
     */
    public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) {
        return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
                                                        vclass,
                                                        fieldName);
    }

    /**
     * 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 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>May <a href="package-summary.html#Spurious">fail spuriously</a>
     * and does not provide ordering guarantees, 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 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) {
        for (;;) {
            V current = get(obj);
            if (compareAndSet(obj, current, newValue))
                return current;
        }
    }

    private static final class AtomicReferenceFieldUpdaterImpl<T,V>
        extends AtomicReferenceFieldUpdater<T,V> {
        private static final Unsafe unsafe = Unsafe.getUnsafe();
        private final long offset;
        private final Class<T> tclass;
        private final Class<V> vclass;
        private final Class<?> cclass;

        /*
         * 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,
                                        Class<V> vclass,
                                        final String fieldName) {
            final Field field;
            final Class<?> fieldClass;
            final Class<?> caller;
            final int modifiers;
            try {
                field = AccessController.doPrivileged(
                    new PrivilegedExceptionAction<Field>() {
                        public Field run() throws NoSuchFieldException {
                            return tclass.getDeclaredField(fieldName);
                        }
                    });
                caller = sun.reflect.Reflection.getCallerClass(3);
                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 (!Modifier.isVolatile(modifiers))
                throw new IllegalArgumentException("Must be volatile type");

            this.cclass = (Modifier.isProtected(modifiers) &&
                           caller != tclass) ? caller : null;
            this.tclass = tclass;
            if (vclass == Object.class)
                this.vclass = null;
            else
                this.vclass = vclass;
            offset = unsafe.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;
        }

        void targetCheck(T obj) {
            if (!tclass.isInstance(obj))
                throw new ClassCastException();
            if (cclass != null)
                ensureProtectedAccess(obj);
        }

        void updateCheck(T obj, V update) {
            if (!tclass.isInstance(obj) ||
                (update != null && vclass != null && !vclass.isInstance(update)))
                throw new ClassCastException();
            if (cclass != null)
                ensureProtectedAccess(obj);
        }

        public boolean compareAndSet(T obj, V expect, V update) {
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (update != null && vclass != null &&
                 vclass != update.getClass()))
                updateCheck(obj, update);
            return unsafe.compareAndSwapObject(obj, offset, expect, update);
        }

        public boolean weakCompareAndSet(T obj, V expect, V update) {
            // same implementation as strong form for now
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (update != null && vclass != null &&
                 vclass != update.getClass()))
                updateCheck(obj, update);
            return unsafe.compareAndSwapObject(obj, offset, expect, update);
        }

        public void set(T obj, V newValue) {
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (newValue != null && vclass != null &&
                 vclass != newValue.getClass()))
                updateCheck(obj, newValue);
            unsafe.putObjectVolatile(obj, offset, newValue);
        }

        public void lazySet(T obj, V newValue) {
            if (obj == null || obj.getClass() != tclass || cclass != null ||
                (newValue != null && vclass != null &&
                 vclass != newValue.getClass()))
                updateCheck(obj, newValue);
            unsafe.putOrderedObject(obj, offset, newValue);
        }

        @SuppressWarnings("unchecked")
        public V get(T obj) {
            if (obj == null || obj.getClass() != tclass || cclass != null)
                targetCheck(obj);
            return (V)unsafe.getObjectVolatile(obj, offset);
        }

        private void ensureProtectedAccess(T obj) {
            if (cclass.isInstance(obj)) {
                return;
            }
            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()
                )
            );
        }
    }
}
Revision:	1.2
Committed:	Wed Jan 2 06:29:00 2013 UTC (11 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.1:	+2 -2 lines
Log Message:	@return javadoc style
#	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	import sun.misc.Unsafe;
9	import java.lang.reflect.Field;
10	import java.lang.reflect.Modifier;
11	import java.security.AccessController;
12	import java.security.PrivilegedExceptionAction;
13	import java.security.PrivilegedActionException;
14
15	/**
16	* A reflection-based utility that enables atomic updates to
17	* designated {@code volatile} reference fields of designated
18	* classes. This class is designed for use in atomic data structures
19	* in which several reference fields of the same node are
20	* independently subject to atomic updates. For example, a tree node
21	* might be declared as
22	*
23	* <pre> {@code
24	* class Node {
25	* private volatile Node left, right;
26	*
27	* private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
28	* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
29	* private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
30	* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
31	*
32	* Node getLeft() { return left; }
33	* boolean compareAndSetLeft(Node expect, Node update) {
34	* return leftUpdater.compareAndSet(this, expect, update);
35	* }
36	* // ... and so on
37	* }}</pre>
38	*
39	* <p>Note that the guarantees of the {@code compareAndSet}
40	* method in this class are weaker than in other atomic classes.
41	* Because this class cannot ensure that all uses of the field
42	* are appropriate for purposes of atomic access, it can
43	* guarantee atomicity only with respect to other invocations of
44	* {@code compareAndSet} and {@code set} on the same updater.
45	*
46	* @since 1.5
47	* @author Doug Lea
48	* @param <T> The type of the object holding the updatable field
49	* @param <V> The type of the field
50	*/
51	public abstract class AtomicReferenceFieldUpdater<T, V> {
52
53	/**
54	* Creates and returns an updater for objects with the given field.
55	* The Class arguments are needed to check that reflective types and
56	* generic types match.
57	*
58	* @param tclass the class of the objects holding the field.
59	* @param vclass the class of the field
60	* @param fieldName the name of the field to be updated.
61	* @return the updater
62	* @throws IllegalArgumentException if the field is not a volatile reference type.
63	* @throws RuntimeException with a nested reflection-based
64	* exception if the class does not hold field or is the wrong type,
65	* or the field is inaccessible to the caller according to Java language
66	* access control
67	*/
68	public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) {
69	return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
70	vclass,
71	fieldName);
72	}
73
74	/**
75	* Protected do-nothing constructor for use by subclasses.
76	*/
77	protected AtomicReferenceFieldUpdater() {
78	}
79
80	/**
81	* Atomically sets the field of the given object managed by this updater
82	* to the given updated value if the current value {@code ==} the
83	* expected value. This method is guaranteed to be atomic with respect to
84	* other calls to {@code compareAndSet} and {@code set}, but not
85	* necessarily with respect to other changes in the field.
86	*
87	* @param obj An object whose field to conditionally set
88	* @param expect the expected value
89	* @param update the new value
90	* @return true if successful
91	*/
92	public abstract boolean compareAndSet(T obj, V expect, V update);
93
94	/**
95	* Atomically sets the field of the given object managed by this updater
96	* to the given updated value if the current value {@code ==} the
97	* expected value. This method is guaranteed to be atomic with respect to
98	* other calls to {@code compareAndSet} and {@code set}, but not
99	* necessarily with respect to other changes in the field.
100	*
101	* <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
102	* and does not provide ordering guarantees, so is only rarely an
103	* appropriate alternative to {@code compareAndSet}.
104	*
105	* @param obj An object whose field to conditionally set
106	* @param expect the expected value
107	* @param update the new value
108	* @return true if successful
109	*/
110	public abstract boolean weakCompareAndSet(T obj, V expect, V update);
111
112	/**
113	* Sets the field of the given object managed by this updater to the
114	* given updated value. This operation is guaranteed to act as a volatile
115	* store with respect to subsequent invocations of {@code compareAndSet}.
116	*
117	* @param obj An object whose field to set
118	* @param newValue the new value
119	*/
120	public abstract void set(T obj, V newValue);
121
122	/**
123	* Eventually sets the field of the given object managed by this
124	* updater to the given updated value.
125	*
126	* @param obj An object whose field to set
127	* @param newValue the new value
128	* @since 1.6
129	*/
130	public abstract void lazySet(T obj, V newValue);
131
132	/**
133	* Gets the current value held in the field of the given object managed
134	* by this updater.
135	*
136	* @param obj An object whose field to get
137	* @return the current value
138	*/
139	public abstract V get(T obj);
140
141	/**
142	* Atomically sets the field of the given object managed by this updater
143	* to the given value and returns the old value.
144	*
145	* @param obj An object whose field to get and set
146	* @param newValue the new value
147	* @return the previous value
148	*/
149	public V getAndSet(T obj, V newValue) {
150	for (;;) {
151	V current = get(obj);
152	if (compareAndSet(obj, current, newValue))
153	return current;
154	}
155	}
156
157	private static final class AtomicReferenceFieldUpdaterImpl<T,V>
158	extends AtomicReferenceFieldUpdater<T,V> {
159	private static final Unsafe unsafe = Unsafe.getUnsafe();
160	private final long offset;
161	private final Class<T> tclass;
162	private final Class<V> vclass;
163	private final Class<?> cclass;
164
165	/*
166	* Internal type checks within all update methods contain
167	* internal inlined optimizations checking for the common
168	* cases where the class is final (in which case a simple
169	* getClass comparison suffices) or is of type Object (in
170	* which case no check is needed because all objects are
171	* instances of Object). The Object case is handled simply by
172	* setting vclass to null in constructor. The targetCheck and
173	* updateCheck methods are invoked when these faster
174	* screenings fail.
175	*/
176
177	AtomicReferenceFieldUpdaterImpl(final Class<T> tclass,
178	Class<V> vclass,
179	final String fieldName) {
180	final Field field;
181	final Class<?> fieldClass;
182	final Class<?> caller;
183	final int modifiers;
184	try {
185	field = AccessController.doPrivileged(
186	new PrivilegedExceptionAction<Field>() {
187	public Field run() throws NoSuchFieldException {
188	return tclass.getDeclaredField(fieldName);
189	}
190	});
191	caller = sun.reflect.Reflection.getCallerClass(3);
192	modifiers = field.getModifiers();
193	sun.reflect.misc.ReflectUtil.ensureMemberAccess(
194	caller, tclass, null, modifiers);
195	ClassLoader cl = tclass.getClassLoader();
196	ClassLoader ccl = caller.getClassLoader();
197	if ((ccl != null) && (ccl != cl) &&
198	((cl == null) \|\| !isAncestor(cl, ccl))) {
199	sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
200	}
201	fieldClass = field.getType();
202	} catch (PrivilegedActionException pae) {
203	throw new RuntimeException(pae.getException());
204	} catch (Exception ex) {
205	throw new RuntimeException(ex);
206	}
207
208	if (vclass != fieldClass)
209	throw new ClassCastException();
210
211	if (!Modifier.isVolatile(modifiers))
212	throw new IllegalArgumentException("Must be volatile type");
213
214	this.cclass = (Modifier.isProtected(modifiers) &&
215	caller != tclass) ? caller : null;
216	this.tclass = tclass;
217	if (vclass == Object.class)
218	this.vclass = null;
219	else
220	this.vclass = vclass;
221	offset = unsafe.objectFieldOffset(field);
222	}
223
224	/**
225	* Returns true if the second classloader can be found in the first
226	* classloader's delegation chain.
227	* Equivalent to the inaccessible: first.isAncestor(second).
228	*/
229	private static boolean isAncestor(ClassLoader first, ClassLoader second) {
230	ClassLoader acl = first;
231	do {
232	acl = acl.getParent();
233	if (second == acl) {
234	return true;
235	}
236	} while (acl != null);
237	return false;
238	}
239
240	void targetCheck(T obj) {
241	if (!tclass.isInstance(obj))
242	throw new ClassCastException();
243	if (cclass != null)
244	ensureProtectedAccess(obj);
245	}
246
247	void updateCheck(T obj, V update) {
248	if (!tclass.isInstance(obj) \|\|
249	(update != null && vclass != null && !vclass.isInstance(update)))
250	throw new ClassCastException();
251	if (cclass != null)
252	ensureProtectedAccess(obj);
253	}
254
255	public boolean compareAndSet(T obj, V expect, V update) {
256	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
257	(update != null && vclass != null &&
258	vclass != update.getClass()))
259	updateCheck(obj, update);
260	return unsafe.compareAndSwapObject(obj, offset, expect, update);
261	}
262
263	public boolean weakCompareAndSet(T obj, V expect, V update) {
264	// same implementation as strong form for now
265	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
266	(update != null && vclass != null &&
267	vclass != update.getClass()))
268	updateCheck(obj, update);
269	return unsafe.compareAndSwapObject(obj, offset, expect, update);
270	}
271
272	public void set(T obj, V newValue) {
273	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
274	(newValue != null && vclass != null &&
275	vclass != newValue.getClass()))
276	updateCheck(obj, newValue);
277	unsafe.putObjectVolatile(obj, offset, newValue);
278	}
279
280	public void lazySet(T obj, V newValue) {
281	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
282	(newValue != null && vclass != null &&
283	vclass != newValue.getClass()))
284	updateCheck(obj, newValue);
285	unsafe.putOrderedObject(obj, offset, newValue);
286	}
287
288	@SuppressWarnings("unchecked")
289	public V get(T obj) {
290	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null)
291	targetCheck(obj);
292	return (V)unsafe.getObjectVolatile(obj, offset);
293	}
294
295	private void ensureProtectedAccess(T obj) {
296	if (cclass.isInstance(obj)) {
297	return;
298	}
299	throw new RuntimeException(
300	new IllegalAccessException("Class " +
301	cclass.getName() +
302	" can not access a protected member of class " +
303	tclass.getName() +
304	" using an instance of " +
305	obj.getClass().getName()
306	)
307	);
308	}
309	}
310	}