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
     * @param <U> the type of instances of tclass
     * @param <W> the type of instances of vclass
     * @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><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 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:	1.6
Committed:	Sat Jul 20 20:12:37 2013 UTC (10 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.5:	+2 -0 lines
Log Message:	sync javadoc fixes from src/main
#	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	* @param <U> the type of instances of tclass
62	* @param <W> the type of instances of vclass
63	* @return the updater
64	* @throws IllegalArgumentException if the field is not a volatile reference type
65	* @throws RuntimeException with a nested reflection-based
66	* exception if the class does not hold field or is the wrong type,
67	* or the field is inaccessible to the caller according to Java language
68	* access control
69	*/
70	public static <U, W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass, Class<W> vclass, String fieldName) {
71	return new AtomicReferenceFieldUpdaterImpl<U,W>(tclass,
72	vclass,
73	fieldName);
74	}
75
76	/**
77	* Protected do-nothing constructor for use by subclasses.
78	*/
79	protected AtomicReferenceFieldUpdater() {
80	}
81
82	/**
83	* Atomically sets the field of the given object managed by this updater
84	* to the given updated value if the current value {@code ==} the
85	* expected value. This method is guaranteed to be atomic with respect to
86	* other calls to {@code compareAndSet} and {@code set}, but not
87	* necessarily with respect to other changes in the field.
88	*
89	* @param obj An object whose field to conditionally set
90	* @param expect the expected value
91	* @param update the new value
92	* @return true if successful
93	*/
94	public abstract boolean compareAndSet(T obj, V expect, V update);
95
96	/**
97	* Atomically sets the field of the given object managed by this updater
98	* to the given updated value if the current value {@code ==} the
99	* expected value. This method is guaranteed to be atomic with respect to
100	* other calls to {@code compareAndSet} and {@code set}, but not
101	* necessarily with respect to other changes in the field.
102	*
103	* <p><a href="package-summary.html#weakCompareAndSet">May fail
104	* spuriously and does not provide ordering guarantees</a>, so is
105	* only rarely an appropriate alternative to {@code compareAndSet}.
106	*
107	* @param obj An object whose field to conditionally set
108	* @param expect the expected value
109	* @param update the new value
110	* @return true if successful
111	*/
112	public abstract boolean weakCompareAndSet(T obj, V expect, V update);
113
114	/**
115	* Sets the field of the given object managed by this updater to the
116	* given updated value. This operation is guaranteed to act as a volatile
117	* store with respect to subsequent invocations of {@code compareAndSet}.
118	*
119	* @param obj An object whose field to set
120	* @param newValue the new value
121	*/
122	public abstract void set(T obj, V newValue);
123
124	/**
125	* Eventually sets the field of the given object managed by this
126	* updater to the given updated value.
127	*
128	* @param obj An object whose field to set
129	* @param newValue the new value
130	* @since 1.6
131	*/
132	public abstract void lazySet(T obj, V newValue);
133
134	/**
135	* Gets the current value held in the field of the given object managed
136	* by this updater.
137	*
138	* @param obj An object whose field to get
139	* @return the current value
140	*/
141	public abstract V get(T obj);
142
143	/**
144	* Atomically sets the field of the given object managed by this updater
145	* to the given value and returns the old value.
146	*
147	* @param obj An object whose field to get and set
148	* @param newValue the new value
149	* @return the previous value
150	*/
151	public V getAndSet(T obj, V newValue) {
152	for (;;) {
153	V current = get(obj);
154	if (compareAndSet(obj, current, newValue))
155	return current;
156	}
157	}
158
159	private static final class AtomicReferenceFieldUpdaterImpl<T,V>
160	extends AtomicReferenceFieldUpdater<T,V> {
161	private static final Unsafe unsafe = Unsafe.getUnsafe();
162	private final long offset;
163	private final Class<T> tclass;
164	private final Class<V> vclass;
165	private final Class<?> cclass;
166
167	/*
168	* Internal type checks within all update methods contain
169	* internal inlined optimizations checking for the common
170	* cases where the class is final (in which case a simple
171	* getClass comparison suffices) or is of type Object (in
172	* which case no check is needed because all objects are
173	* instances of Object). The Object case is handled simply by
174	* setting vclass to null in constructor. The targetCheck and
175	* updateCheck methods are invoked when these faster
176	* screenings fail.
177	*/
178
179	AtomicReferenceFieldUpdaterImpl(final Class<T> tclass,
180	Class<V> vclass,
181	final String fieldName) {
182	final Field field;
183	final Class<?> fieldClass;
184	final Class<?> caller;
185	final int modifiers;
186	try {
187	field = AccessController.doPrivileged(
188	new PrivilegedExceptionAction<Field>() {
189	public Field run() throws NoSuchFieldException {
190	return tclass.getDeclaredField(fieldName);
191	}
192	});
193	caller = sun.reflect.Reflection.getCallerClass(3);
194	modifiers = field.getModifiers();
195	sun.reflect.misc.ReflectUtil.ensureMemberAccess(
196	caller, tclass, null, modifiers);
197	ClassLoader cl = tclass.getClassLoader();
198	ClassLoader ccl = caller.getClassLoader();
199	if ((ccl != null) && (ccl != cl) &&
200	((cl == null) \|\| !isAncestor(cl, ccl))) {
201	sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
202	}
203	fieldClass = field.getType();
204	} catch (PrivilegedActionException pae) {
205	throw new RuntimeException(pae.getException());
206	} catch (Exception ex) {
207	throw new RuntimeException(ex);
208	}
209
210	if (vclass != fieldClass)
211	throw new ClassCastException();
212
213	if (!Modifier.isVolatile(modifiers))
214	throw new IllegalArgumentException("Must be volatile type");
215
216	this.cclass = (Modifier.isProtected(modifiers) &&
217	caller != tclass) ? caller : null;
218	this.tclass = tclass;
219	if (vclass == Object.class)
220	this.vclass = null;
221	else
222	this.vclass = vclass;
223	offset = unsafe.objectFieldOffset(field);
224	}
225
226	/**
227	* Returns true if the second classloader can be found in the first
228	* classloader's delegation chain.
229	* Equivalent to the inaccessible: first.isAncestor(second).
230	*/
231	private static boolean isAncestor(ClassLoader first, ClassLoader second) {
232	ClassLoader acl = first;
233	do {
234	acl = acl.getParent();
235	if (second == acl) {
236	return true;
237	}
238	} while (acl != null);
239	return false;
240	}
241
242	void targetCheck(T obj) {
243	if (!tclass.isInstance(obj))
244	throw new ClassCastException();
245	if (cclass != null)
246	ensureProtectedAccess(obj);
247	}
248
249	void updateCheck(T obj, V update) {
250	if (!tclass.isInstance(obj) \|\|
251	(update != null && vclass != null && !vclass.isInstance(update)))
252	throw new ClassCastException();
253	if (cclass != null)
254	ensureProtectedAccess(obj);
255	}
256
257	public boolean compareAndSet(T obj, V expect, V update) {
258	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
259	(update != null && vclass != null &&
260	vclass != update.getClass()))
261	updateCheck(obj, update);
262	return unsafe.compareAndSwapObject(obj, offset, expect, update);
263	}
264
265	public boolean weakCompareAndSet(T obj, V expect, V update) {
266	// same implementation as strong form for now
267	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
268	(update != null && vclass != null &&
269	vclass != update.getClass()))
270	updateCheck(obj, update);
271	return unsafe.compareAndSwapObject(obj, offset, expect, update);
272	}
273
274	public void set(T obj, V newValue) {
275	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
276	(newValue != null && vclass != null &&
277	vclass != newValue.getClass()))
278	updateCheck(obj, newValue);
279	unsafe.putObjectVolatile(obj, offset, newValue);
280	}
281
282	public void lazySet(T obj, V newValue) {
283	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
284	(newValue != null && vclass != null &&
285	vclass != newValue.getClass()))
286	updateCheck(obj, newValue);
287	unsafe.putOrderedObject(obj, offset, newValue);
288	}
289
290	@SuppressWarnings("unchecked")
291	public V get(T obj) {
292	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null)
293	targetCheck(obj);
294	return (V)unsafe.getObjectVolatile(obj, offset);
295	}
296
297	private void ensureProtectedAccess(T obj) {
298	if (cclass.isInstance(obj)) {
299	return;
300	}
301	throw new RuntimeException(
302	new IllegalAccessException("Class " +
303	cclass.getName() +
304	" can not access a protected member of class " +
305	tclass.getName() +
306	" using an instance of " +
307	obj.getClass().getName()
308	)
309	);
310	}
311	}
312	}