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:	1.9
Committed:	Sun Jan 18 20:17:32 2015 UTC (9 years, 4 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.8:	+1 -0 lines
Log Message:	exactly one blank line before and after package statements
#	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 sun.misc.Unsafe;
10	import java.lang.reflect.Field;
11	import java.lang.reflect.Modifier;
12	import java.security.AccessController;
13	import java.security.PrivilegedExceptionAction;
14	import java.security.PrivilegedActionException;
15
16	/**
17	* A reflection-based utility that enables atomic updates to
18	* designated {@code volatile} reference fields of designated
19	* classes. This class is designed for use in atomic data structures
20	* in which several reference fields of the same node are
21	* independently subject to atomic updates. For example, a tree node
22	* might be declared as
23	*
24	* <pre> {@code
25	* class Node {
26	* private volatile Node left, right;
27	*
28	* private static final AtomicReferenceFieldUpdater<Node, Node> leftUpdater =
29	* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "left");
30	* private static AtomicReferenceFieldUpdater<Node, Node> rightUpdater =
31	* AtomicReferenceFieldUpdater.newUpdater(Node.class, Node.class, "right");
32	*
33	* Node getLeft() { return left; }
34	* boolean compareAndSetLeft(Node expect, Node update) {
35	* return leftUpdater.compareAndSet(this, expect, update);
36	* }
37	* // ... and so on
38	* }}</pre>
39	*
40	* <p>Note that the guarantees of the {@code compareAndSet}
41	* method in this class are weaker than in other atomic classes.
42	* Because this class cannot ensure that all uses of the field
43	* are appropriate for purposes of atomic access, it can
44	* guarantee atomicity only with respect to other invocations of
45	* {@code compareAndSet} and {@code set} on the same updater.
46	*
47	* @since 1.5
48	* @author Doug Lea
49	* @param <T> The type of the object holding the updatable field
50	* @param <V> The type of the field
51	*/
52	public abstract class AtomicReferenceFieldUpdater<T,V> {
53
54	/**
55	* Creates and returns an updater for objects with the given field.
56	* The Class arguments are needed to check that reflective types and
57	* generic types match.
58	*
59	* @param tclass the class of the objects holding the field
60	* @param vclass the class of the field
61	* @param fieldName the name of the field to be updated
62	* @param <U> the type of instances of tclass
63	* @param <W> the type of instances of vclass
64	* @return the updater
65	* @throws IllegalArgumentException if the field is not a volatile reference type
66	* @throws RuntimeException with a nested reflection-based
67	* exception if the class does not hold field or is the wrong type,
68	* or the field is inaccessible to the caller according to Java language
69	* access control
70	*/
71	public static <U,W> AtomicReferenceFieldUpdater<U,W> newUpdater(Class<U> tclass,
72	Class<W> vclass,
73	String fieldName) {
74	return new AtomicReferenceFieldUpdaterImpl<U,W>
75	(tclass, vclass, fieldName);
76	}
77
78	/**
79	* Protected do-nothing constructor for use by subclasses.
80	*/
81	protected AtomicReferenceFieldUpdater() {
82	}
83
84	/**
85	* Atomically sets the field of the given object managed by this updater
86	* to the given updated value if the current value {@code ==} the
87	* expected value. This method is guaranteed to be atomic with respect to
88	* other calls to {@code compareAndSet} and {@code set}, but not
89	* necessarily with respect to other changes in the field.
90	*
91	* @param obj An object whose field to conditionally set
92	* @param expect the expected value
93	* @param update the new value
94	* @return true if successful
95	*/
96	public abstract boolean compareAndSet(T obj, V expect, V update);
97
98	/**
99	* Atomically sets the field of the given object managed by this updater
100	* to the given updated value if the current value {@code ==} the
101	* expected value. This method is guaranteed to be atomic with respect to
102	* other calls to {@code compareAndSet} and {@code set}, but not
103	* necessarily with respect to other changes in the field.
104	*
105	* <p><a href="package-summary.html#weakCompareAndSet">May fail
106	* spuriously and does not provide ordering guarantees</a>, so is
107	* only rarely an appropriate alternative to {@code compareAndSet}.
108	*
109	* @param obj An object whose field to conditionally set
110	* @param expect the expected value
111	* @param update the new value
112	* @return true if successful
113	*/
114	public abstract boolean weakCompareAndSet(T obj, V expect, V update);
115
116	/**
117	* Sets the field of the given object managed by this updater to the
118	* given updated value. This operation is guaranteed to act as a volatile
119	* store with respect to subsequent invocations of {@code compareAndSet}.
120	*
121	* @param obj An object whose field to set
122	* @param newValue the new value
123	*/
124	public abstract void set(T obj, V newValue);
125
126	/**
127	* Eventually sets the field of the given object managed by this
128	* updater to the given updated value.
129	*
130	* @param obj An object whose field to set
131	* @param newValue the new value
132	* @since 1.6
133	*/
134	public abstract void lazySet(T obj, V newValue);
135
136	/**
137	* Gets the current value held in the field of the given object managed
138	* by this updater.
139	*
140	* @param obj An object whose field to get
141	* @return the current value
142	*/
143	public abstract V get(T obj);
144
145	/**
146	* Atomically sets the field of the given object managed by this updater
147	* to the given value and returns the old value.
148	*
149	* @param obj An object whose field to get and set
150	* @param newValue the new value
151	* @return the previous value
152	*/
153	public V getAndSet(T obj, V newValue) {
154	for (;;) {
155	V current = get(obj);
156	if (compareAndSet(obj, current, newValue))
157	return current;
158	}
159	}
160
161	private static final class AtomicReferenceFieldUpdaterImpl<T,V>
162	extends AtomicReferenceFieldUpdater<T,V> {
163	private static final Unsafe unsafe = Unsafe.getUnsafe();
164	private final long offset;
165	private final Class<T> tclass;
166	private final Class<V> vclass;
167	private final Class<?> cclass;
168
169	/*
170	* Internal type checks within all update methods contain
171	* internal inlined optimizations checking for the common
172	* cases where the class is final (in which case a simple
173	* getClass comparison suffices) or is of type Object (in
174	* which case no check is needed because all objects are
175	* instances of Object). The Object case is handled simply by
176	* setting vclass to null in constructor. The targetCheck and
177	* updateCheck methods are invoked when these faster
178	* screenings fail.
179	*/
180
181	AtomicReferenceFieldUpdaterImpl(final Class<T> tclass,
182	Class<V> vclass,
183	final String fieldName) {
184	final Field field;
185	final Class<?> fieldClass;
186	final Class<?> caller;
187	final int modifiers;
188	try {
189	field = AccessController.doPrivileged(
190	new PrivilegedExceptionAction<Field>() {
191	public Field run() throws NoSuchFieldException {
192	return tclass.getDeclaredField(fieldName);
193	}
194	});
195	caller = sun.reflect.Reflection.getCallerClass(3);
196	modifiers = field.getModifiers();
197	sun.reflect.misc.ReflectUtil.ensureMemberAccess(
198	caller, tclass, null, modifiers);
199	ClassLoader cl = tclass.getClassLoader();
200	ClassLoader ccl = caller.getClassLoader();
201	if ((ccl != null) && (ccl != cl) &&
202	((cl == null) \|\| !isAncestor(cl, ccl))) {
203	sun.reflect.misc.ReflectUtil.checkPackageAccess(tclass);
204	}
205	fieldClass = field.getType();
206	} catch (PrivilegedActionException pae) {
207	throw new RuntimeException(pae.getException());
208	} catch (Exception ex) {
209	throw new RuntimeException(ex);
210	}
211
212	if (vclass != fieldClass)
213	throw new ClassCastException();
214
215	if (!Modifier.isVolatile(modifiers))
216	throw new IllegalArgumentException("Must be volatile type");
217
218	this.cclass = (Modifier.isProtected(modifiers) &&
219	caller != tclass) ? caller : null;
220	this.tclass = tclass;
221	if (vclass == Object.class)
222	this.vclass = null;
223	else
224	this.vclass = vclass;
225	offset = unsafe.objectFieldOffset(field);
226	}
227
228	/**
229	* Returns true if the second classloader can be found in the first
230	* classloader's delegation chain.
231	* Equivalent to the inaccessible: first.isAncestor(second).
232	*/
233	private static boolean isAncestor(ClassLoader first, ClassLoader second) {
234	ClassLoader acl = first;
235	do {
236	acl = acl.getParent();
237	if (second == acl) {
238	return true;
239	}
240	} while (acl != null);
241	return false;
242	}
243
244	void targetCheck(T obj) {
245	if (!tclass.isInstance(obj))
246	throw new ClassCastException();
247	if (cclass != null)
248	ensureProtectedAccess(obj);
249	}
250
251	void updateCheck(T obj, V update) {
252	if (!tclass.isInstance(obj) \|\|
253	(update != null && vclass != null && !vclass.isInstance(update)))
254	throw new ClassCastException();
255	if (cclass != null)
256	ensureProtectedAccess(obj);
257	}
258
259	public boolean compareAndSet(T obj, V expect, V update) {
260	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
261	(update != null && vclass != null &&
262	vclass != update.getClass()))
263	updateCheck(obj, update);
264	return unsafe.compareAndSwapObject(obj, offset, expect, update);
265	}
266
267	public boolean weakCompareAndSet(T obj, V expect, V update) {
268	// same implementation as strong form for now
269	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
270	(update != null && vclass != null &&
271	vclass != update.getClass()))
272	updateCheck(obj, update);
273	return unsafe.compareAndSwapObject(obj, offset, expect, update);
274	}
275
276	public void set(T obj, V newValue) {
277	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
278	(newValue != null && vclass != null &&
279	vclass != newValue.getClass()))
280	updateCheck(obj, newValue);
281	unsafe.putObjectVolatile(obj, offset, newValue);
282	}
283
284	public void lazySet(T obj, V newValue) {
285	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null \|\|
286	(newValue != null && vclass != null &&
287	vclass != newValue.getClass()))
288	updateCheck(obj, newValue);
289	unsafe.putOrderedObject(obj, offset, newValue);
290	}
291
292	@SuppressWarnings("unchecked")
293	public V get(T obj) {
294	if (obj == null \|\| obj.getClass() != tclass \|\| cclass != null)
295	targetCheck(obj);
296	return (V)unsafe.getObjectVolatile(obj, offset);
297	}
298
299	private void ensureProtectedAccess(T obj) {
300	if (cclass.isInstance(obj)) {
301	return;
302	}
303	throw new RuntimeException(
304	new IllegalAccessException("Class " +
305	cclass.getName() +
306	" can not access a protected member of class " +
307	tclass.getName() +
308	" using an instance of " +
309	obj.getClass().getName()
310	)
311	);
312	}
313	}
314	}