jsr166e/extra/AtomicDouble.java

/*
 * Written by Doug Lea and Martin Buchholz 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 jsr166e.extra;
import static java.lang.Double.doubleToRawLongBits;
import static java.lang.Double.longBitsToDouble;

/**
 * A {@code double} value that may be updated atomically.  See the
 * {@link java.util.concurrent.atomic} package specification for
 * description of the properties of atomic variables.  An {@code
 * AtomicDouble} is used in applications such as atomic accumulation,
 * and cannot be used as a replacement for a {@link Double}.  However,
 * this class does extend {@code Number} to allow uniform access by
 * tools and utilities that deal with numerically-based classes.
 *
 * <p>This class differs from the primitive double {@code ==} operator
 * and from {@link Double#equals} in that it uses purely bitwise
 * equality in methods such as {@link #compareAndSet}, as if
 * implemented by:
 *  <pre> {@code
 * boolean bitEquals(double x, double y) {
 *   long xBits = Double.doubleToRawLongBits(x);
 *   long yBits = Double.doubleToRawLongBits(y);
 *   return xBits == yBits;
 * }}</pre>
 *
 * @author Doug Lea
 * @author Martin Buchholz
 */
public class AtomicDouble extends Number implements java.io.Serializable {
    static final long serialVersionUID = -8405198993435143622L;

    // setup to use Unsafe.compareAndSwapLong for updates
    private static final sun.misc.Unsafe unsafe = getUnsafe();
    private static final long valueOffset;

    /**
     * Records whether the underlying JVM supports lockless
     * compareAndSwap for longs. While the Unsafe.compareAndSwapLong
     * method works in either case, some constructions should be
     * handled at Java level to avoid locking user-visible locks.
     */
    static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();

    /**
     * Returns whether underlying JVM supports lockless CompareAndSet
     * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
     */
    private static boolean VMSupportsCS8() {
        final Class<?> klazz = java.util.concurrent.atomic.AtomicLong.class;
        return java.security.AccessController.doPrivileged
            (new java.security.PrivilegedAction<Boolean>() {
                public Boolean run() {
                    try {
                        java.lang.reflect.Method m =
                            klazz.getDeclaredMethod("VMSupportsCS8", new Class<?>[0]);
                        m.setAccessible(true);
                        return (Boolean) m.invoke(new Class<?>[0]);
                    } catch (Throwable t) { throw new Error(t); }
                }});
    }

    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicDouble.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    private volatile long value;

    /**
     * Creates a new AtomicDouble with the given initial value.
     *
     * @param initialValue the initial value
     */
    public AtomicDouble(double initialValue) {
        value = doubleToRawLongBits(initialValue);
    }

    /**
     * Creates a new AtomicDouble with initial value {@code 0.0}.
     */
    public AtomicDouble() { this(0.0); }

    /**
     * Gets the current value.
     *
     * @return the current value
     */
    public final double get() {
        return longBitsToDouble(value);
    }

    /**
     * Sets to the given value.
     *
     * @param newValue the new value
     */
    public final void set(double newValue) {
        value = doubleToRawLongBits(newValue);
    }

    /**
     * Eventually sets to the given value.
     *
     * @param newValue the new value
     */
    public final void lazySet(double newValue) {
        unsafe.putOrderedLong(this, valueOffset, doubleToRawLongBits(newValue));
    }

    /**
     * Atomically sets to the given value and returns the old value.
     *
     * @param newValue the new value
     * @return the previous value
     */
    public final double getAndSet(double newValue) {
        long newBits = doubleToRawLongBits(newValue);
        while (true) {
            long currentBits = value;
            if (unsafe.compareAndSwapLong(this, valueOffset,
                                          currentBits, newBits))
                return longBitsToDouble(currentBits);
        }
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * @param expect the expected value
     * @param update the new value
     * @return true if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(double expect, double update) {
        return unsafe.compareAndSwapLong(this, valueOffset,
                                         doubleToRawLongBits(expect),
                                         doubleToRawLongBits(update));
    }

    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     *
     * <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 expect the expected value
     * @param update the new value
     * @return true if successful.
     */
    public final boolean weakCompareAndSet(double expect, double update) {
        return compareAndSet(expect, update);
    }

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the previous value
     */
    public final double getAndAdd(double delta) {
        while (true) {
            long currentBits = value;
            long nextBits = doubleToRawLongBits(longBitsToDouble(currentBits) + delta);
            if (unsafe.compareAndSwapLong(this, valueOffset,
                                          currentBits, nextBits))
                return longBitsToDouble(currentBits);
        }
    }

    /**
     * Atomically adds the given value to the current value.
     *
     * @param delta the value to add
     * @return the updated value
     */
    public final double addAndGet(double delta) {
        for (;;) {
            double current = get();
            double next = current + delta;
            if (compareAndSet(current, next))
                return next;
        }
    }

    /**
     * Returns the String representation of the current value.
     * @return the String representation of the current value.
     */
    public String toString() {
        return Double.toString(get());
    }


    /**
     * Returns the value of this {@code AtomicDouble} as an {@code int}
     * after a narrowing primitive conversion.
     */
    public int intValue() {
        return (int)get();
    }

    /**
     * Returns the value of this {@code AtomicDouble} as a {@code long}.
     */
    public long longValue() {
        return (long)get();
    }

    /**
     * Returns the value of this {@code AtomicDouble} as a {@code float}
     * after a widening primitive conversion.
     */
    public float floatValue() {
        return (float)get();
    }

    /**
     * Returns the value of this {@code AtomicDouble} as a {@code double}
     * after a widening primitive conversion.
     */
    public double doubleValue() {
        return get();
    }

    /**
     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
     * Replace with a simple call to Unsafe.getUnsafe when integrating
     * into a jdk.
     *
     * @return a sun.misc.Unsafe
     */
    private static sun.misc.Unsafe getUnsafe() {
        try {
            return sun.misc.Unsafe.getUnsafe();
        } catch (SecurityException se) {
            try {
                return java.security.AccessController.doPrivileged
                    (new java.security
                     .PrivilegedExceptionAction<sun.misc.Unsafe>() {
                        public sun.misc.Unsafe run() throws Exception {
                            java.lang.reflect.Field f = sun.misc
                                .Unsafe.class.getDeclaredField("theUnsafe");
                            f.setAccessible(true);
                            return (sun.misc.Unsafe) f.get(null);
                        }});
            } catch (java.security.PrivilegedActionException e) {
                throw new RuntimeException("Could not initialize intrinsics",
                                           e.getCause());
            }
        }
    }

}
Revision:	1.3
Committed:	Tue Aug 9 08:28:54 2011 UTC (12 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.2:	+44 -15 lines
Log Message:	unsafe mechanics
#	Content
1	/*
2	* Written by Doug Lea and Martin Buchholz with assistance from
3	* members of JCP JSR-166 Expert Group and released to the public
4	* domain, as explained at
5	* http://creativecommons.org/publicdomain/zero/1.0/
6	*/
7
8	package jsr166e.extra;
9	import static java.lang.Double.doubleToRawLongBits;
10	import static java.lang.Double.longBitsToDouble;
11
12	/**
13	* A {@code double} value that may be updated atomically. See the
14	* {@link java.util.concurrent.atomic} package specification for
15	* description of the properties of atomic variables. An {@code
16	* AtomicDouble} is used in applications such as atomic accumulation,
17	* and cannot be used as a replacement for a {@link Double}. However,
18	* this class does extend {@code Number} to allow uniform access by
19	* tools and utilities that deal with numerically-based classes.
20	*
21	* <p>This class differs from the primitive double {@code ==} operator
22	* and from {@link Double#equals} in that it uses purely bitwise
23	* equality in methods such as {@link #compareAndSet}, as if
24	* implemented by:
25	* <pre> {@code
26	* boolean bitEquals(double x, double y) {
27	* long xBits = Double.doubleToRawLongBits(x);
28	* long yBits = Double.doubleToRawLongBits(y);
29	* return xBits == yBits;
30	* }}</pre>
31	*
32	* @author Doug Lea
33	* @author Martin Buchholz
34	*/
35	public class AtomicDouble extends Number implements java.io.Serializable {
36	static final long serialVersionUID = -8405198993435143622L;
37
38	// setup to use Unsafe.compareAndSwapLong for updates
39	private static final sun.misc.Unsafe unsafe = getUnsafe();
40	private static final long valueOffset;
41
42	/**
43	* Records whether the underlying JVM supports lockless
44	* compareAndSwap for longs. While the Unsafe.compareAndSwapLong
45	* method works in either case, some constructions should be
46	* handled at Java level to avoid locking user-visible locks.
47	*/
48	static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();
49
50	/**
51	* Returns whether underlying JVM supports lockless CompareAndSet
52	* for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
53	*/
54	private static boolean VMSupportsCS8() {
55	final Class<?> klazz = java.util.concurrent.atomic.AtomicLong.class;
56	return java.security.AccessController.doPrivileged
57	(new java.security.PrivilegedAction<Boolean>() {
58	public Boolean run() {
59	try {
60	java.lang.reflect.Method m =
61	klazz.getDeclaredMethod("VMSupportsCS8", new Class<?>[0]);
62	m.setAccessible(true);
63	return (Boolean) m.invoke(new Class<?>[0]);
64	} catch (Throwable t) { throw new Error(t); }
65	}});
66	}
67
68	static {
69	try {
70	valueOffset = unsafe.objectFieldOffset
71	(AtomicDouble.class.getDeclaredField("value"));
72	} catch (Exception ex) { throw new Error(ex); }
73	}
74
75	private volatile long value;
76
77	/**
78	* Creates a new AtomicDouble with the given initial value.
79	*
80	* @param initialValue the initial value
81	*/
82	public AtomicDouble(double initialValue) {
83	value = doubleToRawLongBits(initialValue);
84	}
85
86	/**
87	* Creates a new AtomicDouble with initial value {@code 0.0}.
88	*/
89	public AtomicDouble() { this(0.0); }
90
91	/**
92	* Gets the current value.
93	*
94	* @return the current value
95	*/
96	public final double get() {
97	return longBitsToDouble(value);
98	}
99
100	/**
101	* Sets to the given value.
102	*
103	* @param newValue the new value
104	*/
105	public final void set(double newValue) {
106	value = doubleToRawLongBits(newValue);
107	}
108
109	/**
110	* Eventually sets to the given value.
111	*
112	* @param newValue the new value
113	*/
114	public final void lazySet(double newValue) {
115	unsafe.putOrderedLong(this, valueOffset, doubleToRawLongBits(newValue));
116	}
117
118	/**
119	* Atomically sets to the given value and returns the old value.
120	*
121	* @param newValue the new value
122	* @return the previous value
123	*/
124	public final double getAndSet(double newValue) {
125	long newBits = doubleToRawLongBits(newValue);
126	while (true) {
127	long currentBits = value;
128	if (unsafe.compareAndSwapLong(this, valueOffset,
129	currentBits, newBits))
130	return longBitsToDouble(currentBits);
131	}
132	}
133
134	/**
135	* Atomically sets the value to the given updated value
136	* if the current value {@code ==} the expected value.
137	*
138	* @param expect the expected value
139	* @param update the new value
140	* @return true if successful. False return indicates that
141	* the actual value was not equal to the expected value.
142	*/
143	public final boolean compareAndSet(double expect, double update) {
144	return unsafe.compareAndSwapLong(this, valueOffset,
145	doubleToRawLongBits(expect),
146	doubleToRawLongBits(update));
147	}
148
149	/**
150	* Atomically sets the value to the given updated value
151	* if the current value {@code ==} the expected value.
152	*
153	* <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
154	* and does not provide ordering guarantees, so is only rarely an
155	* appropriate alternative to {@code compareAndSet}.
156	*
157	* @param expect the expected value
158	* @param update the new value
159	* @return true if successful.
160	*/
161	public final boolean weakCompareAndSet(double expect, double update) {
162	return compareAndSet(expect, update);
163	}
164
165	/**
166	* Atomically adds the given value to the current value.
167	*
168	* @param delta the value to add
169	* @return the previous value
170	*/
171	public final double getAndAdd(double delta) {
172	while (true) {
173	long currentBits = value;
174	long nextBits = doubleToRawLongBits(longBitsToDouble(currentBits) + delta);
175	if (unsafe.compareAndSwapLong(this, valueOffset,
176	currentBits, nextBits))
177	return longBitsToDouble(currentBits);
178	}
179	}
180
181	/**
182	* Atomically adds the given value to the current value.
183	*
184	* @param delta the value to add
185	* @return the updated value
186	*/
187	public final double addAndGet(double delta) {
188	for (;;) {
189	double current = get();
190	double next = current + delta;
191	if (compareAndSet(current, next))
192	return next;
193	}
194	}
195
196	/**
197	* Returns the String representation of the current value.
198	* @return the String representation of the current value.
199	*/
200	public String toString() {
201	return Double.toString(get());
202	}
203
204
205	/**
206	* Returns the value of this {@code AtomicDouble} as an {@code int}
207	* after a narrowing primitive conversion.
208	*/
209	public int intValue() {
210	return (int)get();
211	}
212
213	/**
214	* Returns the value of this {@code AtomicDouble} as a {@code long}.
215	*/
216	public long longValue() {
217	return (long)get();
218	}
219
220	/**
221	* Returns the value of this {@code AtomicDouble} as a {@code float}
222	* after a widening primitive conversion.
223	*/
224	public float floatValue() {
225	return (float)get();
226	}
227
228	/**
229	* Returns the value of this {@code AtomicDouble} as a {@code double}
230	* after a widening primitive conversion.
231	*/
232	public double doubleValue() {
233	return get();
234	}
235
236	/**
237	* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
238	* Replace with a simple call to Unsafe.getUnsafe when integrating
239	* into a jdk.
240	*
241	* @return a sun.misc.Unsafe
242	*/
243	private static sun.misc.Unsafe getUnsafe() {
244	try {
245	return sun.misc.Unsafe.getUnsafe();
246	} catch (SecurityException se) {
247	try {
248	return java.security.AccessController.doPrivileged
249	(new java.security
250	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
251	public sun.misc.Unsafe run() throws Exception {
252	java.lang.reflect.Field f = sun.misc
253	.Unsafe.class.getDeclaredField("theUnsafe");
254	f.setAccessible(true);
255	return (sun.misc.Unsafe) f.get(null);
256	}});
257	} catch (java.security.PrivilegedActionException e) {
258	throw new RuntimeException("Could not initialize intrinsics",
259	e.getCause());
260	}
261	}
262	}
263
264	}