concurrent/atomic/AtomicReferenceArray.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.util.Arrays;
import java.lang.reflect.Array;
import sun.misc.Unsafe;

/**
 * An array of object references in which elements may be updated
 * atomically.  See the {@link java.util.concurrent.atomic} package
 * specification for description of the properties of atomic
 * variables.
 * @since 1.5
 * @author Doug Lea
 * @param <E> The base class of elements held in this array
 */
public class AtomicReferenceArray<E> implements java.io.Serializable {
    private static final long serialVersionUID = -6209656149925076980L;

    private static final Unsafe unsafe;
    private static final int base;
    private static final int shift;
    private static final long arrayFieldOffset;
    private final Object[] array; // must have exact type Object[]

    static {
        int scale;
        try {
            unsafe = Unsafe.getUnsafe();
            arrayFieldOffset = unsafe.objectFieldOffset
                (AtomicReferenceArray.class.getDeclaredField("array"));
            base = unsafe.arrayBaseOffset(Object[].class);
            scale = unsafe.arrayIndexScale(Object[].class);
        } catch (Exception e) {
            throw new Error(e);
        }
        if ((scale & (scale - 1)) != 0)
            throw new Error("data type scale not a power of two");
        shift = 31 - Integer.numberOfLeadingZeros(scale);
    }

    private long checkedByteOffset(int i) {
        if (i < 0 || i >= array.length)
            throw new IndexOutOfBoundsException("index " + i);

        return byteOffset(i);
    }

    private static long byteOffset(int i) {
        return ((long) i << shift) + base;
    }

    /**
     * Creates a new AtomicReferenceArray of the given length, with all
     * elements initially null.
     *
     * @param length the length of the array
     */
    public AtomicReferenceArray(int length) {
        array = new Object[length];
    }

    /**
     * Creates a new AtomicReferenceArray with the same length as, and
     * all elements copied from, the given array.
     *
     * @param array the array to copy elements from
     * @throws NullPointerException if array is null
     */
    public AtomicReferenceArray(E[] array) {
        // Visibility guaranteed by final field guarantees
        this.array = Arrays.copyOf(array, array.length, Object[].class);
    }

    /**
     * Returns the length of the array.
     *
     * @return the length of the array
     */
    public final int length() {
        return array.length;
    }

    /**
     * Gets the current value at position {@code i}.
     *
     * @param i the index
     * @return the current value
     */
    public final E get(int i) {
        return getRaw(checkedByteOffset(i));
    }

    @SuppressWarnings("unchecked")
    private E getRaw(long offset) {
        return (E) unsafe.getObjectVolatile(array, offset);
    }

    /**
     * Sets the element at position {@code i} to the given value.
     *
     * @param i the index
     * @param newValue the new value
     */
    public final void set(int i, E newValue) {
        unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue);
    }

    /**
     * Eventually sets the element at position {@code i} to the given value.
     *
     * @param i the index
     * @param newValue the new value
     * @since 1.6
     */
    public final void lazySet(int i, E newValue) {
        unsafe.putOrderedObject(array, checkedByteOffset(i), newValue);
    }

    /**
     * Atomically sets the element at position {@code i} to the given
     * value and returns the old value.
     *
     * @param i the index
     * @param newValue the new value
     * @return the previous value
     */
    public final E getAndSet(int i, E newValue) {
        long offset = checkedByteOffset(i);
        while (true) {
            E current = getRaw(offset);
            if (compareAndSetRaw(offset, current, newValue))
                return current;
        }
    }

    /**
     * Atomically sets the element at position {@code i} to the given
     * updated value if the current value {@code ==} the expected value.
     *
     * @param i the index
     * @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(int i, E expect, E update) {
        return compareAndSetRaw(checkedByteOffset(i), expect, update);
    }

    private boolean compareAndSetRaw(long offset, E expect, E update) {
        return unsafe.compareAndSwapObject(array, offset, expect, update);
    }

    /**
     * Atomically sets the element at position {@code i} 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 i the index
     * @param expect the expected value
     * @param update the new value
     * @return true if successful
     */
    public final boolean weakCompareAndSet(int i, E expect, E update) {
        return compareAndSet(i, expect, update);
    }

    /**
     * Returns the String representation of the current values of array.
     * @return the String representation of the current values of array
     */
    public String toString() {
        int iMax = array.length - 1;
        if (iMax == -1)
            return "[]";

        StringBuilder b = new StringBuilder();
        b.append('[');
        for (int i = 0; ; i++) {
            b.append(getRaw(byteOffset(i)));
            if (i == iMax)
                return b.append(']').toString();
            b.append(',').append(' ');
        }
    }

    /**
     * Reconstitutes the instance from a stream (that is, deserializes it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException,
        java.io.InvalidObjectException {
        // Note: This must be changed if any additional fields are defined
        Object a = s.readFields().get("array", null);
        if (a == null || !a.getClass().isArray())
            throw new java.io.InvalidObjectException("Not array type");
        if (a.getClass() != Object[].class)
            a = Arrays.copyOf((Object[])a, Array.getLength(a), Object[].class);
        unsafe.putObjectVolatile(this, arrayFieldOffset, a);
    }

}
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:	+1 -1 lines
Log Message:	@return javadoc style
#	User	Rev	Content
1	dl	1.1	/*
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.util.Arrays;
10			import java.lang.reflect.Array;
11			import sun.misc.Unsafe;
12
13			/**
14			* An array of object references in which elements may be updated
15			* atomically. See the {@link java.util.concurrent.atomic} package
16			* specification for description of the properties of atomic
17			* variables.
18			* @since 1.5
19			* @author Doug Lea
20			* @param <E> The base class of elements held in this array
21			*/
22			public class AtomicReferenceArray<E> implements java.io.Serializable {
23			private static final long serialVersionUID = -6209656149925076980L;
24
25			private static final Unsafe unsafe;
26			private static final int base;
27			private static final int shift;
28			private static final long arrayFieldOffset;
29			private final Object[] array; // must have exact type Object[]
30
31			static {
32			int scale;
33			try {
34			unsafe = Unsafe.getUnsafe();
35			arrayFieldOffset = unsafe.objectFieldOffset
36			(AtomicReferenceArray.class.getDeclaredField("array"));
37			base = unsafe.arrayBaseOffset(Object[].class);
38			scale = unsafe.arrayIndexScale(Object[].class);
39			} catch (Exception e) {
40			throw new Error(e);
41			}
42			if ((scale & (scale - 1)) != 0)
43			throw new Error("data type scale not a power of two");
44			shift = 31 - Integer.numberOfLeadingZeros(scale);
45			}
46
47			private long checkedByteOffset(int i) {
48			if (i < 0 \|\| i >= array.length)
49			throw new IndexOutOfBoundsException("index " + i);
50
51			return byteOffset(i);
52			}
53
54			private static long byteOffset(int i) {
55			return ((long) i << shift) + base;
56			}
57
58			/**
59			* Creates a new AtomicReferenceArray of the given length, with all
60			* elements initially null.
61			*
62			* @param length the length of the array
63			*/
64			public AtomicReferenceArray(int length) {
65			array = new Object[length];
66			}
67
68			/**
69			* Creates a new AtomicReferenceArray with the same length as, and
70			* all elements copied from, the given array.
71			*
72			* @param array the array to copy elements from
73			* @throws NullPointerException if array is null
74			*/
75			public AtomicReferenceArray(E[] array) {
76			// Visibility guaranteed by final field guarantees
77			this.array = Arrays.copyOf(array, array.length, Object[].class);
78			}
79
80			/**
81			* Returns the length of the array.
82			*
83			* @return the length of the array
84			*/
85			public final int length() {
86			return array.length;
87			}
88
89			/**
90			* Gets the current value at position {@code i}.
91			*
92			* @param i the index
93			* @return the current value
94			*/
95			public final E get(int i) {
96			return getRaw(checkedByteOffset(i));
97			}
98
99			@SuppressWarnings("unchecked")
100			private E getRaw(long offset) {
101			return (E) unsafe.getObjectVolatile(array, offset);
102			}
103
104			/**
105			* Sets the element at position {@code i} to the given value.
106			*
107			* @param i the index
108			* @param newValue the new value
109			*/
110			public final void set(int i, E newValue) {
111			unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue);
112			}
113
114			/**
115			* Eventually sets the element at position {@code i} to the given value.
116			*
117			* @param i the index
118			* @param newValue the new value
119			* @since 1.6
120			*/
121			public final void lazySet(int i, E newValue) {
122			unsafe.putOrderedObject(array, checkedByteOffset(i), newValue);
123			}
124
125			/**
126			* Atomically sets the element at position {@code i} to the given
127			* value and returns the old value.
128			*
129			* @param i the index
130			* @param newValue the new value
131			* @return the previous value
132			*/
133			public final E getAndSet(int i, E newValue) {
134			long offset = checkedByteOffset(i);
135			while (true) {
136			E current = getRaw(offset);
137			if (compareAndSetRaw(offset, current, newValue))
138			return current;
139			}
140			}
141
142			/**
143			* Atomically sets the element at position {@code i} to the given
144			* updated value if the current value {@code ==} the expected value.
145			*
146			* @param i the index
147			* @param expect the expected value
148			* @param update the new value
149			* @return true if successful. False return indicates that
150			* the actual value was not equal to the expected value.
151			*/
152			public final boolean compareAndSet(int i, E expect, E update) {
153			return compareAndSetRaw(checkedByteOffset(i), expect, update);
154			}
155
156			private boolean compareAndSetRaw(long offset, E expect, E update) {
157			return unsafe.compareAndSwapObject(array, offset, expect, update);
158			}
159
160			/**
161			* Atomically sets the element at position {@code i} to the given
162			* updated value if the current value {@code ==} the expected value.
163			*
164			* <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
165			* and does not provide ordering guarantees, so is only rarely an
166			* appropriate alternative to {@code compareAndSet}.
167			*
168			* @param i the index
169			* @param expect the expected value
170			* @param update the new value
171	jsr166	1.2	* @return true if successful
172	dl	1.1	*/
173			public final boolean weakCompareAndSet(int i, E expect, E update) {
174			return compareAndSet(i, expect, update);
175			}
176
177			/**
178			* Returns the String representation of the current values of array.
179			* @return the String representation of the current values of array
180			*/
181			public String toString() {
182			int iMax = array.length - 1;
183			if (iMax == -1)
184			return "[]";
185
186			StringBuilder b = new StringBuilder();
187			b.append('[');
188			for (int i = 0; ; i++) {
189			b.append(getRaw(byteOffset(i)));
190			if (i == iMax)
191			return b.append(']').toString();
192			b.append(',').append(' ');
193			}
194			}
195
196			/**
197			* Reconstitutes the instance from a stream (that is, deserializes it).
198			*/
199			private void readObject(java.io.ObjectInputStream s)
200			throws java.io.IOException, ClassNotFoundException,
201			java.io.InvalidObjectException {
202			// Note: This must be changed if any additional fields are defined
203			Object a = s.readFields().get("array", null);
204			if (a == null \|\| !a.getClass().isArray())
205			throw new java.io.InvalidObjectException("Not array type");
206			if (a.getClass() != Object[].class)
207			a = Arrays.copyOf((Object[])a, Array.getLength(a), Object[].class);
208			unsafe.putObjectVolatile(this, arrayFieldOffset, a);
209			}
210
211			}