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/* * @(#)AtomicReferenceArray.java 1.12 06/06/15 * * Copyright 2006 Sun Microsystems, Inc. All rights reserved. * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. */ package java.util.concurrent.atomic; import sun.misc.Unsafe; import java.util.*; /** * 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 = Unsafe.getUnsafe(); private static final int base = unsafe.arrayBaseOffset(Object[].class); private static final int scale = unsafe.arrayIndexScale(Object[].class); private final Object[] array; private long rawIndex(int i) { if (i < 0 || i >= array.length) throw new IndexOutOfBoundsException("index " + i); return base + i * scale; } /** * Creates a new AtomicReferenceArray of given length. * @param length the length of the array */ public AtomicReferenceArray(int length) { array = new Object[length]; // must perform at least one volatile write to conform to JMM if (length > 0) unsafe.putObjectVolatile(array, rawIndex(0), null); } /** * 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) { if (array == null) throw new NullPointerException(); int length = array.length; this.array = new Object[length]; if (length > 0) { int last = length-1; for (int i = 0; i < last; ++i) this.array[i] = array[i]; // Do the last write as volatile E e = array[last]; unsafe.putObjectVolatile(this.array, rawIndex(last), e); } } /** * 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 (E) unsafe.getObjectVolatile(array, rawIndex(i)); } /** * 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, rawIndex(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, rawIndex(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) { while (true) { E current = get(i); if (compareAndSet(i, 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 unsafe.compareAndSwapObject(array, rawIndex(i), 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() { if (array.length > 0) // force volatile read get(0); return Arrays.toString(array); } }