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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/licenses/publicdomain 5: */ 6: 7: package java.util.concurrent.atomic; 8: import sun.misc.Unsafe; 9: import java.util.*; 10: 11: /** 12: * An array of object references in which elements may be updated 13: * atomically. See the {@link java.util.concurrent.atomic} package 14: * specification for description of the properties of atomic 15: * variables. 16: * @since 1.5 17: * @author Doug Lea 18: * @param <E> The base class of elements held in this array 19: */ 20: public class AtomicReferenceArray<E> implements java.io.Serializable { 21: private static final long serialVersionUID = -6209656149925076980L; 22: 23: private static final Unsafe unsafe = Unsafe.getUnsafe(); 24: private static final int base = unsafe.arrayBaseOffset(Object[].class); 25: private static final int scale = unsafe.arrayIndexScale(Object[].class); 26: private final Object[] array; 27: 28: private long rawIndex(int i) { 29: if (i < 0 || i >= array.length) 30: throw new IndexOutOfBoundsException("index " + i); 31: return base + i * scale; 32: } 33: 34: /** 35: * Creates a new AtomicReferenceArray of given length. 36: * @param length the length of the array 37: */ 38: public AtomicReferenceArray(int length) { 39: array = new Object[length]; 40: // must perform at least one volatile write to conform to JMM 41: if (length > 0) 42: unsafe.putObjectVolatile(array, rawIndex(0), null); 43: } 44: 45: /** 46: * Creates a new AtomicReferenceArray with the same length as, and 47: * all elements copied from, the given array. 48: * 49: * @param array the array to copy elements from 50: * @throws NullPointerException if array is null 51: */ 52: public AtomicReferenceArray(E[] array) { 53: if (array == null) 54: throw new NullPointerException(); 55: int length = array.length; 56: this.array = new Object[length]; 57: if (length > 0) { 58: int last = length-1; 59: for (int i = 0; i < last; ++i) 60: this.array[i] = array[i]; 61: // Do the last write as volatile 62: E e = array[last]; 63: unsafe.putObjectVolatile(this.array, rawIndex(last), e); 64: } 65: } 66: 67: /** 68: * Returns the length of the array. 69: * 70: * @return the length of the array 71: */ 72: public final int length() { 73: return array.length; 74: } 75: 76: /** 77: * Gets the current value at position <tt>i</tt>. 78: * 79: * @param i the index 80: * @return the current value 81: */ 82: public final E get(int i) { 83: return (E) unsafe.getObjectVolatile(array, rawIndex(i)); 84: } 85: 86: /** 87: * Sets the element at position <tt>i</tt> to the given value. 88: * 89: * @param i the index 90: * @param newValue the new value 91: */ 92: public final void set(int i, E newValue) { 93: unsafe.putObjectVolatile(array, rawIndex(i), newValue); 94: } 95: 96: /** 97: * Eventually sets the element at position <tt>i</tt> to the given value. 98: * 99: * @param i the index 100: * @param newValue the new value 101: * @since 1.6 102: */ 103: public final void lazySet(int i, E newValue) { 104: unsafe.putOrderedObject(array, rawIndex(i), newValue); 105: } 106: 107: 108: /** 109: * Atomically sets the element at position <tt>i</tt> to the given 110: * value and returns the old value. 111: * 112: * @param i the index 113: * @param newValue the new value 114: * @return the previous value 115: */ 116: public final E getAndSet(int i, E newValue) { 117: while (true) { 118: E current = get(i); 119: if (compareAndSet(i, current, newValue)) 120: return current; 121: } 122: } 123: 124: /** 125: * Atomically sets the element at position <tt>i</tt> to the given 126: * updated value if the current value <tt>==</tt> the expected value. 127: * @param i the index 128: * @param expect the expected value 129: * @param update the new value 130: * @return true if successful. False return indicates that 131: * the actual value was not equal to the expected value. 132: */ 133: public final boolean compareAndSet(int i, E expect, E update) { 134: return unsafe.compareAndSwapObject(array, rawIndex(i), 135: expect, update); 136: } 137: 138: /** 139: * Atomically sets the element at position <tt>i</tt> to the given 140: * updated value if the current value <tt>==</tt> the expected value. 141: * May fail spuriously and does not provide ordering guarantees, 142: * so is only rarely an appropriate alternative to <tt>compareAndSet</tt>. 143: * 144: * @param i the index 145: * @param expect the expected value 146: * @param update the new value 147: * @return true if successful. 148: */ 149: public final boolean weakCompareAndSet(int i, E expect, E update) { 150: return compareAndSet(i, expect, update); 151: } 152: 153: /** 154: * Returns the String representation of the current values of array. 155: * @return the String representation of the current values of array. 156: */ 157: public String toString() { 158: if (array.length > 0) // force volatile read 159: get(0); 160: return Arrays.toString(array); 161: } 162: 163: }
GNU Classpath (0.98) |