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 7package java.util.concurrent.atomic; 8 9import java.util.Arrays; 10import java.lang.reflect.Array; 11import 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 */ 22public 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 private E getRaw(long offset) { 100 return (E) unsafe.getObjectVolatile(array, offset); 101 } 102 103 /** 104 * Sets the element at position {@code i} to the given value. 105 * 106 * @param i the index 107 * @param newValue the new value 108 */ 109 public final void set(int i, E newValue) { 110 unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue); 111 } 112 113 /** 114 * Eventually sets the element at position {@code i} to the given value. 115 * 116 * @param i the index 117 * @param newValue the new value 118 * @since 1.6 119 */ 120 public final void lazySet(int i, E newValue) { 121 unsafe.putOrderedObject(array, checkedByteOffset(i), newValue); 122 } 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 * @return true if successful. 172 */ 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 * @param s the stream 199 */ 200 private void readObject(java.io.ObjectInputStream s) 201 throws java.io.IOException, ClassNotFoundException, 202 java.io.InvalidObjectException { 203 // Note: This must be changed if any additional fields are defined 204 Object a = s.readFields().get("array", null); 205 if (a == null || !a.getClass().isArray()) 206 throw new java.io.InvalidObjectException("Not array type"); 207 if (a.getClass() != Object[].class) 208 a = Arrays.copyOf((Object[])a, Array.getLength(a), Object[].class); 209 unsafe.putObjectVolatile(this, arrayFieldOffset, a); 210 } 211 212} 213