/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
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*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* 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.lang.reflect.Array;
import java.util.Arrays;
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));
}
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).
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// 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);
}
}