AtomicIntegerArray 源码分析与感悟
Expert Group and released to the public domain, as explained at
http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent.atomic;
import sun.misc.Unsafe;
import java.util.*;
/**
An {@code int} array 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
*/
public class AtomicIntegerArray implements java.io.Serializable {
private static final long serialVersionUID = 2862133569453604235L;
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final int base = unsafe.arrayBaseOffset(int[].class);
private static final int shift;
private final int[] array;
static {
int scale = unsafe.arrayIndexScale(int[].class);
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 AtomicIntegerArray of the given length, with all
elements initially zero.
@param length the length of the array
*/
public AtomicIntegerArray(int length) {
array = new int[length];
}
/**
Creates a new AtomicIntegerArray 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 AtomicIntegerArray(int[] array) {
// Visibility guaranteed by final field guarantees
this.array = array.clone();
}
/**
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 int get(int i) {
return getRaw(checkedByteOffset(i));
}
private int getRaw(long offset) {
return unsafe.getIntVolatile(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, int newValue) {
unsafe.putIntVolatile(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, int newValue) {
unsafe.putOrderedInt(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 int getAndSet(int i, int newValue) {
long offset = checkedByteOffset(i);
while (true) {
int 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 equ 《一线大厂 Java 面试题解析+后端开发学习笔记+最新架构讲解视频+实战项目源码讲义》无偿开源 威信搜索公众号【编程进阶路】 al to the expected value.
*/
public final boolean compareAndSet(int i, int expect, int update) {
return compareAndSetRaw(checkedByteOffset(i), expect, update);
}
private boolean compareAndSetRaw(long offset, int expect, int update) {
return unsafe.compareAndSwapInt(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, int expect, int update) {
return compareAndSet(i, expect, update);
}
/**
Atomically increments by one the element at index {@code i}.
@param i the index
@return the previous value
*/
public final int getAndIncrement(int i) {
return getAndAdd(i, 1);
}
/**
Atomically decrements by one the element at index {@code i}.
@param i the index
@return the previous value
*/
public final int getAndDecrement(int i) {
return getAndAdd(i, -1);
}
/**
Atomically adds the given value to the element at index {@code i}.
@param i the index
@param delta the value to add
@return the previous value
*/
public final int getAndAdd(int i, int delta) {
long offset = checkedByteOffset(i);
while (true) {
int current = getRaw(offset);
if (compareAndSetRaw(offset, current, current + delta))
return current;
}
}
/**
Atomically increments by one the element at index {@code i}.
@param i the index
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