Java多线程系列--“JUC原子类”03之 AtomicLongArray原子类

Java多线程系列-目录

[笔记][Java7并发编程实战手册]系列目录

概要

AtomicIntegerArray, AtomicLongArray, AtomicReferenceArray这3个数组类型的原子类的原理和用法相似。本章以AtomicLongArray对数组类型的原子类进行介绍.

AtomicLongArray介绍和函数列表

在”Java多线程系列–“JUC原子类”02之 AtomicLong原子类“中介绍过，AtomicLong是作用是对长整形进行原子操作。而AtomicLongArray的作用则是对”长整形数组”进行原子操作。

AtomicLongArray函数列表

// 创建给定长度的新 AtomicLongArray。
AtomicLongArray(int length)
// 创建与给定数组具有相同长度的新 AtomicLongArray，并从给定数组复制其所有元素。
AtomicLongArray(long[] array)

// 以原子方式将给定值添加到索引 i 的元素。
long addAndGet(int i, long delta)
// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。
boolean compareAndSet(int i, long expect, long update)
// 以原子方式将索引 i 的元素减1。
long decrementAndGet(int i)
// 获取位置 i 的当前值。
long get(int i)
// 以原子方式将给定值与索引 i 的元素相加。
long getAndAdd(int i, long delta)
// 以原子方式将索引 i 的元素减 1。
long getAndDecrement(int i)
// 以原子方式将索引 i 的元素加 1。
long getAndIncrement(int i)
// 以原子方式将位置 i 的元素设置为给定值，并返回旧值。
long getAndSet(int i, long newValue)
// 以原子方式将索引 i 的元素加1。
long incrementAndGet(int i)
// 最终将位置 i 的元素设置为给定值。
void lazySet(int i, long newValue)
// 返回该数组的长度。
int length()
// 将位置 i 的元素设置为给定值。
void set(int i, long newValue)
// 返回数组当前值的字符串表示形式。
String toString()
// 如果当前值 == 预期值，则以原子方式将该值设置为给定的更新值。
boolean    weakCompareAndSet(int i, long expect, long update)

AtomicLongArray源码分析(基于JDK1.7+)

AtomicLongArray的完整源码

/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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*/

/*
*
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* 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 sun.misc.Unsafe;
import java.util.*;

/**
* A {@code long} 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 AtomicLongArray implements java.io.Serializable {
private static final long serialVersionUID = -2308431214976778248L;

private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final int base = unsafe.arrayBaseOffset(long[].class);
private static final int shift;
private final long[] array;

static {
int scale = unsafe.arrayIndexScale(long[].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 AtomicLongArray of the given length, with all
* elements initially zero.
*
* @param length the length of the array
*/
public AtomicLongArray(int length) {
array = new long[length];
}

/**
* Creates a new AtomicLongArray 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 AtomicLongArray(long[] 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 long get(int i) {
return getRaw(checkedByteOffset(i));
}

private long getRaw(long offset) {
return unsafe.getLongVolatile(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, long newValue) {
unsafe.putLongVolatile(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, long newValue) {
unsafe.putOrderedLong(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 long getAndSet(int i, long newValue) {
long offset = checkedByteOffset(i);
while (true) {
long 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, long expect, long update) {
return compareAndSetRaw(checkedByteOffset(i), expect, update);
}

private boolean compareAndSetRaw(long offset, long expect, long update) {
return unsafe.compareAndSwapLong(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, long expect, long 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 long 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 long 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 long getAndAdd(int i, long delta) {
long offset = checkedByteOffset(i);
while (true) {
long 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
* @return the updated value
*/
public final long incrementAndGet(int i) {
return addAndGet(i, 1);
}

/**
* Atomically decrements by one the element at index {@code i}.
*
* @param i the index
* @return the updated value
*/
public final long decrementAndGet(int i) {
return addAndGet(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 updated value
*/
public long addAndGet(int i, long delta) {
long offset = checkedByteOffset(i);
while (true) {
long current = getRaw(offset);
long next = current + delta;
if (compareAndSetRaw(offset, current, next))
return next;
}
}

/**
* 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(' ');
}
}

}

AtomicLongArray的代码很简单，下面仅以incrementAndGet()为例，对AtomicLong的原理进行说明。

incrementAndGet()源码如下：

public final long incrementAndGet(int i) {
return addAndGet(i, 1);
}

说明：

incrementAndGet()的作用是以原子方式将long数组的索引 i 的元素加1，并返回加1之后的值。

addAndGet()源码如下：

public long addAndGet(int i, long delta) {
// 检查数组是否越界
long offset = checkedByteOffset(i);
while (true) {
// 获取long型数组的索引 offset 的原始值
long current = getRaw(offset);
// 修改long型值
long next = current + delta;
// 通过CAS更新long型数组的索引 offset的值。
if (compareAndSetRaw(offset, current, next))
return next;
}
}

说明：

addAndGet()首先检查数组是否越界。如果没有越界的话，则先获取数组索引i的值；然后通过CAS函数更新i的值。

getRaw()源码如下

private long getRaw(long offset) {
return unsafe.getLongVolatile(array, offset);
}

说明：

unsafe是通过Unsafe.getUnsafe()返回的一个Unsafe对象。通过Unsafe的CAS函数对long型数组的元素进行原子操作。如compareAndSetRaw()就是调用Unsafe的CAS函数，它的源码如下：

private boolean compareAndSetRaw(long offset, long expect, long update) {
return unsafe.compareAndSwapLong(array, offset, expect, update);
}

AtomicLongArray示例

public class LongArrayTest {
public static void main(String[] args) {

// 新建AtomicLongArray对象
long[] arrLong = new long[]{10, 20, 30, 40, 50};
AtomicLongArray ala = new AtomicLongArray(arrLong);

ala.set(0, 100);
for (int i = 0, len = ala.length(); i < len; i++)
System.out.printf("get(%d) : %s\n", i, ala.get(i));

System.out.printf("%20s : %s\n", "getAndDecrement(0)", ala.getAndDecrement(0));
System.out.printf("%20s : %s\n", "decrementAndGet(1)", ala.decrementAndGet(1));
System.out.printf("%20s : %s\n", "getAndIncrement(2)", ala.getAndIncrement(2));
System.out.printf("%20s : %s\n", "incrementAndGet(3)", ala.incrementAndGet(3));

System.out.printf("%20s : %s\n", "addAndGet(100)", ala.addAndGet(0, 100));
System.out.printf("%20s : %s\n", "getAndAdd(100)", ala.getAndAdd(1, 100));

System.out.printf("%20s : %s\n", "compareAndSet()", ala.compareAndSet(2, 31, 1000));
System.out.printf("%20s : %s\n", "get(2)", ala.get(2));
}
}

运行结果

get(0) : 100
get(1) : 20
get(2) : 30
get(3) : 40
get(4) : 50
getAndDecrement(0) : 100
decrementAndGet(1) : 19
getAndIncrement(2) : 30
incrementAndGet(3) : 41
addAndGet(100) : 199
getAndAdd(100) : 19
compareAndSet() : true
get(2) : 1000

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