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

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。内容包括：
AtomicLongArray介绍和函数列表
AtomicLongArray源码分析(基于JDK1.7.0_40)
AtomicLongArray示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3514604.html

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JDK1.7.0_40)

AtomicLongArray的完整源码

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

 // LongArrayTest.java的源码
 import java.util.concurrent.atomic.AtomicLongArray;

 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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更多内容

1. Java多线程系列--“JUC原子类”01之 框架

2. Java多线程系列--“JUC原子类”02之 AtomicLong原子类

3. Java多线程系列目录(共xx篇)