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深入浅析Java中的AtomicLongArray原子类

本篇文章为大家展示了深入浅析Java中的AtomicLongArray原子类，内容简明扼要并且容易理解，绝对能使你眼前一亮，通过这篇文章的详细介绍希望你能有所收获。

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AtomicLongArray介绍和函数列表 

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-
  * Expert Group and released to the public domain, as explained at
  * http://creativecommons.org/publicdomain/zero/./
  */
 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 .
  * @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 - )) != )
       throw new Error("data type scale not a power of two");
     shift = - Integer.numberOfLeadingZeros(scale);
   }
   private long checkedByteOffset(int i) {
     if (i < || 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.
   *
   * 
May fail spuriously
   * 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

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

上述内容就是深入浅析Java中的AtomicLongArray原子类，你们学到知识或技能了吗？如果还想学到更多技能或者丰富自己的知识储备，欢迎关注创新互联行业资讯频道。


文章名称：深入浅析Java中的AtomicLongArray原子类
文章出自：http://scyanting.com/article/gjiohh.html

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