Java concurrency之AtomicLongArray原子类_动力节点Java学院整理

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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/*
 *
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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.
  *
  * <p>May <a href="package-summary.html#Spurious" rel="external nofollow" >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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