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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