Java多线程系列--“JUC原子类”02之 AtomicLong原子类

概要

AtomicInteger, AtomicLong和AtomicBoolean这3个基本类型的原子类的原理和用法相似。本章以AtomicLong对基本类型的原子类进行介绍。内容包括：
AtomicLong介绍和函数列表
AtomicLong源码分析(基于JDK1.7.0_40)
AtomicLong示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3514593.html

AtomicLong介绍和函数列表

AtomicLong是作用是对长整形进行原子操作。
在32位操作系统中，64位的long 和 double 变量由于会被JVM当作两个分离的32位来进行操作，所以不具有原子性。而使用AtomicLong能让long的操作保持原子型。

AtomicLong函数列表

// 构造函数
AtomicLong()
// 创建值为initialValue的AtomicLong对象
AtomicLong(long initialValue)
// 以原子方式设置当前值为newValue。
final void set(long newValue)
// 获取当前值
final long get()
// 以原子方式将当前值减 1，并返回减1后的值。等价于“--num”
final long decrementAndGet()
// 以原子方式将当前值减 1，并返回减1前的值。等价于“num--”
final long getAndDecrement()
// 以原子方式将当前值加 1，并返回加1后的值。等价于“++num”
final long incrementAndGet()
// 以原子方式将当前值加 1，并返回加1前的值。等价于“num++”
final long getAndIncrement()
// 以原子方式将delta与当前值相加，并返回相加后的值。
final long addAndGet(long delta)
// 以原子方式将delta添加到当前值，并返回相加前的值。
final long getAndAdd(long delta)
// 如果当前值 == expect，则以原子方式将该值设置为update。成功返回true，否则返回false，并且不修改原值。
final boolean compareAndSet(long expect, long update)
// 以原子方式设置当前值为newValue，并返回旧值。
final long getAndSet(long newValue)
// 返回当前值对应的int值
int intValue()
// 获取当前值对应的long值
long longValue()
// 以 float 形式返回当前值
float floatValue()
// 以 double 形式返回当前值
double doubleValue()
// 最后设置为给定值。延时设置变量值，这个等价于set()方法，但是由于字段是volatile类型的，因此次字段的修改会比普通字段（非volatile字段）有稍微的性能延时（尽管可以忽略），所以如果不是想立即读取设置的新值，允许在“后台”修改值，那么此方法就很有用。如果还是难以理解，这里就类似于启动一个后台线程如执行修改新值的任务，原线程就不等待修改结果立即返回（这种解释其实是不正确的，但是可以这么理解）。
final void lazySet(long newValue)
// 如果当前值 == 预期值，则以原子方式将该设置为给定的更新值。JSR规范中说：以原子方式读取和有条件地写入变量但不 创建任何 happen-before 排序，因此不提供与除 weakCompareAndSet 目标外任何变量以前或后续读取或写入操作有关的任何保证。大意就是说调用weakCompareAndSet时并不能保证不存在happen-before的发生（也就是可能存在指令重排序导致此操作失败）。但是从Java源码来看，其实此方法并没有实现JSR规范的要求，最后效果和compareAndSet是等效的，都调用了unsafe.compareAndSwapInt()完成操作。
final boolean weakCompareAndSet(long expect, long update)

AtomicLong源码分析(基于JDK1.7.0_40)

AtomicLong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;

 /**
  * A {@code long} value that may be updated atomically.  See the
  * {@link java.util.concurrent.atomic} package specification for
  * description of the properties of atomic variables. An
  * {@code AtomicLong} is used in applications such as atomically
  * incremented sequence numbers, and cannot be used as a replacement
  * for a {@link java.lang.Long}. However, this class does extend
  * {@code Number} to allow uniform access by tools and utilities that
  * deal with numerically-based classes.
  *
  * @since 1.5
  * @author Doug Lea
  */
 public class AtomicLong extends Number implements java.io.Serializable {
     private static final long serialVersionUID = 1927816293512124184L;

     // setup to use Unsafe.compareAndSwapLong for updates
     private static final Unsafe unsafe = Unsafe.getUnsafe();
     private static final long valueOffset;

     /**
      * Records whether the underlying JVM supports lockless
      * compareAndSwap for longs. While the Unsafe.compareAndSwapLong
      * method works in either case, some constructions should be
      * handled at Java level to avoid locking user-visible locks.
      */
     static final boolean VM_SUPPORTS_LONG_CAS = VMSupportsCS8();

     /**
      * Returns whether underlying JVM supports lockless CompareAndSet
      * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
      */
     private static native boolean VMSupportsCS8();

     static {
       try {
         valueOffset = unsafe.objectFieldOffset
             (AtomicLong.class.getDeclaredField("value"));
       } catch (Exception ex) { throw new Error(ex); }
     }

     private volatile long value;

     /**
      * Creates a new AtomicLong with the given initial value.
      *
      * @param initialValue the initial value
      */
     public AtomicLong(long initialValue) {
         value = initialValue;
     }

     /**
      * Creates a new AtomicLong with initial value {@code 0}.
      */
     public AtomicLong() {
     }

     /**
      * Gets the current value.
      *
      * @return the current value
      */
     public final long get() {
         return value;
     }

     /**
      * Sets to the given value.
      *
      * @param newValue the new value
      */
     public final void set(long newValue) {
         value = newValue;
     }

     /**
      * Eventually sets to the given value.
      *
      * @param newValue the new value
      * @since 1.6
      */
     public final void lazySet(long newValue) {
         unsafe.putOrderedLong(this, valueOffset, newValue);
     }

     /**
      * Atomically sets to the given value and returns the old value.
      *
      * @param newValue the new value
      * @return the previous value
      */
     public final long getAndSet(long newValue) {
         while (true) {
             long current = get();
             if (compareAndSet(current, newValue))
                 return current;
         }
     }

     /**
      * Atomically sets the value to the given updated value
      * if the current value {@code ==} the expected value.
      *
      * @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(long expect, long update) {
         return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
     }

     /**
      * Atomically sets the value 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 expect the expected value
      * @param update the new value
      * @return true if successful.
      */
     public final boolean weakCompareAndSet(long expect, long update) {
         return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
     }

     /**
      * Atomically increments by one the current value.
      *
      * @return the previous value
      */
     public final long getAndIncrement() {
         while (true) {
             long current = get();
             long next = current + 1;
             if (compareAndSet(current, next))
                 return current;
         }
     }

     /**
      * Atomically decrements by one the current value.
      *
      * @return the previous value
      */
     public final long getAndDecrement() {
         while (true) {
             long current = get();
             long next = current - 1;
             if (compareAndSet(current, next))
                 return current;
         }
     }

     /**
      * Atomically adds the given value to the current value.
      *
      * @param delta the value to add
      * @return the previous value
      */
     public final long getAndAdd(long delta) {
         while (true) {
             long current = get();
             long next = current + delta;
             if (compareAndSet(current, next))
                 return current;
         }
     }

     /**
      * Atomically increments by one the current value.
      *
      * @return the updated value
      */
     public final long incrementAndGet() {
         for (;;) {
             long current = get();
             long next = current + 1;
             if (compareAndSet(current, next))
                 return next;
         }
     }

     /**
      * Atomically decrements by one the current value.
      *
      * @return the updated value
      */
     public final long decrementAndGet() {
         for (;;) {
             long current = get();
             long next = current - 1;
             if (compareAndSet(current, next))
                 return next;
         }
     }

     /**
      * Atomically adds the given value to the current value.
      *
      * @param delta the value to add
      * @return the updated value
      */
     public final long addAndGet(long delta) {
         for (;;) {
             long current = get();
             long next = current + delta;
             if (compareAndSet(current, next))
                 return next;
         }
     }

     /**
      * Returns the String representation of the current value.
      * @return the String representation of the current value.
      */
     public String toString() {
         return Long.toString(get());
     }

     public int intValue() {
         return (int)get();
     }

     public long longValue() {
         return get();
     }

     public float floatValue() {
         return (float)get();
     }

     public double doubleValue() {
         return (double)get();
     }

 }

AtomicLong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() {
    for (;;) {
        // 获取AtomicLong当前对应的long值
        long current = get();
        // 将current加1
        long next = current + 1;
        // 通过CAS函数，更新current的值
        if (compareAndSet(current, next))
            return next;
    }
}

说明：
(01) incrementAndGet()首先会根据get()获取AtomicLong对应的long值。该值是volatile类型的变量，get()的源码如下：

// value是AtomicLong对应的long值
private volatile long value;
// 返回AtomicLong对应的long值
public final long get() {
    return value;
}

(02) incrementAndGet()接着将current加1,然后通过CAS函数，将新的值赋值给value。
compareAndSet()的源码如下：

public final boolean compareAndSet(long expect, long update) {
    return unsafe.compareAndSwapLong(this, valueOffset, expect, update);
}

compareAndSet()的作用是更新AtomicLong对应的long值。它会比较AtomicLong的原始值是否与expect相等，若相等的话，则设置AtomicLong的值为update。

AtomicLong示例

 // LongTest.java的源码
 import java.util.concurrent.atomic.AtomicLong;

 public class LongTest {

     public static void main(String[] args){

         // 新建AtomicLong对象
         AtomicLong mAtoLong = new AtomicLong();

         mAtoLong.set(0x0123456789ABCDEFL);
         System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
         System.out.printf("%20s : 0x%016X\n", "intValue()", mAtoLong.intValue());
         System.out.printf("%20s : 0x%016X\n", "longValue()", mAtoLong.longValue());
         System.out.printf("%20s : %s\n", "doubleValue()", mAtoLong.doubleValue());
         System.out.printf("%20s : %s\n", "floatValue()", mAtoLong.floatValue());

         System.out.printf("%20s : 0x%016X\n", "getAndDecrement()", mAtoLong.getAndDecrement());
         System.out.printf("%20s : 0x%016X\n", "decrementAndGet()", mAtoLong.decrementAndGet());
         System.out.printf("%20s : 0x%016X\n", "getAndIncrement()", mAtoLong.getAndIncrement());
         System.out.printf("%20s : 0x%016X\n", "incrementAndGet()", mAtoLong.incrementAndGet());

         System.out.printf("%20s : 0x%016X\n", "addAndGet(0x10)", mAtoLong.addAndGet(0x10));
         System.out.printf("%20s : 0x%016X\n", "getAndAdd(0x10)", mAtoLong.getAndAdd(0x10));

         System.out.printf("\n%20s : 0x%016X\n", "get()", mAtoLong.get());

         System.out.printf("%20s : %s\n", "compareAndSet()", mAtoLong.compareAndSet(0x12345679L, 0xFEDCBA9876543210L));
         System.out.printf("%20s : 0x%016X\n", "get()", mAtoLong.get());
     }
 }

运行结果：

               get() : 0x0123456789ABCDEF
          intValue() : 0x0000000089ABCDEF
         longValue() : 0x0123456789ABCDEF
       doubleValue() : 8.1985529216486896E16
        floatValue() : 8.1985531E16
   getAndDecrement() : 0x0123456789ABCDEF
   decrementAndGet() : 0x0123456789ABCDED
   getAndIncrement() : 0x0123456789ABCDED
   incrementAndGet() : 0x0123456789ABCDEF
     addAndGet(0x10) : 0x0123456789ABCDFF
     getAndAdd(0x10) : 0x0123456789ABCDFF

               get() : 0x0123456789ABCE0F
     compareAndSet() : false
               get() : 0x0123456789ABCE0F

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