Java concurrency之AtomicLong原子類_動力節(jié)點Java學院整理

// 構造函數(shù)
AtomicLong()
// 創(chuàng)建值為initialValue的AtomicLong對象
AtomicLong(long initialValue)
// 以原子方式設置當前值為newValue。
final void set(long newValue) 
// 獲取當前值
final long get() 
// 以原子方式將當前值減 1，并返回減1后的值。等價于“--num”
final long decrementAndGet() 
// 以原子方式將當前值減 1，并返回減1前的值。等價于“num--”
final long getAndDecrement() 
// 以原子方式將當前值加 1，并返回加1后的值。等價于“++num”
final long incrementAndGet() 
// 以原子方式將當前值加 1，并返回加1前的值。等價于“num++”
final long getAndIncrement()  
// 以原子方式將delta與當前值相加，并返回相加后的值。
final long addAndGet(long delta) 
// 以原子方式將delta添加到當前值，并返回相加前的值。
final long getAndAdd(long delta) 
// 如果當前值 == expect，則以原子方式將該值設置為update。成功返回true，否則返回false，并且不修改原值。
final boolean compareAndSet(long expect, long update)
// 以原子方式設置當前值為newValue，并返回舊值。
final long getAndSet(long newValue)
// 返回當前值對應的int值
int intValue() 
// 獲取當前值對應的long值
long longValue()  
// 以 float 形式返回當前值
float floatValue()  
// 以 double 形式返回當前值
double doubleValue()  
// 最后設置為給定值。延時設置變量值，這個等價于set()方法，但是由于字段是volatile類型的，因此次字段的修改會比普通字段（非volatile字段）有稍微的性能延時（盡管可以忽略），所以如果不是想立即讀取設置的新值，允許在“后臺”修改值，那么此方法就很有用。如果還是難以理解，這里就類似于啟動一個后臺線程如執(zhí)行修改新值的任務，原線程就不等待修改結果立即返回（這種解釋其實是不正確的，但是可以這么理解）。
final void lazySet(long newValue)
// 如果當前值 == 預期值，則以原子方式將該設置為給定的更新值。JSR規(guī)范中說：以原子方式讀取和有條件地寫入變量但不 創(chuàng)建任何 happen-before 排序，因此不提供與除 weakCompareAndSet 目標外任何變量以前或后續(xù)讀取或寫入操作有關的任何保證。大意就是說調用weakCompareAndSet時并不能保證不存在happen-before的發(fā)生（也就是可能存在指令重排序導致此操作失敗）。但是從Java源碼來看，其實此方法并沒有實現(xiàn)JSR規(guī)范的要求，最后效果和compareAndSet是等效的，都調用了unsafe.compareAndSwapInt()完成操作。
final boolean weakCompareAndSet(long expect, long update)

  /*
  * 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;
 /**
  * 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 .
  * @author Doug Lea
  */
 public class AtomicLong extends Number implements java.io.Serializable {
   private static final long serialVersionUID = L;
   // 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 = VMSupportsCS();
   /**
    * Returns whether underlying JVM supports lockless CompareAndSet
    * for longs. Called only once and cached in VM_SUPPORTS_LONG_CAS.
    */
   private static native boolean VMSupportsCS();
   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 }.
    */
   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" rel="external nofollow" >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();
   }
 }

public final long incrementAndGet() {
  for (;;) {
    // 獲取AtomicLong當前對應的long值
    long current = get();
    // 將current加1
    long next = current + 1;
    // 通過CAS函數(shù)，更新current的值
    if (compareAndSet(current, next))
      return next;
  }
}

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

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

 // LongTest.java的源碼
 import java.util.concurrent.atomic.AtomicLong;
 public class LongTest {
   public static void main(String[] args){
     // 新建AtomicLong對象
     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());
   }
 }

  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