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

// 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)

  /*  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  *  */ /*  *  *  *  *  *  * 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 (;;) {    // 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;  }}

// 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;}

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

 // 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());   } }

  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