通过 UNSAFE 来实现一个 Atomic 的 CAS 辅助类【原创】详解编程语言

public abstract class AbstractUnSafeCas<T> { 
 
  @SuppressWarnings("unused") 
  private volatile Object value; 
 
  private static final AtomicHelper ATOMIC_HELPER; 
 
  private final CountDownLatch countDownLatch = new CountDownLatch(1); 
 
  static { 
    ATOMIC_HELPER = new AtomicHelper(); 
  } 
 
  private static class AtomicHelper { 
 
    private static final sun.misc.Unsafe UNSAFE; 
 
    private static final long VALUE_OFFSET; 
 
    static { 
      sun.misc.Unsafe unsafe; 
      try { 
        unsafe = sun.misc.Unsafe.getUnsafe(); 
      } catch (SecurityException tryReflectionInstead) { 
        unsafe = AccessController.doPrivileged(new PrivilegedAction<Unsafe>() { 
          @Override 
          @SneakyThrows 
          public Unsafe run() { 
            Class<sun.misc.Unsafe> unsafeClass = sun.misc.Unsafe.class; 
            for (Field f : unsafeClass.getDeclaredFields()) { 
              f.setAccessible(true); 
              Object o = f.get(null); 
              if (unsafeClass.isInstance(o)) { 
                return unsafeClass.cast(o); 
              } 
            } 
            throw new NoSuchFieldError("the Unsafe"); 
          } 
        }); 
      } 
      try { 
        VALUE_OFFSET = unsafe.objectFieldOffset(AbstractUnSafeAtomicHelper.class.getDeclaredField("value")); 
      } catch (Exception e) { 
        throw Throwables.propagate(e); 
      } 
      UNSAFE = unsafe; 
    } 
 
    public boolean casValue(AbstractUnSafeAtomicHelper<?> obj, Object expect, Object update) { 
      return UNSAFE.compareAndSwapObject(obj, VALUE_OFFSET, expect, update); 
    } 
 
  } 
 
  public boolean set(T value) { 
    Preconditions.checkArgument(null != value, "The value to be setted is null"); 
    if (ATOMIC_HELPER.casValue(this, null, value)) { 
      countDownLatch.countDown(); 
      return true; 
    } 
    return false; 
  } 
 
  @SneakyThrows 
  public T get(boolean blockUntilValueIsNotNull) { 
    if (blockUntilValueIsNotNull) { 
      countDownLatch.await(); 
    } 
    @SuppressWarnings("unchecked") 
    T asT = (T) value; 
    return asT; 
  } 
 
  @SneakyThrows 
  public T get(Long timeout, TimeUnit timeUnit) { 
    countDownLatch.await(timeout, timeUnit); 
    @SuppressWarnings("unchecked") 
    T asT = (T) value; 
    return asT; 
  } 
 
}