Java中的CAS和ABA问题说明

public final native boolean compareAndSwapObject(Object var1, long var2, Object var3, Object var4);
    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);

    
    public final int getAndSet(int newValue) {
        return unsafe.getAndSetInt(this, valueOffset, newValue);
    }

package java.util.concurrent.atomic;
import java.util.function.IntUnaryOperator;
import java.util.function.IntBinaryOperator;
import sun.misc.Unsafe;
 
public class AtomicInteger extends Number implements java.io.Serializable {
    private static final long serialVersionUID = 6214790243416807050L;
 
    // 获取UnSafe对象实例
    private static final Unsafe unsafe = Unsafe.getUnsafe();
    //对象在内存中的偏移量
    private static final long valueOffset;
    
    //初始化valueOffset
    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicInteger.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }
    
    //对象属性值
    private volatile int value;
 
    public AtomicInteger(int initialValue) {
        value = initialValue;
    }
 
    public AtomicInteger() {
    }
 
    
    public final int getAndSet(int newValue) {
        return unsafe.getAndSetInt(this, valueOffset, newValue);
    }
 
    //调用UnSafe的compareAndSwapInt方法保证CAS
    public final boolean compareAndSet(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }
 
    //调用UnSafe的compareAndSwapInt方法保证CAS
    public final boolean weakCompareAndSet(int expect, int update) {
        return unsafe.compareAndSwapInt(this, valueOffset, expect, update);
    }
 
    //调用UnSafe的getAndAddInt再调用UnSafe的getAndSetInt方法保证CAS
    public final int getAndIncrement() {
        return unsafe.getAndAddInt(this, valueOffset, 1);
    }
 
    
    public final int getAndDecrement() {
        return unsafe.getAndAddInt(this, valueOffset, -1);
    }
    .........
}

package java.util.concurrent.atomic;
public class AtomicStampedReference<V> {
   
    private static class Pair<T> {
        final T reference;
        final int stamp;
        private Pair(T reference, int stamp) {
            this.reference = reference;
            this.stamp = stamp;
        }
        static <T> Pair<T> of(T reference, int stamp) {
            return new Pair<T>(reference, stamp);
        }
    }
 
    private volatile Pair<V> pair;
 
    
    public AtomicStampedReference(V initialRef, int initialStamp) {
        pair = Pair.of(initialRef, initialStamp);
    }
    
    
    public boolean compareAndSet(V   expectedReference,
                                 V   newReference,
                                 int expectedStamp,
                                 int newStamp) {
        Pair<V> current = pair;  //赋值当前对象
        return
            expectedReference == current.reference &&
            expectedStamp == current.stamp &&
            ((newReference == current.reference &&
              newStamp == current.stamp) ||
             casPair(current, Pair.of(newReference, newStamp)));
    }
 
    
    // Unsafe mechanics
 
    private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
    private static final long pairOffset =
        objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);
 
    private boolean casPair(Pair<V> cmp, Pair<V> val) {
        return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
    }
 
    static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
                                  String field, Class<?> klazz) {
        try {
            return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
        } catch (NoSuchFieldException e) {
            // Convert Exception to corresponding Error
            NoSuchFieldError error = new NoSuchFieldError(field);
            error.initCause(e);
            throw error;
        }
    }
}