JavaCAS原子操作详解

    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);
    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 class CASTest {
    public static void main(String[] args) {
        Entity entity = new Entity();
        Unsafe unsafe = UnsafeFactory.getUnsafe();
        long offset = UnsafeFactory.getFieldOffset(unsafe, Entity.class, "x");
        //12
        System.out.println(offset);
        boolean successful;
        // 4个参数分别是：对象实例、字段的内存偏移量、字段期望值、字段更新值
        //x是不是等于0，如果等于0就把它修改为3
        successful = unsafe.compareAndSwapInt(entity, offset, 0, 3);
        System.out.println(successful + "-----" + entity.x);
        successful = unsafe.compareAndSwapInt(entity, offset, 3, 5);
        System.out.println(successful + "-----" + entity.x);
        successful = unsafe.compareAndSwapInt(entity, offset, 3, 8);
        System.out.println(successful + "-----" + entity.x);
    }
}
class Entity{
    int x;
}
public class UnsafeFactory {
    
    public static Unsafe getUnsafe() {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            return (Unsafe) field.get(null);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
    
    public static long getFieldOffset(Unsafe unsafe, Class clazz, String fieldName) {
        try {
            return unsafe.objectFieldOffset(clazz.getDeclaredField(fieldName));
        } catch (NoSuchFieldException e) {
            throw new Error(e);
        }
    }
}

#unsafe.cpp 
UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jo bject obj, jlong offset, jint e, jint x)) 
UnsafeWrapper("Unsafe_CompareAndSwapInt"); 
oop p = JNIHandles::resolve(obj); 
// 根据偏移量，计算value的地址 
jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);
// Atomic::cmpxchg(x, addr, e) cas逻辑 x:要交换的值 e:要比较的值 
//cas成功，返回期望值e，等于e,此方法返回true 
//cas失败，返回内存中的value值，不等于e，此方法返回false 
return (jint)(Atomic::cmpxchg(x, addr, e)) == e; 
UNSAFE_END

#atomic_linux_x86.inline.hpp 
inline jint Atomic::cmpxchg (jint exchange_value, volatile jint* dest, jint com pare_value) { 
 //判断当前执行环境是否为多处理器环境 
 int mp = os::is_MP(); 
 //LOCK_IF_MP(%4)在多处理器环境下，为cmpxchgl指令添加lock前缀，以达到内存屏障的效果 
  //cmpxchgl 指令是包含在 x86 架构及 IA‐64 架构中的一个原子条件指令， 
  //它会首先比较 dest 指针指向的内存值是否和 compare_value 的值相等， 
  //如果相等，则双向交换 dest 与 exchange_value，否则就单方面地将dest指向的内存值交给exchange_value。 
  //这条指令完成了整个CAS操作，因此它也被称为CAS指令。 
  __asm__ volatile (LOCK_IF_MP(%4) "cmpxchgl %1,(%3)" 
  : "=a" (exchange_value) 
  : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp) 
  : "cc", "memory"); 
  return exchange_value; 
  }

public class ABATest {
    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(1);
        new Thread(()->{
            int value = atomicInteger.get();
            log.debug("Thread1 read value: " + value);
            // 阻塞1s
            LockSupport.parkNanos(1000000000L);
            // Thread1通过CAS修改value值为3
            if (atomicInteger.compareAndSet(value, 3)) {
                log.debug("Thread1 update from " + value + " to 3");
            } else {
                log.debug("Thread1 update fail!");
            }
        },"Thread1").start();
        new Thread(()->{
            int value = atomicInteger.get();
            log.debug("Thread2 read value: " + value);
            // Thread2通过CAS修改value值为2
            if (atomicInteger.compareAndSet(value, 2)) {
                log.debug("Thread2 update from " + value + " to 2");
                // do something
                value = atomicInteger.get();
                log.debug("Thread2 read value: " + value);
                // Thread2通过CAS修改value值为1
                if (atomicInteger.compareAndSet(value, 1)) {
                    log.debug("Thread2 update from " + value + " to 1");
                }
            }
        },"Thread2").start();
    }
}

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);
        }
    }
    ...

public class AtomicStampedReferenceTest {
    public static void main(String[] args) {
        // 定义AtomicStampedReference    Pair.reference值为1, Pair.stamp为1
        AtomicStampedReference atomicStampedReference = new AtomicStampedReference(1,1);
        new Thread(()->{
            int[] stampHolder = new int[1];
            int value = (int) atomicStampedReference.get(stampHolder);
            int stamp = stampHolder[0];
            log.debug("Thread1 read value: " + value + ", stamp: " + stamp);
            // 阻塞1s
            LockSupport.parkNanos(1000000000L);
            // Thread1通过CAS修改value值为3   stamp是版本，每次修改可以通过+1保证版本唯一性
            if (atomicStampedReference.compareAndSet(value, 3,stamp,stamp+1)) {
                log.debug("Thread1 update from " + value + " to 3");
            } else {
                log.debug("Thread1 update fail!");
            }
        },"Thread1").start();
        new Thread(()->{
            int[] stampHolder = new int[1];
            int value = (int)atomicStampedReference.get(stampHolder);
            int stamp = stampHolder[0];
            log.debug("Thread2 read value: " + value+ ", stamp: " + stamp);
            // Thread2通过CAS修改value值为2
            if (atomicStampedReference.compareAndSet(value, 2,stamp,stamp+1)) {
                log.debug("Thread2 update from " + value + " to 2");
                // do something
                value = (int) atomicStampedReference.get(stampHolder);
                stamp = stampHolder[0];
                log.debug("Thread2 read value: " + value+ ", stamp: " + stamp);
                // Thread2通过CAS修改value值为1
                if (atomicStampedReference.compareAndSet(value, 1,stamp,stamp+1)) {
                    log.debug("Thread2 update from " + value + " to 1");
                }
            }
        },"Thread2").start();
    }
}