一个基于反射的工具类，它能对指定类的指定的volatile字段进行原子更新
下面是netty源码中AbstractReferenceCountedByteBuf类的使用
private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> refCntUpdater;
static {
AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> updater = PlatformDependent.newAtomicIntegerFieldUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
if (updater == null) {
updater = AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");
}
refCntUpdater = updater;
}
// 必须用volative声明，保证多线程的可见性
private volatile int refCnt = 1;
@Override
public ByteBuf retain() {
//失败重试
for (;;) {
int refCnt = this.refCnt;
if (refCnt == 0) {
throw new IllegalReferenceCountException(0, 1);
}
if (refCnt == Integer.MAX_VALUE) {
throw new IllegalReferenceCountException(Integer.MAX_VALUE, 1);
}
//使用CAS对refcnt线程安全的操作
if (refCntUpdater.compareAndSet(this, refCnt, refCnt + 1)) {
break;
}
}
return this;
}
@Override
public final boolean release() {
for (;;) {
int refCnt = this.refCnt;
if (refCnt == 0) {
throw new IllegalReferenceCountException(0, -1);
}
// CAS 操作
if (refCntUpdater.compareAndSet(this, refCnt, refCnt - 1)) {
if (refCnt == 1) {
deallocate();
return true;
}
return false;
}
}
}