RefCnt Class — netty Architecture

Source Code

common/src/main/java/io/netty/util/internal/RefCnt.java lines 31–476

@SuppressWarnings("deprecation")
public final class RefCnt {

    private static final int UNSAFE = 0;
    private static final int VAR_HANDLE = 1;
    private static final int ATOMIC_UPDATER = 2;
    private static final int REF_CNT_IMPL;

    static {
        if (PlatformDependent.hasUnsafe()) {
            REF_CNT_IMPL = UNSAFE;
        } else if (PlatformDependent.hasVarHandle()) {
            REF_CNT_IMPL = VAR_HANDLE;
        } else {
            REF_CNT_IMPL = ATOMIC_UPDATER;
        }
    }

    /*
     * Implementation notes:
     *
     * For the updated int field:
     *   Even => "real" refcount is (refCnt >>> 1)
     *   Odd  => "real" refcount is 0
     *
     * This field is package-private so that the AtomicRefCnt implementation can reach it, even on native-image.
     */
    volatile int value;

    public RefCnt() {
        switch (REF_CNT_IMPL) {
        case UNSAFE:
            UnsafeRefCnt.init(this);
            break;
        case VAR_HANDLE:
            VarHandleRefCnt.init(this);
            break;
        case ATOMIC_UPDATER:
        default:
            AtomicRefCnt.init(this);
            break;
        }
    }

    /**
     * Returns the current reference count of the given {@code RefCnt} instance with a load acquire semantic.
     *
     * @param ref the target RefCnt instance
     * @return the reference count
     */
    public static int refCnt(RefCnt ref) {
        switch (REF_CNT_IMPL) {
        case UNSAFE:
            return UnsafeRefCnt.refCnt(ref);
        case VAR_HANDLE:
            return VarHandleRefCnt.refCnt(ref);
        case ATOMIC_UPDATER:
        default:
            return AtomicRefCnt.refCnt(ref);
        }
    }

    /**
     * Increases the reference count of the given {@code RefCnt} instance by 1.
     *
     * @param ref the target RefCnt instance
     */
    public static void retain(RefCnt ref) {
        switch (REF_CNT_IMPL) {
        case UNSAFE:
            UnsafeRefCnt.retain(ref);
            break;
        case VAR_HANDLE:
            VarHandleRefCnt.retain(ref);
            break;
        case ATOMIC_UPDATER:
        default:
            AtomicRefCnt.retain(ref);
            break;
        }
    }