Double Checked Locking

DoubleCheckedLocking.java

/** private final DCLValue<Foo> foo = new DCLValue.create();
 *  Foo result = foo.getOrCreate(syncOn, () -> new Foo());
 *  Foo result = foo.get(); */
interface DCLValue<T> {
    T getOrCreate(Object syncOn, Supplier<T> creator);
    T get();
    void set(T value);

    static <T> DCLValue<T> create() {
        return new DCLValueRegular<>();
    }
}

class DCLValueRegular<T> implements DCLValue<T> {

    private volatile T value;

    @Override
    public T getOrCreate(Object syncOn, Supplier<T> creator) {
        T local = this.value;
        if (local == null) {
            synchronized (syncOn) {
                local = this.value;
                if (local == null) {
                    local = creator.get();
                    this.value = local;
                }
            }
        }
        return local;
    }

    @Override
    public T get() {
        return this.value;
    }

    @Override
    public void set(T value) {
        this.value = value;
    }
}

/** Implemented using VarHandle */
class DCLValueVarHandle<T> implements DCLValue<T> {

    private final VarHandle varHandle;

    private volatile T valueField;

    public DCLValueVarHandle() {
        try {
            MethodHandles.Lookup lookup = MethodHandles.lookup();
            varHandle = lookup.findVarHandle(DCLValueVarHandle.class, "valueField", Object.class);
        } catch (ReflectiveOperationException e) {
            throw new IllegalStateException(e);
        }
    }

    @Override
    public T getOrCreate(Object syncOn, Supplier<T> creator) {
        var obj = getAcquire();
        if (obj == null) {
            synchronized (syncOn) {
                obj = getAcquire();
                if (obj == null) {
                    obj = creator.get();
                    setRelease(obj);
                }
            }
        }
        return obj;
    }

    private T getAcquire() {
        return (T) varHandle.getAcquire(this);
    }

    private void setRelease(T value) {
        varHandle.setRelease(this, value);
    }

    @Override
    public T get() {
        return getAcquire();
    }

    @Override
    public void set(T value) {
        setRelease(value);
    }
}

// import java.lang.invoke.MethodHandles;
// import java.lang.invoke.VarHandle;
// import java.util.function.Supplier;