Almost final (first attempt)

import java.lang.invoke.*;

public class AlmostFinal<T> {
        private final MethodHandle getter;
        private SwitchPoint switchPoint;
        private VolatileCallSite callSite;

        public AlmostFinal(T initial) {
                switchPoint = new SwitchPoint();
                callSite = new VolatileCallSite(MethodType.genericMethodType(0));
                getter = switchPoint.guardWithTest(MethodHandles.constant(Object.class, initial), callSite.dynamicInvoker());
        }

        public T value() {
                try {
                        return (T) getter.invokeExact();
                } catch (Throwable t) {
                        throw new Error(t); // Shouldn't happen
                }
        }

        public synchronized void setValue(T val) {
                final SwitchPoint[] invalid = { switchPoint };
                switchPoint = new SwitchPoint();
                final VolatileCallSite previousSite = callSite;
                callSite = new VolatileCallSite(MethodType.genericMethodType(0));
                previousSite.setTarget(switchPoint.guardWithTest(MethodHandles.constant(Object.class, val), callSite.dynamicInvoker()));
                SwitchPoint.invalidateAll(invalid);
        }
}