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);
    }
}