8236925: (dc) Upgrade DatagramChannel socket adaptor to extend MulticastSocket

[Alan Bateman]

On 16/01/2020 11:45, Chris Hegarty wrote:
> :
> Generally, I agree with updating the socket adapter to support
> multicast. It will certainly help with future work in this area.
>
> The instanceof checks in the constructors highlight that there is an
> abstraction missing here - to support creating a custom MulticastSocket
> implementation. If / When DatagramSocketImpl is deprecated, the advise
> will likely be to subclass MulticastSocket if one wants to provide a
> custom implementation. I believe that this cannot be done efficiently
> with the current API - and that is the issue that this change is running
> into that results in the instanceof checks in the constructors.
The main API issues date back to choices made in JDK 1.0 but it's not 
clear to me that it's worth trying to fix them now. The extensibility 
could have been looked in JDK 1.4 when the protected constructor was 
added but even then, it's not clear that there was any interest in 
completely different implementations outside of the JDK. So if someone 
really wants to this now then they could create a DatagramSocketImpl and 
use the factory method to set it system-wide. I don't think we've ever 
seen anyone do that, maybe because that mechanism is very under 
specified, maybe because it would require accessing the internal 
representation of FileDescriptor and DatagramPacket. If that mechanism 
is deprecated and eventually removed (which is probably the right thing 
to do) then the only choice would be to extend and override all methods. 
Yes, I agree the check for the sub-class is ugly. The legacy 
implementation also has to check if it's a MulticastSocket so that the 
right impl is chosen.

-Alan