What is the 'wildcard address' In the cont

问题:

Referring to the Java 6 API docs for the DatagramSocket class:

UDP broadcasts sends are always enabled on a DatagramSocket. In order to receive broadcast packets a DatagramSocket should be bound to the wildcard address. In some implementations, broadcast packets may also be received when a DatagramSocket is bound to a more specific address.

Could someone tell me what the 'wildcard address' is? And is the following valid for listening for UDP broadcasts:

MulticastSocket socket = new MulticastSocket(new InetSocketAddress(InetAddress.getByName("0.0.0.0"),4445);

回答1:

The wildcard address is 0.0.0.0. Not to be confused with the broadcast-to-all-subnets address, which is 255.255.255.255. It is more correctly called the 'any' address, after INADDR_ANY.

In Java it is most easily used by supplying null as a bind-address, or omitting the parameter altogether, e.g. new InetSocketAddress(null, 0) or new InetSocketAddress(0) respectively. In other words it is the default when binding, and therefore implicitly 'good practice'.