Is binding to 0.0.0.0 in Java guaranteed to bind to all network interfaces?

It has nothing to do with Java. 0.0.0.0 is INADDR_ANY, which is is a special address that is guaranteed to receive from any network interface by the C sockets API, which is called by Java.

Using 0.0.0.0 will only bind to IPv4-enabled interfaces. However, if you bind to ::, that should cover all IPv4 and IPv6 interfaces, assuming your TCP/IP stack (and Java) have IPv4-compatible IPv6 sockets enabled.

You'll need to look to the kernel (or socket libraries, if you're on Windows) for an explanation of "why". On my OS X system, the man pages explain it.

From man 4 inet`:

Sockets may be created with the local address INADDR_ANY to effect 'wildcard' matching on incoming messages. The address in a connect(2) or sendto(2) call may be given as INADDR_ANY to mean 'this host'. The distinguished address INADDR_BROADCAST is allowed as a shorthand for the broadcast address on the primary network if the first network configured supports broadcast.

From man 4 inet6:

Sockets may be created with the local address :: (which is equal to IPv6 address 0:0:0:0:0:0:0:0) to affect 'wildcard' matching on incoming messages.