This is observed in Java 5 and Java 6.

When called with -Djava.net.preferIPv6Addresses=true the call to DatagramSocket getLocalSocketAddress returns an all-zero address, instead of ::1.

           InetAddress iNet = InetAddress.getByName("localhost");
            DatagramSocket soc = new DatagramSocket( 12345, iNet);
            SocketAddress sa = soc. getLocalSocketAddress();

            System.out.println("iNet: " + iNet);
            System.out.println("Expected: " + iNet + ":12345");
            System.out.println("Returned: " + ((InetSocketAddress)sa).getAddress());
workaround: Here's a fix:
In net_util_md.h, the macro IN6_IS_ADDR_ANY is defined as:
#define IN6_IS_ADDR_ANY(a) \
    (((a)->s6_words[0] == 0) && ((a)->s6_words[1] == 0) && \
    ((a)->s6_words[2] == 0) && ((a)->s6_words[3] == 0) && \
    ((a)->s6_words[4] == 0) && ((a)->s6_words[5] == 0))

It should be:
#define IN6_IS_ADDR_ANY(a) \
    (((a)->s6_words[0] == 0) && ((a)->s6_words[1] == 0) && \
    ((a)->s6_words[2] == 0) && ((a)->s6_words[3] == 0) && \
    ((a)->s6_words[4] == 0) && ((a)->s6_words[5] == 0) && \
    ((a)->s6_words[6] == 0) && ((a)->s6_words[7] == 0))

IPV6 addresses are 16 bytes long, the macros only tests 12 bytes.