今天同事说道，使用InetAddress类无法在mac上获取到本机ip和主机名，而在服务器linux上就可以。
下面以jdk_1.7.0_25加以说明。
首先，对于主机名的问题，linux服务器上在/etc/hostname文件中做了设置，所以能查出；而在mac上，作为开发机，并没有做相关配置，所以查不到。
然后，在InetAddress.getLocalHost方法中，会根据主机名做判断，如果是localhost就返回ip为127.0.0.1的接口对象，否则会通过名字服务器来查找本地的ip，并返回相关对象。如下所示：
public static InetAddress getLocalHost() throws UnknownHostException {
SecurityManager security = System.getSecurityManager();
try {
String local = impl.getLocalHostName();
if (security != null) {
security.checkConnect(local, -1);
}
if (local.equals("localhost")) {
return impl.loopbackAddress();
}
InetAddress ret = null;
synchronized (cacheLock) {
long now = System.currentTimeMillis();
if (cachedLocalHost != null) {
if ((now - cacheTime) < maxCacheTime) // Less than 5s old?
ret = cachedLocalHost;
else
cachedLocalHost = null;
}
// we are calling getAddressesFromNameService directly
// to avoid getting localHost from cache
if (ret == null) {
InetAddress[] localAddrs;
try {
localAddrs =
InetAddress.getAddressesFromNameService(local, null);
} catch (UnknownHostException uhe) {
// Rethrow with a more informative error message.
UnknownHostException uhe2 =
new UnknownHostException(local + ": " +
uhe.getMessage());
uhe2.initCause(uhe);
throw uhe2;
}
cachedLocalHost = localAddrs[0];
cacheTime = now;
ret = localAddrs[0];
}
}
return ret;
} catch (java.lang.SecurityException e) {
return impl.loopbackAddress();
}
}
针对上面的问题，可以使用NetworkInterface类来处理，该类用于获取本机网络接口信息，可以正确的拿到ip和主机名信息。
NetworkInterface类的getAll方法中会获取本地所有网络接口，是一个本地方法，其在bsd上的实现NetworkInterface.c是：
/*
* Class: java_net_NetworkInterface
* Method: getAll
* Signature: ()[Ljava/net/NetworkInterface;
*/
JNIEXPORT jobjectArray JNICALL Java_java_net_NetworkInterface_getAll(JNIEnv *env, jclass cls) {
netif *ifs, *curr;
jobjectArray netIFArr;
jint arr_index, ifCount;
ifs = enumInterfaces(env);
if (ifs == NULL) {
return NULL;
}
/* count the interface */
ifCount = 0;
curr = ifs;
while (curr != NULL) {
ifCount++;
curr = curr->next;
}
/* allocate a NetworkInterface array */
netIFArr = (*env)->NewObjectArray(env, ifCount, cls, NULL);
if (netIFArr == NULL) {
freeif(ifs);
return NULL;
}
/*
* Iterate through the interfaces, create a NetworkInterface instance
* for each array element and populate the object.
*/
curr = ifs;
arr_index = 0;
while (curr != NULL) {
jobject netifObj;
netifObj = createNetworkInterface(env, curr);
if (netifObj == NULL) {
freeif(ifs);
return NULL;
}
/* put the NetworkInterface into the array */
(*env)->SetObjectArrayElement(env, netIFArr, arr_index++, netifObj);
curr = curr->next;
}
freeif(ifs);
return netIFArr;
}
相关的主要方法是enumIPv4Interfaces，该方法会在enumInterfaces方法中被调用。enumIPv4Interfaces方法的主要功能是遍历所有IPv4网络接口，创建相关对象并返回，其具体实现为：
/*
* Enumerates and returns all IPv4 interfaces
*/
static netif *enumIPv4Interfaces(JNIEnv *env, int sock, netif *ifs) {
struct ifaddrs *ifa, *origifa;
if (getifaddrs(&origifa) != 0) {
NET_ThrowByNameWithLastError(env , JNU_JAVANETPKG "SocketException",
"getifaddrs() function failed");
return ifs;
}
for (ifa = origifa; ifa != NULL; ifa = ifa->ifa_next) {
/*
* Skip non-AF_INET entries.
*/
if (ifa->ifa_addr == NULL || ifa->ifa_addr->sa_family != AF_INET)
continue;
/*
* Add to the list.
*/
ifs = addif(env, sock, ifa->ifa_name, ifs, ifa->ifa_addr, AF_INET, 0);
/*
* If an exception occurred then free the list.
*/
if ((*env)->ExceptionOccurred(env)) {
freeifaddrs(origifa);
freeif(ifs);
return NULL;
}
}
/*
* Free socket and buffer
*/
freeifaddrs(origifa);
return ifs;
}