Socket兼容IP和域名的原因

//将172.16.12.171 替换为 xingyun25 打印结果一致
Socket sock = new Socket("172.16.12.171",12331); 
InetAddress netAddress = sock.getInetAddress();
System.out.println(netAddress.getHostName());
System.out.println(netAddress.getHostAddress());
sock.close();

xingyun25
172.16.12.171

 public Socket(String host, int port) throws UnknownHostException, IOException
    {
       //host != null 因此第一个参数为new InetSocketAddress(host, port)
       this(host != null ? new InetSocketAddress(host, port) :
             new InetSocketAddress(InetAddress.getByName(null), 
             //第二个参数 (SocketAddress) null 
             //第三个参数 true             (SocketAddress) null, true);
    }

private Socket(SocketAddress address, SocketAddress localAddr,
                   boolean stream) throws IOException {
    setImpl();
    // backward compatibility
if (address == null)
        throw new NullPointerException();
    try {
        createImpl(stream);
if (localAddr != null)
            //localhost/127.0.0.1
            bind(localAddr);
        //建立连接
        connect(address);
    } catch (IOException | IllegalArgumentException | SecurityException e) {
        try {
            close();
        } catch (IOException ce) {
            e.addSuppressed(ce);
        }
        throw e;
    }
}

void setImpl() {
if (factory != null) {
        impl = factory.createSocketImpl();
        checkOldImpl();
    } else {
        // No need to do a checkOldImpl() here, we know it's an up to date        // SocketImpl!        impl = new SocksSocketImpl();    }    //Impl SET Socket     if (impl != null)        impl.setSocket(this);}

// SocketImpl impl ：The implementation of this Socket. 
void createImpl(boolean stream) throws SocketException {
if (impl == null)
        setImpl();
    try {
        impl.create(stream);
        created =true;
    } catch (IOException e) {
        throw new SocketException(e.getMessage());
    }
}

 public InetSocketAddress(String hostname, int port) {
        checkHost(hostname);
        InetAddress addr = null;
    String host = null;
        try {
            addr = InetAddress.getByName(hostname);
        } catch(UnknownHostException e){
           host = hostname;
        }
        //实际构造了 InetSocketAddressHolder 
        holder = new InetSocketAddressHolder(host, addr, checkPort(port));
    }

connect 过程中实际使用的是 InetAddress
 private InetSocketAddressHolder(String hostname, InetAddress addr, int port) {
            this.hostname = hostname;
             this.addr = addr;
            this.port = port;
}