public Socket accept() throws IOException {
if (isClosed())
throw new SocketException("Socket is closed");
if (!isBound())
throw new SocketException("Socket is not bound yet");
//通过系统默认类型的 SocketImpl 创建未连接套接字
Socket s = new Socket((SocketImpl) null);
//ServerSocket 的子类使用此方法重写 accept() 以返回它们自己的套接字子类
implAccept(s);
return s;
}