要想使用串口包进行串口通信，
除了设置好环境变量之外，
还要将rxtxSerial.dll复制到<JRE>\bin或者<JDK>\bin目录下；
将RXTXcomm.jar复制到<JRE>\lib或者<JDK>\lib.

If you want to use a serial port to a serial port communication, 
in addition to set up environment variables, 
you should copy "rxtxSerial.dll" to the "<JRE> \bin" directory or the "<JDK> \bin" directory; 
and copy "RXTXcomm.jar" to the "<JRE> \lib" directory or the " <JDK> \lib" directory.