/**
 * 将异常信息转化成字符串
 * @param t
 * @return
 * @throws IOException 
 */
private static String exception(Throwable t) throws IOException{
    if(t == null)
        return null;
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    try{
        t.printStackTrace(new PrintStream(baos));
    }finally{
        baos.close();
    }
    return baos.toString();
}