1，获取文件的流

InportStream importStream = file.getInputStream()；

2，把文件流转化为字节流

//字节数组输出流
ByteArrayOutputStream baos = new ByteArrayOutputStream();
try {
    byte[] buffer = new byte[1024];
    int len;
    while ((len = inputStream.read(buffer)) > -1) {
        baos.write(buffer, 0, len);
    }
    baos.flush();
}catch(IOException e){
    e.printStackTrace();
}
return new ByteArrayInputStream(baos.toByteArray());

3，根据字节流的判断文件的编码

//默认编码
String charset = "GBK";
byte[] first3Bytes = new byte[3];
//缓存流
BufferedInputStream bis = new BufferedInputStream(inputStream1);
try {
    boolean checked = false;
    //标记
    bis.mark(0);
    //读取文件的前3个字符
    int read = bis.read(first3Bytes, 0, 3);
    if (read == -1) return charset;
    if (first3Bytes[0] == (byte) 0xFF && first3Bytes[1] == (byte) 0xFE) {
        charset = "UTF-16LE";
        checked = true;
    } else if (first3Bytes[0] == (byte) 0xFE && first3Bytes[1] == (byte) 0xFF) {
        charset = "UTF-16BE";
        checked = true;
    } else if (first3Bytes[0] == (byte) 0xEF && first3Bytes[1] == (byte) 0xBB && first3Bytes[2] == (byte) 0xBF) {
        charset = "UTF-8";
        checked = true;
    }
    bis.reset();
    if (!checked) {

        int loc = 0;
        while ((read = bis.read()) != -1) {
            loc++;
            if (read >= 0xF0) break;
            if (0x80 <= read && read <= 0xBF) // 单独出现BF以下的，也算是GBK
                break;
            if (0xC0 <= read && read <= 0xDF) {
                read = bis.read();
                if (0x80 <= read && read <= 0xBF) // 双字节 (0xC0 - 0xDF) (0x80
                    continue;
                else break;
            } else if (0xE0 <= read && read <= 0xEF) {// 也有可能出错，但是几率较小
                read = bis.read();
                if (0x80 <= read && read <= 0xBF) {
                    read = bis.read();
                    if (0x80 <= read && read <= 0xBF) {
                        charset = "UTF-8";
                        break;
                    } else break;
                } else break;
            }
        }
    }
} catch (Exception e) {
    e.printStackTrace();
} finally {
    try {
        bis.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}