public class MD5Check {
/**
* 默認的密碼字符串組合，用來將字節轉換成 16 進制表示的字符,apache校驗下載的文件的正確性用的就是默認的這個組合
*/
    protected char hexDigits[] = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
    protected  MessageDigest messagedigest = null;

    {
        try {
            messagedigest = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
    }

    public String getFileMD5String(File file) throws IOException {
        InputStream fis;
        fis = new FileInputStream(file);
        byte[] buffer = new byte[1024];
        int numRead = 0;
        while ((numRead = fis.read(buffer)) > 0) {
            messagedigest.update(buffer, 0, numRead);
        }
        fis.close();
        return bufferToHex(messagedigest.digest());
    }

    public String getFileMD5String(InputStream in) throws IOException {
        byte[] buffer = new byte[1024];
        int numRead = 0;
        while ((numRead = in.read(buffer)) > 0) {
            messagedigest.update(buffer, 0, numRead);
        }
        in.close();
        return bufferToHex(messagedigest.digest());
    }

    private String bufferToHex(byte bytes[]) {
        return bufferToHex(bytes, 0, bytes.length);
    }

    private String bufferToHex(byte bytes[], int m, int n) {
        StringBuffer stringbuffer = new StringBuffer(2 * n);
        int k = m + n;
        for (int l = m; l < k; l++) {
            appendHexPair(bytes[l], stringbuffer);
        }
        return stringbuffer.toString();
    }

    private void appendHexPair(byte bt, StringBuffer stringbuffer) {
        char c0 = hexDigits[(bt & 0xf0) >> 4];// 取字節中高 4 位的數字轉換
        // 為邏輯右移，將符號位一起右移,此處未發現兩種符號有何不同
        char c1 = hexDigits[bt & 0xf];// 取字節中低 4 位的數字轉換
        stringbuffer.append(c0);
        stringbuffer.append(c1);
    }

}