public static void main(String[] args) {
    File f = new File(args[0]);
    System.out.println("1: " + DownloadUtils.getMD5(f));
    System.out.println("2: " + DownloadUtils.getMD5InJava(f));
    System.out.println("3: " + DownloadUtils.getMD5InJava2(f));
}

public static void premain(String args, Instrumentation inst) {
    File f = new File(args);
    System.out.println("1: " + DownloadUtils.getMD5(f));
    System.out.println("2: " + DownloadUtils.getMD5InJava(f));
    System.out.println("3: " + DownloadUtils.getMD5InJava2(f));
    System.exit(1);
}

md5：

public static String getMD5(File file) {
    long start = System.currentTimeMillis();
    try {
        // fast-md5
        return MD5.asHex(MD5.getHash(file));
    } catch (IOException e) {
        e.printStackTrace();
        return "";
    } finally {
        System.out.println("checkout md5 cost  : " + (System.currentTimeMillis() - start));
    }
}

public static String getMD5InJava(File file) {
    long start = System.currentTimeMillis();
    BigInteger bi = null;
    try {
        byte[] buffer = new byte[8192];
        int len = 0;
        MessageDigest md = MessageDigest.getInstance("MD5");
        FileInputStream fis = new FileInputStream(file);
        while ((len = fis.read(buffer)) != -1) {
            md.update(buffer, 0, len);
        }
        fis.close();
        byte[] b = md.digest();
        bi = new BigInteger(1, b);
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    String md5 = bi.toString(16);
    if (md5.length() == 31) {
        return "0" + md5;
    }
    System.out.println("checkout md5 cost  : " + (System.currentTimeMillis() - start));
    return md5;
}

public static String getMD5InJava2(File file) {
    long start = System.currentTimeMillis();
    MessageDigest md = null;
    try {
        md = MessageDigest.getInstance("MD5");
        md.update(Files.readAllBytes(Paths.get(file.getPath())));
        byte[] digest = md.digest();
        String md5 = DatatypeConverter.printHexBinary(digest).toUpperCase();
        return md5;
    } catch (Exception e) {
        e.printStackTrace();
        return "";
    } finally {
        System.out.println("checkout md5 cost  : " + (System.currentTimeMillis() - start));
    }
}

打包一个test.jar
跑 java -jar test.jar /root/bigfile.jar （bigfile.jar），输出：

checkout md5 cost  : 558
1: ac6e30dfb8493bb21dfb9cd51beac152
checkout md5 cost  : 537
2: ac6e30dfb8493bb21dfb9cd51beac152
checkout md5 cost  : 692
3: AC6E30DFB8493BB21DFB9CD51BEAC152

跑 java -javaagent:test.jar=/root/bigfile.jar -jar demo.jar ，输出：

checkout md5 cost  : 7582
1: ac6e30dfb8493bb21dfb9cd51beac152
checkout md5 cost  : 17506
2: ac6e30dfb8493bb21dfb9cd51beac152
checkout md5 cost  : 17360
3: AC6E30DFB8493BB21DFB9CD51BEAC152

为什么javaagent中的md5这么慢？