Java:如何为文件创建SHA-1？

Question

在纯Java6中为非常大的文件创建SHA-1的最佳方法是什么？如何实现此方法:

public abstract String createSha1(java.io.File file);

Answer 1

MessageDigest逐个使用类和提供数据.下面的示例忽略了将byte []转换为字符串并关闭文件等细节,但应该给出一般的想法.

public byte[] createSha1(File file) throws Exception  {
    MessageDigest digest = MessageDigest.getInstance("SHA-1");
    InputStream fis = new FileInputStream(file);
    int n = 0;
    byte[] buffer = new byte[8192];
    while (n != -1) {
        n = fis.read(buffer);
        if (n > 0) {
            digest.update(buffer, 0, n);
        }
    }
    return digest.digest();
}

Answer 2

操作系统要求函数返回SHA1的字符串,所以我接受了@jeffs的答案,并将缺少的转换添加到String:

/**
 * Read the file and calculate the SHA-1 checksum
 * 
 * @param file
 *            the file to read
 * @return the hex representation of the SHA-1 using uppercase chars
 * @throws FileNotFoundException
 *             if the file does not exist, is a directory rather than a
 *             regular file, or for some other reason cannot be opened for
 *             reading
 * @throws IOException
 *             if an I/O error occurs
 * @throws NoSuchAlgorithmException
 *             should never happen
 */
private static String calcSHA1(File file) throws FileNotFoundException,
        IOException, NoSuchAlgorithmException {

    MessageDigest sha1 = MessageDigest.getInstance("SHA-1");
    try (InputStream input = new FileInputStream(file)) {

        byte[] buffer = new byte[8192];
        int len = input.read(buffer);

        while (len != -1) {
            sha1.update(buffer, 0, len);
            len = input.read(buffer);
        }

        return new HexBinaryAdapter().marshal(sha1.digest());
    }
}