以下是DigestUtils源码
package org.springframework.util;
import java.io.IOException;
import java.io.InputStream;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public abstract class DigestUtils {
    private static final String MD5_ALGORITHM_NAME = "MD5";
    private static final char[] HEX_CHARS = new char[]{'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
    public DigestUtils() {
    }
    public static byte[] md5Digest(byte[] bytes) {
        return digest("MD5", bytes);
    }
    public static byte[] md5Digest(InputStream inputStream) throws IOException {
        return digest("MD5", inputStream);
    }
    public static String md5DigestAsHex(byte[] bytes) {
        return digestAsHexString("MD5", bytes);
    }
    public static String md5DigestAsHex(InputStream inputStream) throws IOException {
        return digestAsHexString("MD5", inputStream);
    }
    public static StringBuilder appendMd5DigestAsHex(byte[] bytes, StringBuilder builder) {
        return appendDigestAsHex("MD5", bytes, builder);
    }
    public static StringBuilder appendMd5DigestAsHex(InputStream inputStream, StringBuilder builder) throws IOException {
        return appendDigestAsHex("MD5", inputStream, builder);
    }
    private static MessageDigest getDigest(String algorithm) {
        try {
            return MessageDigest.getInstance(algorithm);
        } catch (NoSuchAlgorithmException var2) {
            throw new IllegalStateException("Could not find MessageDigest with algorithm \"" + algorithm + "\"", var2);
        }
    }
    private static byte[] digest(String algorithm, byte[] bytes) {
        return getDigest(algorithm).digest(bytes);
    }
    private static byte[] digest(String algorithm, InputStream inputStream) throws IOException {
        MessageDigest messageDigest = getDigest(algorithm);
        if (inputStream instanceof UpdateMessageDigestInputStream) {
            ((UpdateMessageDigestInputStream)inputStream).updateMessageDigest(messageDigest);
            return messageDigest.digest();
        } else {
            byte[] buffer = new byte[4096];
            boolean var4 = true;
            int bytesRead;
            while((bytesRead = inputStream.read(buffer)) != -1) {
                messageDigest.update(buffer, 0, bytesRead);
            }
            return messageDigest.digest();
        }
    }
    private static String digestAsHexString(String algorithm, byte[] bytes) {
        char[] hexDigest = digestAsHexChars(algorithm, bytes);
        return new String(hexDigest);
    }
    private static String digestAsHexString(String algorithm, InputStream inputStream) throws IOException {
        char[] hexDigest = digestAsHexChars(algorithm, inputStream);
        return new String(hexDigest);
    }
    private static StringBuilder appendDigestAsHex(String algorithm, byte[] bytes, StringBuilder builder) {
        char[] hexDigest = digestAsHexChars(algorithm, bytes);
        return builder.append(hexDigest);
    }
    private static StringBuilder appendDigestAsHex(String algorithm, InputStream inputStream, StringBuilder builder) throws IOException {
        char[] hexDigest = digestAsHexChars(algorithm, inputStream);
        return builder.append(hexDigest);
    }
    private static char[] digestAsHexChars(String algorithm, byte[] bytes) {
        byte[] digest = digest(algorithm, bytes);
        return encodeHex(digest);
    }
    private static char[] digestAsHexChars(String algorithm, InputStream inputStream) throws IOException {
        byte[] digest = digest(algorithm, inputStream);
        return encodeHex(digest);
    }
    private static char[] encodeHex(byte[] bytes) {
        char[] chars = new char[32];
        for(int i = 0; i < chars.length; i += 2) {
            byte b = bytes[i / 2];
            chars[i] = HEX_CHARS[b >>> 4 & 15];
            chars[i + 1] = HEX_CHARS[b & 15];
        }
        return chars;
    }
}