求解释下列md5算法中的相关代码

Question

本人一见md5/sha/rsa等加密算法就头疼, 在app自动更新时, 需要对apk进行md5校验, 因此在网上弄了一段代码, 进过本人改写后, 如下. 但是不是很明白其中toHexString方法的意思. 因此求大神解释一下toHexString方法的含意, 以及分析整个类, 为何toHexString方法要这样写 ?

 package com.example;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.security.MessageDigest;


public final class MD5Checksum {

    private static final char HEX_DIGITS[] =
            {
                    '0', '1', '2', '3', '4',
                    '5', '6', '7', '8', '9',
                    'a', 'b', 'c', 'd', 'e', 'f'
            };

    public static String toHexString(byte[] b) {
        StringBuilder sb = new StringBuilder(b.length * 2);
        for (int i = 0; i < b.length; i++) {
            sb.append(HEX_DIGITS[(b[i] & 0xf0) >>> 4]);
            sb.append(HEX_DIGITS[b[i] & 0x0f]);
        }
        return sb.toString();
    }

    public static String md5sum(String absoluteFilePath) {
        InputStream inputStream = null;
        ByteArrayOutputStream outputStream = null;

        byte[] buffer = new byte[1024];
        int readedLength;
        MessageDigest md5;
        try{
            inputStream = new FileInputStream(absoluteFilePath);
            outputStream = new ByteArrayOutputStream();
            md5 = MessageDigest.getInstance("MD5");
            while((readedLength = inputStream.read(buffer)) > 0) {
                outputStream.write(buffer, 0, readedLength);
            }
            byte[] bytes = outputStream.toByteArray();
            md5.update(bytes, 0, bytes.length);
            return toHexString(md5.digest());
        } catch (Exception e) {
            System.out.println("get file \"" + absoluteFilePath+ "\" md5 failed !");
            return null;
        } finally {
            try {
                if(outputStream != null)  outputStream.close();
                if(inputStream != null)  inputStream.close();
            } catch (Exception e) {}
        }
    }

    public static void main(String[] args) {
        System.out.println(md5sum(new File("java-demo2/demo.json").getAbsolutePath()));
    }
}

Answer 1

toHexString将字节转换为字符串。
比如字节数组是 { 0x10, 0x00, 0xf1, 0x3d, ... }
结果就是 1000f13d...