Java傳輸檔案使用Base64優化傳輸速率。

問題：使用http進行檔案傳輸時，將檔案的byte[]轉為json佔用的記憶體空間會增加三倍左右。
解決：將byte[]陣列通過Base64轉成String型別，上傳String型別，伺服器端接收到String之後再通過Base64解密成byte[]，最後儲存為檔案。

//使用者端
String base64Str = Base64.getEncoder().encodeToString(byteArray);
//伺服器端
byte [] byteArray = Base64.getDecoder().decode(base64Str);

如果使用者端是android應用，太低的android系統版本不支援自帶的Base64。可以使用以下程式碼。所有版本都適配。

package com.eryuan.util;

import android.text.TextUtils;

import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;

public class Base64 {
    private static final char last2byte = (char) Integer
            .parseInt("00000011", 2);
    private static final char last4byte = (char) Integer
            .parseInt("00001111", 2);
    private static final char last6byte = (char) Integer
            .parseInt("00111111", 2);
    private static final char lead6byte = (char) Integer
            .parseInt("11111100", 2);
    private static final char lead4byte = (char) Integer
            .parseInt("11110000", 2);
    private static final char lead2byte = (char) Integer
            .parseInt("11000000", 2);
    private static final char[] encodeTable = new char[]{'A', 'B', 'C', 'D',
            'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q',
            'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd',
            'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q',
            'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3',
            '4', '5', '6', '7', '8', '9', '+', '/'};

    /**
     * base64加密
     *
     * @param from
     * @return
     */
    public static String encode(byte[] from) {
        StringBuffer to = new StringBuffer((int) (from.length * 1.34) + 3);
        int num = 0;
        char currentByte = 0;
        for (int i = 0; i < from.length; i++) {
            num = num % 8;
            while (num < 8) {
                switch (num) {
                    case 0:
                        currentByte = (char) (from[i] & lead6byte);
                        currentByte = (char) (currentByte >>> 2);
                        break;
                    case 2:
                        currentByte = (char) (from[i] & last6byte);
                        break;
                    case 4:
                        currentByte = (char) (from[i] & last4byte);
                        currentByte = (char) (currentByte << 2);
                        if ((i + 1) < from.length) {
                            currentByte |= (from[i + 1] & lead2byte) >>> 6;
                        }
                        break;
                    case 6:
                        currentByte = (char) (from[i] & last2byte);
                        currentByte = (char) (currentByte << 4);
                        if ((i + 1) < from.length) {
                            currentByte |= (from[i + 1] & lead4byte) >>> 4;
                        }
                        break;
                }
                to.append(encodeTable[currentByte]);
                num += 6;
            }
        }
        if (to.length() % 4 != 0) {
            for (int i = 4 - to.length() % 4; i > 0; i--) {
                to.append("=");
            }
        }
        return to.toString();
    }

    /**
     * md5加密
     *
     * @param string
     * @return
     */
    public static String md5(String string) {
        if (TextUtils.isEmpty(string)) {
            return "";
        }
        MessageDigest md5 = null;
        try {
            md5 = MessageDigest.getInstance("MD5");
            byte[] bytes = md5.digest(string.getBytes());
            String result = "";
            for (byte b : bytes) {
                String temp = Integer.toHexString(b & 0xff);
                if (temp.length() == 1) {
                    temp = "0" + temp;
                }
                result += temp;
            }
            return result;
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
        return "";
    }

}