不借助工具手动按原理实现图片转BASE64字符串编码

//把有符号转成无符号
public static int convertBinaryUnsigned(byte b){
return ((int)b) & 0XFF;
}

/**
* 把三个字节按6位一个字节转成四个字节
* @param bytes
* @return
*/
public static int[] convertBinaryBase64(byte[] bytes){
int[] result = new int[4];

if(bytes.length==3){
byte a = bytes[0];
byte b = bytes[1];
byte c = bytes[2];
byte a_1 = (byte) (a & 0XFC);//取左六位，0XFC用计算器得出，下同
int out1 = (byte) (convertBinaryUnsigned(a_1) >> 2);//转成无符号数字
result[0] = out1;

byte a_2 = (byte) ((a & 0X3) << 6);//取右两位
byte b_1 =  (byte) ((b & 0XF0) >> 2);//取第二个字符左四位
int out2 = (convertBinaryUnsigned((byte) (a_2 | b_1)) >> 2);//得出的两位与四位组合，生成新的base64数字编码，下面基本大同小异
result[1] = out2;

byte b_2 = (byte) ((b & 0XF) << 4);
byte c_1 = (byte) ((c & 0XC0) >>4);
int out3 = (convertBinaryUnsigned((byte) (b_2 | c_1)) >> 2);
result[2] = out3;

byte c_2 = (byte) (c & 0X3F);
byte out4 = c_2;
result[3] = out4;
}else if(bytes.length==1){
byte a = bytes[0];
byte a_1 = (byte) (a & 0XFC);
int out1 = (byte) (convertBinaryUnsigned(a_1) >> 2);
result[0] = out1;

byte a_2 = (byte) ((a & 0X3) << 6);
int out2 = (convertBinaryUnsigned((byte) (a_2)) >> 2);
result[1] = out2;

result[2] = 64;
result[3] = 64;
}else if(bytes.length==2){
byte a = bytes[0];
byte b = bytes[1];
byte a_1 = (byte) (a & 0XFC);
int out1 = (byte) (convertBinaryUnsigned(a_1) >> 2);
result[0] = out1;

byte a_2 = (byte) ((a & 0X3) << 6);
byte b_1 =  (byte) ((b & 0XF0) >> 2);
int out2 = (convertBinaryUnsigned((byte) (a_2 | b_1)) >> 2);
result[1] = out2;

byte b_2 = (byte) ((b & 0XF) << 4);
int out3 = (convertBinaryUnsigned((byte) (b_2)) >> 2);
result[2] = out3;
result[3] = 64;
}
return result;
}

/**
*这个方法是为了把base64编码转成图片或二进制数据准备的
*/
public static int indexOfBase64(char c){
int result = -1;
for (int i = 0; i < BASE_64_STR.length; i++) {
if(c == BASE_64_STR[i]){
result = i;
break;
}
}
return result;
}

    /**
* byte数组转base64字符串的方法
* @param bytes
* @return
*/
public static String convertBase64StrForBytes(byte[] bytes){
int remainder = bytes.length % 3;

StringBuffer buffer = new StringBuffer();
for(int i=0;i<bytes.length-2;i+=3){
int[] temp = convertBinaryBase64(new byte[]{bytes[i],bytes[i+1],bytes[i+2]});
for (int j : temp) {
buffer.append(BASE_64_STR[j]);
}

}

if(remainder!=0){
int doFor = (bytes.length - remainder) / 3;
int index = doFor * 3;
int[] end = null;
if(bytes.length-index==1){
end = convertBinaryBase64(new byte[]{bytes[index]});
}else if(bytes.length-index==2){
end = convertBinaryBase64(new byte[]{bytes[index],bytes[index+1]});
}
for (int i : end) {
buffer.append(BASE_64_STR[i]);
}
}

return buffer.toString();

}


/**
*base64编码库
*/
public static final char[] BASE_64_STR = {
'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',
'+','/','='
};


/**
*最后读图片文件
*/
public static void main(String[] args) throws Exception {
File f = new File("D:/a1.jpg");
InputStream in = new FileInputStream(new File("D:/a1.jpg"));
int length = (int) f.length();
byte[] a = new byte[length];
in.read(a);
/*
for (int i = 0; i < 3; i++) {
System.out.println(a[i]);
}
*/
String str = convertBase64StrForBytes(a);
System.out.println(str);
}


控制台输出



复制字符串到解析的网站去验证，注意，有的网站解析图片前面要加上“data:image/jpeg;base64,”字符串，后面再跟着base字符串。html的image标签src也要加这个字符串，只要在解析出来后把这个字符串加到编码的前面就可以了。效果图



如果能不借助工具自己手写实现图片转base64编码，同样也能写出base64编码转图片。我就懒得写了