自己写的一个base64转换用途Java程序

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.StringWriter;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

public class MyBase64 {
private static final char[] set = ".0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz"
.toCharArray();
/*
* 结尾表示填充0x0字符数目
*/
static final char blank = '-';
/*
* 间隔三个字符分段
*/
static final int step = 3;
/*
* 源数组结尾填0
*/
static final byte fill = 0x0;

private static MyBase64A myBase64 = null;

public static MyBase64A getInstance() {
if (myBase64 == null) {
synchronized (MyBase64A.class) {
if (myBase64 == null) {
myBase64 = new MyBase64A();
}
}
}
return myBase64;
// return threadLocal.get();
}

private static Map<Character, Integer> initMap() {
Map<Character, Integer> hashMap = new HashMap<Character, Integer>(
set.length);
for (int i = 0; i < set.length; i++) {
hashMap.put(set[i], i);
}
return hashMap;
}

private final Map<Character, Integer> map;

private MyBase64A() {
map = initMap();
}

public byte[] reverseTrans(String string) throws IOException {
char[] chars = string.toCharArray();

final int step = 4;

int approximation = chars.length * 3 / 4;
ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(
approximation);

long swap;
char c1, c2, c3, c4;
byte b1, b2, b3;

int begin = 0;
int end = chars.length;

int offset = begin;
while (offset + 3 < end) {
c1 = chars[offset];
c2 = chars[offset + 1];
c3 = chars[offset + 2];
c4 = chars[offset + 3];

swap = (map.get(c1) & 0xff) << 18 | (map.get(c2) & 0xff) << 12
| (map.get(c3) & 0xff) << 6 | (map.get(c4) & 0xff);

b1 = (byte) ((swap >> 16) & 0xff);
b2 = (byte) ((swap >> 8) & 0xff);
b3 = (byte) (swap & 0xff);

byteArrayOutputStream.write(b1);
byteArrayOutputStream.write(b2);
byteArrayOutputStream.write(b3);

offset += step;
}
// logger.debug(end - offset);
byte[] bytes = Arrays.copyOf(byteArrayOutputStream.toByteArray(),
byteArrayOutputStream.size() - (end - offset));
return bytes;
}

/**
*
* @param source
*            总之是一个字节队列。
* @return url相容的 自制 base64编码
*/
public String trans(byte[] source) {
int begin = 0, end = source.length;
/*
* (source.length + 2) / 3 处以三，余数上限取整。 + 2 ， 可能追加至多两个填充占位符。
*/
int approximation = (source.length + 2) / 3 * 4 + 2;
StringWriter stringWriter = new StringWriter(approximation);

int offset;
long swap;
byte a, b, c;
int num1, num2, num3, num4;
int app = 0;

offset = begin;
while (offset < end) {
a = source[offset];
if (offset + 1 < end) {
b = source[offset + 1];
} else {
b = fill;
app++;
}
if (offset + 2 < end) {
c = source[offset + 2];
} else {
c = fill;
app++;
}

swap = ((a & 0xff) << 16) | ((b & 0xff) << 8) | c & 0xff;

// logger.debug(String.format("%x", swap));

num1 = (int) ((swap >> 18) & 63);
num2 = (int) ((swap >> 12) & 63);
num3 = (int) ((swap >> 6) & 63);
num4 = (int) (swap & 63);

stringWriter.append(set[num1]);
stringWriter.append(set[num2]);
stringWriter.append(set[num3]);
stringWriter.append(set[num4]);

offset += step;
}
if (app == 1) {
stringWriter.append(blank);
}
if (app == 2) {
stringWriter.append(blank);
stringWriter.append(blank);
}
return stringWriter.toString();
}
}