Java 获取文件的md5

public static String getMd5ByFile(File file) {
String value = null;
try {
FileInputStream in = new FileInputStream(file);
MappedByteBuffer byteBuffer = in.getChannel().map(FileChannel.MapMode.READ_ONLY, 0, file.length());
MessageDigest md5 = MessageDigest.getInstance("MD5");
md5.update(byteBuffer);
BigInteger bi = new BigInteger(1, md5.digest());
value = bi.toString(16);
//value.toUpperCase();将英文全部转化为大写
if (in != null) {
in.close();
byteBuffer.clear();
}
} catch (Exception e) {
e.printStackTrace();
}
return value;
}

开始用的上述方法，基本能够正常使用，但是久了就发现一个问题，BigInteger bi = new BigInteger(1, md5.digest());

若md5值开头为数字 0 的话，转换后会丢掉0，得到的是一个31位的md5值，网上找了另一种方法，不过太复杂了，整理了下，解决了上述问题

public static String getFileMD5String(File file){
StringBuffer stringbuffer=null;
try {
char hexDigits[] = { '0', '1', '2', '3', '4', '5', '6','7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };
FileInputStream in = new FileInputStream(file);
FileChannel ch = in.getChannel();
MappedByteBuffer byteBuffer = ch.map(FileChannel.MapMode.READ_ONLY, 0,file.length());
MessageDigest messagedigest = MessageDigest.getInstance("MD5");
messagedigest.update(byteBuffer);
byte[] bytes=messagedigest.digest();
int n=bytes.length;
stringbuffer = new StringBuffer(2 * n);
for (int l = 0; l < n; l++) {
byte bt=bytes[l];
char c0 = hexDigits[(bt & 0xf0) >> 4];
char c1 = hexDigits[bt & 0xf];
stringbuffer.append(c0);
stringbuffer.append(c1);
}
} catch (Exception e) {
e.printStackTrace();
}
return stringbuffer.toString();
}