public static String toHexString(byte[] bytes) {

    StringBuilder sb = new StringBuilder();

    if (bytes != null) 
        for (byte b:bytes) {

            final String hexString = Integer.toHexString(b & 0xff);

            if(hexString.length()==1)
                sb.append('0');

            sb.append(hexString);//.append(' ');
        }

      return sb.toString();//.toUpperCase();
}

使用DatatypeConverter:

public String toHexString(byte... bytes) {

    return Optional.ofNullable(bytes)
            .filter(bs->bs.length>0)
            .map(DatatypeConverter::printHexBinary)
            .map(str->IntStream.range(0, str.length())
                    .filter(i->(i%2)==0)        // take every second index
                    .mapToObj(i->"0x" + str.substring(i, i+2))
                    .collect(Collectors.joining(" ")))
            .orElse("");
}

Java 17最终包含HexFormat类，所以你可以简单地做:

HexFormat.of().formatHex(bytes);

它支持配置为小写/大写，分隔符，前缀，后缀等。

@maybewecouldstealavan提出的解决方案的一个小变种，它让你在输出十六进制字符串中可视化地捆绑N个字节:

 final static char[] HEX_ARRAY = "0123456789ABCDEF".toCharArray();
 final static char BUNDLE_SEP = ' ';

public static String bytesToHexString(byte[] bytes, int bundleSize /*[bytes]*/]) {
        char[] hexChars = new char[(bytes.length * 2) + (bytes.length / bundleSize)];
        for (int j = 0, k = 1; j < bytes.length; j++, k++) {
                int v = bytes[j] & 0xFF;
                int start = (j * 2) + j/bundleSize;

                hexChars[start] = HEX_ARRAY[v >>> 4];
                hexChars[start + 1] = HEX_ARRAY[v & 0x0F];

                if ((k % bundleSize) == 0) {
                        hexChars[start + 2] = BUNDLE_SEP;
                }   
        }   
        return new String(hexChars).trim();    
}

那就是:

bytesToHexString("..DOOM..".toCharArray().getBytes(), 2);
2E2E 444F 4F4D 2E2E

bytesToHexString("..DOOM..".toCharArray().getBytes(), 4);
2E2E444F 4F4D2E2E

Converts bytes data to hex characters

@param bytes byte array to be converted to hex string
@return byte String in hex format

private static String bytesToHex(byte[] bytes) {
    char[] hexChars = new char[bytes.length * 2];
    int v;
    for (int j = 0; j < bytes.length; j++) {
        v = bytes[j] & 0xFF;
        hexChars[j * 2] = HEX_ARRAY[v >>> 4];
        hexChars[j * 2 + 1] = HEX_ARRAY[v & 0x0F];
    }
    return new String(hexChars);
}

我发现了三种不同的方法: http://www.rgagnon.com/javadetails/java-0596.html

他还提到，我认为最优雅的一个是:

static final String HEXES = "0123456789ABCDEF";
public static String getHex( byte [] raw ) {
    if ( raw == null ) {
        return null;
    }
    final StringBuilder hex = new StringBuilder( 2 * raw.length );
    for ( final byte b : raw ) {
        hex.append(HEXES.charAt((b & 0xF0) >> 4))
            .append(HEXES.charAt((b & 0x0F)));
    }
    return hex.toString();
}

对于固定长度，我会使用这样的东西，比如哈希值:

md5sum = String.format("%032x", new BigInteger(1, md.digest()));