Encode byte[] as String

问题:

Heyho,

I want to convert byte data, which can be anything, to a String. My question is, whether it is "secure" to encode the byte data with UTF-8 for example:

String s1 = new String(data, "UTF-8");

or by using base64:

String s2 = Base64.encodeToString(data, false); //migbase64

I'm just afraid that using the first method has negative side effects. I mean both variants work p̶e̶r̶f̶e̶c̶t̶l̶y̶ , but s1 can contain any character of the UTF-8 charset, s2 only uses "readable" characters. I'm just not sure if it's really need to use base64. Basically I just need to create a String send it over the network and receive it again. (There is no other way in my situation :/)

The question is only about negative side effects, not if it's possible!

回答1:

You should absolutely use base64 or possibly hex. (Either will work; base64 is more compact but harder for humans to read.)

You claim "both variants work perfectly" but that's actually not true. If you use the first approach and data is not actually a valid UTF-8 sequence, you will lose data. You're not trying to convert UTF-8-encoded text into a String, so don't write code which tries to do so.

Using ISO-8859-1 as an encoding will preserve all the data - but in very many cases the string that is returned will not be easily transported across other protocols. It may very well contain unprintable control characters, for example.

Only use the String(byte[], String) constructor when you've got inherently textual data, which you happen to have in an encoded form (where the encoding is specified as the second argument). For anything else - music, video, images, encrypted or compressed data, just for example - you should use an approach which treats the incoming data as "arbitrary binary data" and finds a textual encoding of it... which is precisely what base64 and hex do.

回答2:

You can store a byte in a String, though it's not a good idea. You can't use UTF-8 as this will mange the bytes but a faster and more efficient way is to use ISO-8859-1 encoding or plain 8-bit. The simplest way to do this is to use

String s1 = new String(data, 0);

or

String s1 = new String(data, "ISO-8859-1");

From UTF-8 on Wikipedia, As Jon Skeet notes, these encodings are not valid under the standard. Their behaviour in Java varies. DataInputStream treats them as the same for the first three version and the next two throw an exception. The Charset decoder treats them as separate characters silently.

00000000 is \0
11000000 10000000 is \0
11100000 10000000 10000000 is \0
11110000 10000000 10000000 10000000 is \0
11111000 10000000 10000000 10000000 10000000 is \0
11111100 10000000 10000000 10000000 10000000 10000000 is \0

This means if you see \0 in you String, you have no way of knowing for sure what the original byte[] values were. DataOutputStream uses the second option for compatibility with C which sees \0 as a terminator.

BTW DataOutputStream is not aware of code points so writes high code point characters in UTF-16 and then UTF-8 encoding.

0xFE and 0xFF are not valid to appear in a character. Values 0x11000000+ can only appear at the start of a character, not inside a multi-byte character.

回答3:

Confirmed the accepted answer with Java. To repeat, UTF-8, UTF-16 do not preserve all the byte values. ISO-8859-1 does preserve all the byte values. But if the encoded bytes is to be transported beyond the JVM, use Base64.

@Test
public void testBase64() {
    final byte[] original = enumerate();
    final String encoded = Base64.encodeBase64String( original );
    final byte[] decoded = Base64.decodeBase64( encoded );
    assertTrue( "Base64 preserves bytes", Arrays.equals( original, decoded ) );
}

@Test
public void testIso8859() {
    final byte[] original = enumerate();
    String s = new String( original, StandardCharsets.ISO_8859_1 );
    final byte[] decoded = s.getBytes( StandardCharsets.ISO_8859_1 );
    assertTrue( "ISO-8859-1 preserves bytes", Arrays.equals( original, decoded ) );
}

@Test
public void testUtf16() {
    final byte[] original = enumerate();
    String s = new String( original, StandardCharsets.UTF_16 );
    final byte[] decoded = s.getBytes( StandardCharsets.UTF_16 );
    assertFalse( "UTF-16 does not preserve bytes", Arrays.equals( original, decoded ) );
}

@Test
public void testUtf8() {
    final byte[] original = enumerate();
    String s = new String( original, StandardCharsets.UTF_8 );
    final byte[] decoded = s.getBytes( StandardCharsets.UTF_8 );
    assertFalse( "UTF-8 does not preserve bytes", Arrays.equals( original, decoded ) );
}

@Test
public void testEnumerate() {
    final Set<Byte> byteSet = new HashSet<>();
    final byte[] bytes = enumerate();
    for ( byte b : bytes ) {
        byteSet.add( b );
    }
    assertEquals( "Expecting 256 distinct values of byte.", 256, byteSet.size() );
}

/**
 * Enumerates all the byte values.
 */
private byte[] enumerate() {
    final int length = Byte.MAX_VALUE - Byte.MIN_VALUE + 1;
    final byte[] bytes = new byte[length];
    for ( int i = 0; i < length; i++ ) {
        bytes[i] = (byte)(i + Byte.MIN_VALUE);
    }
    return bytes;
}