java.io.DataOutputStream 中的 writeUTF

Question

I know that a symbol with utf8 needs 1-4 bytes in Java. But when I used the method readUTF/writeUTF in java.io.DataInputStream/DataOutputStream, I found that the method just solve the situation about a symbol needs 1-3 bytes.

static int writeUTF(String str, DataOutput out) throws IOException {
    int strlen = str.length();
    int utflen = 0;
    int c, count = 0;

   /* use charAt instead of copying String to char array */
    for (int i = 0; i < strlen; i++) {
        c = str.charAt(i);
        if ((c >= 0x0001) && (c <= 0x007F)) {
            utflen++;
        } else if (c > 0x07FF) {
            utflen += 3;
        } else {
            utflen += 2;
        }
    }

    if (utflen > 65535)
        throw new UTFDataFormatException(
            "encoded string too long: " + utflen + " bytes");>

    byte[] bytearr = null;
    if (out instanceof DataOutputStream) {
        DataOutputStream dos = (DataOutputStream)out;
        if(dos.bytearr == null || (dos.bytearr.length < (utflen+2)))
            dos.bytearr = new byte[(utflen*2) + 2];
        bytearr = dos.bytearr;
    } else {
        bytearr = new byte[utflen+2];
    }

    bytearr[count++] = (byte) ((utflen >>> 8) & 0xFF);
    bytearr[count++] = (byte) ((utflen >>> 0) & 0xFF);

    int i=0;
    for (i=0; i<strlen; i++) {
       c = str.charAt(i);
       if (!((c >= 0x0001) && (c <= 0x007F))) break;
       bytearr[count++] = (byte) c;
    }

    for (;i < strlen; i++){
        c = str.charAt(i);
        if ((c >= 0x0001) && (c <= 0x007F)) {
            bytearr[count++] = (byte) c;

        } else if (c > 0x07FF) {
            bytearr[count++] = (byte) (0xE0 | ((c >> 12) & 0x0F));
            bytearr[count++] = (byte) (0x80 | ((c >>  6) & 0x3F));
            bytearr[count++] = (byte) (0x80 | ((c >>  0) & 0x3F));
        } else {
            bytearr[count++] = (byte) (0xC0 | ((c >>  6) & 0x1F));
            bytearr[count++] = (byte) (0x80 | ((c >>  0) & 0x3F));
        }
    }
    out.write(bytearr, 0, utflen+2);
    return utflen + 2;
}

Why not solve the situation when a symbol needs 4 bytes?

Answer 1

It's all explained in the docs, though you have to go through an extra click.

The docs for DataOutputStream#writeUTF mentions that it uses a " modified UTF-8 encoding." That link is in the original JavaDocs (I didn't just add it for this answer), and if you follow it, you get a page explaining that encoding. Note specifically the part near the bottom of the summary (before you get into the method summary section):

The differences between this format and the standard UTF-8 format are the following:

...

• Only the 1-byte, 2-byte, and 3-byte formats are used.

So, while you're right in thinking that UTF-8 uses up to 4 bytes, writeUTF uses a modified version, and one of the modifications is that it only supports up to 3 bytes.