io - Java的DataInputStream的readUTF方法是怎么读取字符串的？？？

Answer 1

DataOutputStream

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];
    }

    // 将字符串的字节长度写入流中
    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);
    // 写入的长度在字符串中增加了2，即字节长度标识所占用的资源
    return utflen + 2;
}

DataInputStream

public final static String readUTF(DataInput in) throws IOException {
    // 读取字符串字节长度
    int utflen = in.readUnsignedShort();
    //...
}

In fact, when calling writeUTF写入时jdk, the number of bytes of the string is written into the stream. When reading, the byte length is first read, and the corresponding string is read according to the specified byte length.

Answer 2

看源码，调用的第一句就获取了长度
int utflen = in.readUnsignedShort();
这个方法的Doc ：

Reads two input bytes and returns an int value in the range 0 through

1. Let a be the first byte read and b be the second byte. The value returned is:

(((a & 0xff) << 8) | (b & 0xff)) This method is suitable for reading
the bytes written by the writeShort method of interface DataOutput if
the argument to writeShort was intended to be a value in the range 0
through 65535. Returns: the unsigned 16-bit value read. Throws:
EOFException - if this stream reaches the end before reading all the

1. IOException - if an I/O error occurs.

readUTF的Doc：

Reads from the stream in a representation of a Unicode character
string encoded in modified UTF-8 format; this string of characters is
then returned as a String. The details of the modified UTF-8
representation are exactly the same as for the readUTF method of
DataInput.