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();
    //...
}

实际上在调用writeUTF写入时jdk内部有将字符串的字节数写入流中，读取时先读取到字节长度，按照指定的字节长度读取出相应的字符串。

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.