关于DataInputStream中readUTF()的返回值疑问？？？

Question

BufferedReader类中的方法readLine()为读取一行字符串，DataInputStream类中的方法readUTF()的返回值为String,也为字符串，那readUTF()读取的字符串到底能有多长呀？

Answer 1

http://202.201.112.11/jpk/apply/teacher/preface/53/api/java/io/DataInput.html#readUTF()

Answer 2

/**
     * Reads from the
     * stream <code>in</code> a representation
     * of a Unicode  character string encoded in
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a> format;
     * this string of characters is then returned as a <code>String</code>.
     * The details of the modified UTF-8 representation
     * are  exactly the same as for the <code>readUTF</code>
     * method of <code>DataInput</code>.
     *
     * @param      in   a data input stream.
     * @return     a Unicode string.
     * @exception  EOFException            if the input stream reaches the end
     *               before all the bytes.
     * @exception  IOException   the stream has been closed and the contained
     *     input stream does not support reading after close, or
     *     another I/O error occurs.
     * @exception  UTFDataFormatException  if the bytes do not represent a
     *               valid modified UTF-8 encoding of a Unicode string.
     * @see        java.io.DataInputStream#readUnsignedShort()
     */
    public final static String readUTF(DataInput in) throws IOException {
        int utflen = in.readUnsignedShort();
        byte[] bytearr = null;
        char[] chararr = null;
        if (in instanceof DataInputStream) {
            DataInputStream dis = (DataInputStream)in;
            if (dis.bytearr.length < utflen){
                dis.bytearr = new byte[utflen*2];
                dis.chararr = new char[utflen*2];
            }
            chararr = dis.chararr;
            bytearr = dis.bytearr;
        } else {
            bytearr = new byte[utflen];
            chararr = new char[utflen];
        }        int c, char2, char3;
        int count = 0;
        int chararr_count=0;        in.readFully(bytearr, 0, utflen);        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;      
            if (c > 127) break;
            count++;
            chararr[chararr_count++]=(char)c;
        }        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;
            switch (c >> 4) {
                case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
                    /* 0xxxxxxx*/
                    count++;
                    chararr[chararr_count++]=(char)c;
                    break;
                case 12: case 13:
                    /* 110x xxxx   10xx xxxx*/
                    count += 2;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-1];
                    if ((char2 & 0xC0) != 0x80)
                        throw new UTFDataFormatException(
                            "malformed input around byte " + count); 
                    chararr[chararr_count++]=(char)(((c & 0x1F) << 6) | 
                                                    (char2 & 0x3F));  
                    break;
                case 14:
                    /* 1110 xxxx  10xx xxxx  10xx xxxx */
                    count += 3;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-2];
                    char3 = (int) bytearr[count-1];
                    if (((char2 & 0xC0) != 0x80) || ((char3 & 0xC0) != 0x80))
                        throw new UTFDataFormatException(
                            "malformed input around byte " + (count-1));
                    chararr[chararr_count++]=(char)(((c     & 0x0F) << 12) |
                                                    ((char2 & 0x3F) << 6)  |
                                                    ((char3 & 0x3F) << 0));
                    break;
                default:
                    /* 10xx xxxx,  1111 xxxx */
                    throw new UTFDataFormatException(
                        "malformed input around byte " + count);
            }
        }
        // The number of chars produced may be less than utflen
        return new String(chararr, 0, chararr_count);
    }这是源码中readUTF最终掉的方法，LZ自己看看就明白了

Answer 3

简单来说readUTF()就是先读两字节，得到数据的长度，再读相应长度的数据，并用UTF-8解码成String

Answer 4

java　ａｐｉ里面的实现，可以看出是一次读两字节
public final static String readUTF(DataInput in) throws IOException {
        int utflen = in.readUnsignedShort();
        byte[] bytearr = null;
        char[] chararr = null;
        if (in instanceof DataInputStream) {
            DataInputStream dis = (DataInputStream)in;
            if (dis.bytearr.length < utflen){
                dis.bytearr = new byte[utflen*2];
                dis.chararr = new char[utflen*2];
            }
            chararr = dis.chararr;
            bytearr = dis.bytearr;
        } else {
            bytearr = new byte[utflen];
            chararr = new char[utflen];
        }        int c, char2, char3;
        int count = 0;
        int chararr_count=0;        in.readFully(bytearr, 0, utflen);        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;      
            if (c > 127) break;
            count++;
            chararr[chararr_count++]=(char)c;
        }        while (count < utflen) {
            c = (int) bytearr[count] & 0xff;
            switch (c >> 4) {
                case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7:
                    /* 0xxxxxxx*/
                    count++;
                    chararr[chararr_count++]=(char)c;
                    break;
                case 12: case 13:
                    /* 110x xxxx   10xx xxxx*/
                    count += 2;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-1];
                    if ((char2 & 0xC0) != 0x80)
                        throw new UTFDataFormatException(
                            "malformed input around byte " + count); 
                    chararr[chararr_count++]=(char)(((c & 0x1F) << 6) | 
                                                    (char2 & 0x3F));  
                    break;
                case 14:
                    /* 1110 xxxx  10xx xxxx  10xx xxxx */
                    count += 3;
                    if (count > utflen)
                        throw new UTFDataFormatException(
                            "malformed input: partial character at end");
                    char2 = (int) bytearr[count-2];
                    char3 = (int) bytearr[count-1];
                    if (((char2 & 0xC0) != 0x80) || ((char3 & 0xC0) != 0x80))
                        throw new UTFDataFormatException(
                            "malformed input around byte " + (count-1));
                    chararr[chararr_count++]=(char)(((c     & 0x0F) << 12) |
                                                    ((char2 & 0x3F) << 6)  |
                                                    ((char3 & 0x3F) << 0));
                    break;
                default:
                    /* 10xx xxxx,  1111 xxxx */
                    throw new UTFDataFormatException(
                        "malformed input around byte " + count);
            }
        }
        // The number of chars produced may be less than utflen
        return new String(chararr, 0, chararr_count);
    }

Answer 5

   读入一个已使用 UTF-8 修改版格式编码的字符串。readUTF 的常规协定是：该方法读取使用 UTF-8 修改版格式编码的 Unicode 字符串的表示形式；然后以 String 的形式返回此字符串。 
    首先读取两个字节，并使用它们构造一个无符号 16 位整数，构造方式与 readUnsignedShort 方法的方式完全相同。该整数值被称为 UTF 长度，它指定要读取的额外字节数。然后成组地将这些字节转换为字符。每组的长度根据该组第一个字节的值计算。紧跟在某个组后面的字节（如果有）是下一组的第一个字节。 

Answer 6

兄弟们，都挺专业呀，谢谢啦，O(∩_∩)O哈哈~
诚心希望各位Coder百尺竿头更进一步！！！！！！

Answer 7

 System.out.println("servlet:\n"+data_In.readUTF()); 
                                        tb = data_In.readUTF();
                                        System.out.println(tb);
                                        carId.setString(tb);
                                        System.out.println(carId.getString());
为什么tb什么也没有？输出语句木有反应