dataoutputstream 写字符串时, 为什么要写入字符串长度

[LinboLen]

java 里写入字符串bytes如下
https://github.com/openjdk-mirror/jdk/blob/jdk8u/jdk8u/master/src/share/classes/java/io/DataOutputStream.java#L273

 public final void writeBytes(String s) throws IOException {
        int len = s.length();
        for (int i = 0 ; i < len ; i++) {
            out.write((byte)s.charAt(i));
        }
        incCount(len);
    }

而js实现却写入了长度信息

proto._writeString = function(str) {
  // newString:
  //   TC_STRING newHandle (utf)
  //   TC_LONGSTRING newHandle (long-utf)

  var out = this.out;
  var bf = new Buffer(str);
  var len = bf.length;

  this._newHandle(str);

  if (len <= 0xffff) {
    out.put(cons.TC_STRING);
    out.putUInt16(len);
  } else {
    out.put(cons.TC_LONGSTRING);
    out.putInt64(len);
  }

  // out.putString always use long to represent string length
  out.put(bf);
}

[LinboLen]

我测试了一下
上面java 的writeBytes(String str)在多字节字符串中会只写入字符对应unicode的低八位. java中写入多字节字符串时会用
到writeChars

[LinboLen]

java dataoutputstream 中的 writeUTF写入的长度信息
https://github.com/openjdk-mirror/jdk/blob/jdk8u/jdk8u/master/src/share/classes/java/io/DataOutputStream.java#L346

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];
        }

       //写入两字节长度信息 高位在前, 低位在后
        bytearr[count++] = (byte) ((utflen >>> 8) & 0xFF);
        bytearr[count++] = (byte) ((utflen >>> 0) & 0xFF);

       //如下是unicode转utf-8
        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;
    }

用两个字节来存储长度, 并获取字符串的unicode再编码成utf-8字节码并写入outputstream

现在我开始困惑于java.io是否是用于完全实现java 中java.io中的部分包, 那么函数的相应行为签名是否要保持一致.

否则用java.io这个名字, 却与java中的java.io大相径庭是否合理.

[gxcsoccer]

能否提供相应的测试代码

[gxcsoccer]

java.io 并没有完整实现 java 原生序列化，只实现了我们用到的部分

[LinboLen]

这是模拟writeBytes

public static void test001() {
        String str = "这是一个字符串";
        byte[] bytes = new byte[str.length()];
        for (int index = 0; index < str.length(); index++) {
            char c = str.charAt(index);
            System.out.print("char: `" + c + "` hex: " + Integer.toString((int) c, 16) + " ");

            System.out.print(" | ");
            bytes[index] = (byte) str.charAt(index);
            String byteString = org.apache.commons.codec.binary.Hex.encodeHexString(new byte[]{bytes[index]});
            System.out.println("byte: `" + bytes[index] + "` hex: " + byteString + " ");

        }
        System.out.println(org.apache.commons.codec.binary.Hex.encodeHexString(bytes));
    }

result

char: 这 hex: 8fd9 | byte: -39 hex: d9
char: 是 hex: 662f | byte: 47 hex: 2f
char: 一 hex: 4e00 | byte: 0 hex: 00
char: 个 hex: 4e2a | byte: 42 hex: 2a
char: 字 hex: 5b57 | byte: 87 hex: 57
char: 符 hex: 7b26 | byte: 38 hex: 26
char: 串 hex: 4e32 | byte: 50 hex: 32
d92f002a572632

上面int显示成-39是打印出来, int转成字符串后的显示问题, 在内存中与0xd9二进制一致