为什么DataOutputStream.writeUTF（）在开头添加额外的2个字节？

当我尝试使用sax在套接字上解析xml时，我遇到了一个奇怪的现象 . 经过分析，我注意到DataOutputStream在我的数据前添加了2个字节 .

DataOutputStream发送的消息：

0020  50 18 00 20 0f df 00 00  00 9d 3c 3f 78 6d 6c 20   P.. .... ..<?xml 
0030  76 65 72 73 69 6f 6e 3d  22 31 2e 30 22 3f 3e 3c   version= "1.0"?><
0040  63 6f 6d 70 61 6e 79 3e  3c 73 74 61 66 66 3e 3c   company> <staff><
0050  66 69 72 73 74 6e 61 6d  65 3e 79 6f 6e 67 3c 2f   firstnam e>yong</
0060  66 69 72 73 74 6e 61 6d  65 3e 3c 6c 61 73 74 6e   firstnam e><lastn
0070  61 6d 65 3e 6d 6f 6f 6b  20 6b 69 6d 3c 2f 6c 61   ame>mook  kim</la
0080  73 74 6e 61 6d 65 3e 3c  6e 69 63 6b 6e 61 6d 65   stname>< nickname
0090  3e c2 a7 3c 2f 6e 69 63  6b 6e 61 6d 65 3e 3c 73   >..</nic kname><s
00a0  61 6c 61 72 79 3e 31 30  30 30 30 30 3c 2f 73 61   alary>10 0000</sa
00b0  6c 61 72 79 3e 3c 2f 73  74 61 66 66 3e 3c 2f 63   lary></s taff></c
00c0  6f 6d 70 61 6e 79 3e                               ompany>

使用Transformer发送消息：

0020  50 18 00 20 b6 b1 00 00  3c 3f 78 6d 6c 20 76 65   P.. .... <?xml ve
0030  72 73 69 6f 6e 3d 22 31  2e 30 22 20 65 6e 63 6f   rsion="1 .0" enco
0040  64 69 6e 67 3d 22 75 74  66 2d 38 22 3f 3e 3c 63   ding="ut f-8"?><c
0050  6f 6d 70 61 6e 79 3e 3c  73 74 61 66 66 3e 3c 66   ompany>< staff><f
0060  69 72 73 74 6e 61 6d 65  3e 79 6f 6e 67 3c 2f 66   irstname >yong</f
0070  69 72 73 74 6e 61 6d 65  3e 3c 6c 61 73 74 6e 61   irstname ><lastna
0080  6d 65 3e 6d 6f 6f 6b 20  6b 69 6d 3c 2f 6c 61 73   me>mook  kim</las
0090  74 6e 61 6d 65 3e 3c 6e  69 63 6b 6e 61 6d 65 3e   tname><n ickname>
00a0  c2 a7 3c 2f 6e 69 63 6b  6e 61 6d 65 3e 3c 73 61   ..</nick name><sa
00b0  6c 61 72 79 3e 31 30 30  30 30 30 3c 2f 73 61 6c   lary>100 000</sal
00c0  61 72 79 3e 3c 2f 73 74  61 66 66 3e 3c 2f 63 6f   ary></st aff></co
00d0  6d 70 61 6e 79 3e                                  mpany>

正如人们可能会注意到DataOutputStream在消息前面添加了两个字节 . 因此，sax解析器抛出异常“org.xml.sax.SAXParseException：prolog中不允许使用内容 . ”但是，当我跳过这两个字节时，sax解析器工作得很好 . 另外我注意到DataInputStream无法读取Transformer消息 .

我的问题是：为什么DataOutputStream会添加这些字节，为什么不是Transformer呢？

对于那些有兴趣复制问题的人，这里有一些代码：

使用DataInputStream的服务器：

String data = "<?xml version=\"1.0\"?><company><staff><firstname>yong</firstname><lastname>mook kim</lastname><nickname>§</nickname><salary>100000</salary></staff></company>";
ServerSocket server = new ServerSocket(60000);
Socket socket = server.accept();
DataOutputStream os = new DataOutputStream(socket.getOutputStream());
os.writeUTF(data);
os.close();
socket.close();

使用Transformer的服务器：

ServerSocket server = new ServerSocket(60000);
Socket socket = server.accept();
Document doc = createDocument();
printXML(doc, os);
os.close();
socket.close();

public synchronized static void printXML(Document document, OutputStream stream) throws TransformerException
{
    DOMSource domSource = new DOMSource(document);
    StreamResult streamResult = new StreamResult(stream);
    TransformerFactory tf = TransformerFactory.newInstance();
    Transformer serializer = tf.newTransformer();
    serializer.setOutputProperty(OutputKeys.ENCODING, "utf-8");
    serializer.setOutputProperty(OutputKeys.INDENT, "no");
    serializer.transform(domSource, streamResult);
}

private static Document createDocument() throws ParserConfigurationException
{
    Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument();
    Element company = document.createElement("company");
    Element staff = document.createElement("staff");
    Element firstname = document.createElement("firstname");
    Element lastname = document.createElement("lastname");
    Element nickname = document.createElement("nickname");
    Element salary = document.createElement("salary");
    Text firstnameText = document.createTextNode("yong");
    Text lastnameText = document.createTextNode("mook kim");
    Text nicknameText = document.createTextNode("§");
    Text salaryText = document.createTextNode("100000");
    document.appendChild(company);
    company.appendChild(staff);
    staff.appendChild(firstname);
    staff.appendChild(lastname);
    staff.appendChild(nickname);
    staff.appendChild(salary);
    firstname.appendChild(firstnameText);
    lastname.appendChild(lastnameText);
    nickname.appendChild(nicknameText);
    salary.appendChild(salaryText);
    return document;
}

使用SAX Parser的客户：

SAXParserFactory factory = SAXParserFactory.newInstance();
SAXParser saxParser = factory.newSAXParser();
DefaultHandler handler = new MyHandler();
Socket socket = new Socket("localhost", 60000);
InputSource is = new InputSource(new InputStreamReader(socket.getInputStream()));
is.setEncoding("UTF-8");
//socket.getInputStream().skip(2); // skip over the 2 bytes from the DataInputStream
saxParser.parse(is, handler);

使用DataInputStream的客户端：

Socket socket = new Socket("localhost", 60000);
DataInputStream os = new DataInputStream(socket.getInputStream());
while(true) {
    String data = os.readUTF();
    System.out.println("Data: " + data);
}